< draft-dkg-openpgp-abuse-resistant-keystore-02.txt   draft-dkg-openpgp-abuse-resistant-keystore-03.txt >
openpgp D. Gillmor openpgp D. Gillmor
Internet-Draft ACLU Internet-Draft ACLU
Intended status: Informational April 15, 2019 Intended status: Informational April 19, 2019
Expires: October 17, 2019 Expires: October 21, 2019
Abuse-Resistant OpenPGP Keystores Abuse-Resistant OpenPGP Keystores
draft-dkg-openpgp-abuse-resistant-keystore-02 draft-dkg-openpgp-abuse-resistant-keystore-03
Abstract Abstract
OpenPGP transferable public keys are composite certificates, made up OpenPGP transferable public keys are composite certificates, made up
of primary keys, direct key signatures, user IDs, identity of primary keys, direct key signatures, user IDs, identity
certifications ("signature packets"), subkeys, and so on. They are certifications ("signature packets"), subkeys, and so on. They are
often assembled by merging multiple certificates that all share the often assembled by merging multiple certificates that all share the
same primary key, and are distributed in public keystores. same primary key, and are distributed in public keystores.
Unfortunately, since many keystores permit any third-party to add a Unfortunately, since many keystores permit any third-party to add a
certification with any content to any OpenPGP certificate, the certification with any content to any OpenPGP certificate, the
assembled/merged form of a certificate can become unwieldy or assembled/merged form of a certificate can become unwieldy or
undistributable. Furthermore, keystores that are searched by user ID undistributable. Furthermore, keystores that are searched by user ID
can be made unusable for specific names or addresses by public or fingerprint can be made unusable for specific searches by public
submission of bogus data. And finally, keystores open to public submission of bogus certificates. And finally, keystores open to
submission can also face simple resource exhaustion from flooding public submission can also face simple resource exhaustion from
with bogus submissions, or legal or other risks from uploads of toxic flooding with bogus submissions, or legal or other risks from uploads
data. of toxic data.
This draft documents techniques that an archive of OpenPGP This draft documents techniques that an archive of OpenPGP
certificates can use to mitigate the impact of these various attacks. certificates can use to mitigate the impact of these various attacks,
and the implications of these concerns and mitigations for the rest
of the OpenPGP ecosystem.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 17, 2019. This Internet-Draft will expire on October 21, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Certificate Flooding . . . . . . . . . . . . . . . . . . 7 2.1. Certificate Flooding . . . . . . . . . . . . . . . . . . 7
2.2. User ID Flooding . . . . . . . . . . . . . . . . . . . . 7 2.2. User ID Flooding . . . . . . . . . . . . . . . . . . . . 8
2.3. Keystore Flooding . . . . . . . . . . . . . . . . . . . . 7 2.3. Fingerprint Flooding . . . . . . . . . . . . . . . . . . 8
3. Toxic Data . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.4. Keystore Flooding . . . . . . . . . . . . . . . . . . . . 9
4. Simple Mitigations . . . . . . . . . . . . . . . . . . . . . 8 2.5. Toxic Data . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Decline Large Packets . . . . . . . . . . . . . . . . . . 8 3. Keystore Interfaces . . . . . . . . . . . . . . . . . . . . . 9
4.2. Enforce Strict User IDs . . . . . . . . . . . . . . . . . 9 3.1. Certificate Update . . . . . . . . . . . . . . . . . . . 10
4.3. Scoped User IDs . . . . . . . . . . . . . . . . . . . . . 9 3.2. Certificate Discovery . . . . . . . . . . . . . . . . . . 10
4.4. Strip or Standardize Unhashed Subpackets . . . . . . . . 9 3.3. Certificate Lookup . . . . . . . . . . . . . . . . . . . 11
4.5. Decline User Attributes . . . . . . . . . . . . . . . . . 10 3.3.1. Full User ID Lookup . . . . . . . . . . . . . . . . . 11
4.6. Decline Non-exportable Certifications . . . . . . . . . . 10 3.3.2. E-mail Address Lookup . . . . . . . . . . . . . . . . 12
4.7. Decline Data From the Future . . . . . . . . . . . . . . 10 3.3.3. Other Lookup Mechanisms . . . . . . . . . . . . . . . 12
4.8. Accept Only Profiled Certifications . . . . . . . . . . . 10 3.4. Certificate Validation . . . . . . . . . . . . . . . . . 12
4.9. Accept Only Certificates Issued by Designated Authorities 11 3.5. Certificate Submission . . . . . . . . . . . . . . . . . 14
4.10. Decline Packets by Blocklist . . . . . . . . . . . . . . 11 4. Simple Mitigations . . . . . . . . . . . . . . . . . . . . . 14
5. Retrieval-time Mitigations . . . . . . . . . . . . . . . . . 12 4.1. Decline Large Packets . . . . . . . . . . . . . . . . . . 14
5.1. Redacting User IDs . . . . . . . . . . . . . . . . . . . 12 4.2. Enforce Strict User IDs . . . . . . . . . . . . . . . . . 15
5.1.1. Certificate Update with Redacted User IDs . . . . . . 12 4.3. Scoped User IDs . . . . . . . . . . . . . . . . . . . . . 15
5.1.2. Certificate Discovery with Redacted User IDs . . . . 13 4.4. Strip or Standardize Unhashed Subpackets . . . . . . . . 15
5.1.3. Hinting Redacted User IDs . . . . . . . . . . . . . . 13 4.4.1. Issuer Fingerprint . . . . . . . . . . . . . . . . . 15
5.1.4. User ID Recovery by Client Brute Force . . . . . . . 14 4.4.2. Cross-sigs . . . . . . . . . . . . . . . . . . . . . 15
6. Contextual Mitigations . . . . . . . . . . . . . . . . . . . 14 4.4.3. First-party Attestations . . . . . . . . . . . . . . 16
6.1. Accept Only Cryptographically-verifiable Certifications . 14 4.5. Decline User Attributes . . . . . . . . . . . . . . . . . 16
6.2. Accept Only Certificates Issued by Known Certificates . . 14 4.6. Decline Non-exportable Certifications . . . . . . . . . . 16
6.3. Rate-limit Submissions by IP Address . . . . . . . . . . 15 4.7. Decline Data From the Future . . . . . . . . . . . . . . 16
6.4. Accept Certificates Based on Exterior Process . . . . . . 15 4.8. Accept Only Profiled Certifications . . . . . . . . . . . 16
6.5. Accept Certificates by E-mail Validation . . . . . . . . 15 4.9. Accept Only Certificates Issued by Designated Authorities 17
4.10. Decline Packets by Blocklist . . . . . . . . . . . . . . 17
7. Non-append-only mitigations . . . . . . . . . . . . . . . . . 16 5. Retrieval-time Mitigations . . . . . . . . . . . . . . . . . 18
7.1. Drop Superseded Signatures . . . . . . . . . . . . . . . 16 5.1. Redacting User IDs . . . . . . . . . . . . . . . . . . . 19
7.2. Drop Expired Signatures . . . . . . . . . . . . . . . . . 17 5.1.1. Certificate Update with Redacted User IDs . . . . . . 19
7.3. Drop Dangling User IDs, User Attributes, and Subkeys . . 17 5.1.2. Certificate Discovery with Redacted User IDs . . . . 19
7.4. Drop All Other Elements of a Directly-Revoked Certificate 17 5.1.3. Certificate Lookup with Redacted User IDs . . . . . . 20
7.5. Implicit Expiration Date . . . . . . . . . . . . . . . . 18 5.1.4. Hinting Redacted User IDs . . . . . . . . . . . . . . 20
8. Updates-only Keystores . . . . . . . . . . . . . . . . . . . 18 5.1.5. User ID Recovery by Client Brute Force . . . . . . . 21
9. First-party-only Keystores . . . . . . . . . . . . . . . . . 19 5.2. Primary-key Only Certificate Update . . . . . . . . . . . 21
9.1. First-party-only Without User IDs . . . . . . . . . . . . 19 5.3. Require Valid Cross-Sigs for Certificate Discovery . . . 21
10. First-party-attested Third-party Certifications . . . . . . . 19 6. Contextual Mitigations . . . . . . . . . . . . . . . . . . . 22
10.1. Key Server Preferences "No-modify" . . . . . . . . . . . 21 6.1. Accept Only Cryptographically-verifiable Certifications . 22
10.2. Client Interactions . . . . . . . . . . . . . . . . . . 21 6.2. Accept Only Certificates Issued by Known Certificates . . 23
11. Side Effects and Ecosystem Impacts . . . . . . . . . . . . . 21 6.3. Rate-limit Submissions by IP Address . . . . . . . . . . 23
11.1. Designated Revoker . . . . . . . . . . . . . . . . . . . 21 6.4. Accept Certificates Based on Exterior Process . . . . . . 23
11.2. Certification-capable Subkeys . . . . . . . . . . . . . 22 6.5. Accept Certificates by E-mail Validation . . . . . . . . 24
11.3. Assessing Certificates in the Past . . . . . . . . . . . 22 7. Non-append-only mitigations . . . . . . . . . . . . . . . . . 24
11.3.1. Point-in-time Certificate Evaluation . . . . . . . . 22 7.1. Drop Superseded Signatures . . . . . . . . . . . . . . . 25
11.3.2. Signature Verification and Non-append-only Keystores 23 7.2. Drop Expired Signatures . . . . . . . . . . . . . . . . . 25
11.4. Global Append-only Ledgers ("Blockchain") . . . . . . . 23 7.3. Drop Dangling User IDs, User Attributes, and Subkeys . . 25
11.5. Certificate Discovery for Identity Monitoring . . . . . 24 7.4. Drop All Other Elements of a Directly-Revoked Certificate 26
12. OpenPGP details . . . . . . . . . . . . . . . . . . . . . . . 25 7.5. Implicit Expiration Date . . . . . . . . . . . . . . . . 26
12.1. Revocations . . . . . . . . . . . . . . . . . . . . . . 25 8. Updates-only Keystores . . . . . . . . . . . . . . . . . . . 27
12.2. User ID Conventions . . . . . . . . . . . . . . . . . . 26 9. First-party-only Keystores . . . . . . . . . . . . . . . . . 27
13. Security Considerations . . . . . . . . . . . . . . . . . . . 27 9.1. First-party-only Without User IDs . . . . . . . . . . . . 28
13.1. Tension Between Unrestricted Uploads and Certificate 10. First-party-attested Third-party Certifications . . . . . . . 28
Discovery . . . . . . . . . . . . . . . . . . . . . . . 27 10.1. Key Server Preferences "No-modify" . . . . . . . . . . . 29
14. Privacy Considerations . . . . . . . . . . . . . . . . . . . 27 10.2. Client Interactions . . . . . . . . . . . . . . . . . . 30
14.1. Publishing Identity Information . . . . . . . . . . . . 28 11. Keystore Client Best Practices . . . . . . . . . . . . . . . 30
14.2. Social Graph . . . . . . . . . . . . . . . . . . . . . . 28 11.1. Use Constrained Keystores for Lookup . . . . . . . . . . 30
14.3. Tracking Clients by Queries . . . . . . . . . . . . . . 28 11.2. Normalize Addresses and User IDs for Lookup . . . . . . 30
14.4. Cleartext Queries . . . . . . . . . . . . . . . . . . . 29 11.3. Avoid Fuzzy Lookups . . . . . . . . . . . . . . . . . . 31
14.5. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 29 11.4. Prefer Full Fingerprint for Discovery and Update . . . . 31
15. User Considerations . . . . . . . . . . . . . . . . . . . . . 30 11.5. Use Caution with Keystore-provided Validation . . . . . 31
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30 12. Certificate Generation and Management Best Practices . . . . 32
17. Document Considerations . . . . . . . . . . . . . . . . . . . 30 12.1. Canonicalized E-Mail Addresses . . . . . . . . . . . . . 32
17.1. Document History . . . . . . . . . . . . . . . . . . . . 31 12.2. Normalized User IDs . . . . . . . . . . . . . . . . . . 32
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 12.3. Avoid Large User Attributes . . . . . . . . . . . . . . 32
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 32 12.4. Provide Cross-Sigs . . . . . . . . . . . . . . . . . . . 33
19.1. Normative References . . . . . . . . . . . . . . . . . . 32 12.5. Provide Issuer Fingerprint Subpackets . . . . . . . . . 33
19.2. Informative References . . . . . . . . . . . . . . . . . 33 12.6. Put Cross-Sigs and Issuer Fingerprint in Hashed
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 34 Subpackets . . . . . . . . . . . . . . . . . . . . . . . 33
12.7. Submit Certificates to Restricted, Lookup-Capable
Keystores . . . . . . . . . . . . . . . . . . . . . . . 33
13. Side Effects and Ecosystem Impacts . . . . . . . . . . . . . 33
13.1. Designated Revoker . . . . . . . . . . . . . . . . . . . 33
13.2. Key IDs vs. Fingerprints in Certificate Discovery . . . 34
13.3. In-band Certificates . . . . . . . . . . . . . . . . . . 34
13.3.1. In-band Certificate Minimization and Validity . . . 35
13.4. Certification-capable Subkeys . . . . . . . . . . . . . 36
13.5. Assessing Certificates in the Past . . . . . . . . . . . 36
13.5.1. Point-in-time Certificate Evaluation . . . . . . . . 37
13.5.2. Signature Verification and Non-append-only Keystores 37
13.6. Global Append-only Ledgers ("Blockchain") . . . . . . . 37
13.7. Certificate Lookup for Identity Monitoring . . . . . . . 39
14. OpenPGP details . . . . . . . . . . . . . . . . . . . . . . . 39
14.1. Revocations . . . . . . . . . . . . . . . . . . . . . . 39
14.2. User ID Conventions . . . . . . . . . . . . . . . . . . 40
14.3. E-mail Address Canonicalization . . . . . . . . . . . . 41
14.3.1. Disallowing Non-UTF-8 Local Parts . . . . . . . . . 41
14.3.2. Domain Canonicalization . . . . . . . . . . . . . . 41
14.3.3. Local Part Canonicalization . . . . . . . . . . . . 41
15. Security Considerations . . . . . . . . . . . . . . . . . . . 41
15.1. Tension Between Unrestricted Uploads and Certificate
Lookup . . . . . . . . . . . . . . . . . . . . . . . . . 42
16. Privacy Considerations . . . . . . . . . . . . . . . . . . . 42
16.1. Publishing Identity Information . . . . . . . . . . . . 42
16.2. Social Graph . . . . . . . . . . . . . . . . . . . . . . 43
16.3. Tracking Clients by Queries . . . . . . . . . . . . . . 43
16.4. "Live" Certificate Validation Leaks Client Activity . . 44
16.5. Certificate Discovery Leaks Client Activity . . . . . . 44
16.6. Certificate Update Leaks Client Activity . . . . . . . . 45
16.7. Distinct Keystore Interfaces Leak Client Context and
Intent . . . . . . . . . . . . . . . . . . . . . . . . . 45
16.8. Cleartext Queries . . . . . . . . . . . . . . . . . . . 46
16.9. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 46
17. User Considerations . . . . . . . . . . . . . . . . . . . . . 46
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
19. Document Considerations . . . . . . . . . . . . . . . . . . . 47
19.1. Document History . . . . . . . . . . . . . . . . . . . . 47
20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 49
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
21.1. Normative References . . . . . . . . . . . . . . . . . . 50
21.2. Informative References . . . . . . . . . . . . . . . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction 1. Introduction
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
1.2. Terminology 1.2. Terminology
skipping to change at page 4, line 41 skipping to change at page 5, line 34
certificate, and binds itself to a user ID in the certificate. certificate, and binds itself to a user ID in the certificate.
That is, the issuer is the same as the subject. This is sometimes That is, the issuer is the same as the subject. This is sometimes
referred to as a "self-sig". referred to as a "self-sig".
o A "third-party certification" is a made over a primary key and o A "third-party certification" is a made over a primary key and
user ID by some other certification-capable primary key. That is, user ID by some other certification-capable primary key. That is,
the issuer is different than the subject. (The elusive "second- the issuer is different than the subject. (The elusive "second-
party" is presumed to be the verifier who is trying to interpret party" is presumed to be the verifier who is trying to interpret
the certificate) the certificate)
o All subkeys are bound to the primary key with an [RFC4880] Subkey
Binding Signature. Some subkeys also reciprocate by binding
themselves back to the primary key with an [RFC4880] Primary Key
Binding Signature. The Primary Key Binding Signature is also
known as a "cross-signature" or "cross-sig".
o A "keystore" is any collection of OpenPGP certificates. Keystores o A "keystore" is any collection of OpenPGP certificates. Keystores
typically receive mergeable updates over the course of their typically receive mergeable updates over the course of their
lifetime which might add to the set of OpenPGP certificates they lifetime which might add to the set of OpenPGP certificates they
hold, or update the certificates. hold, or update the certificates.
o "Certificate discovery" is the process whereby a user retrieves an
OpenPGP certificate based on user ID (see Section 12.2). A user
attempting to discover a certificate from a keystore will search
for a substring of the known user IDs, most typically an e-mail
address if the user ID is an [RFC5322] name-addr or addr-spec.
Some certificate discovery mechanisms look for an exact match on
the known user IDs. [I-D.koch-openpgp-webkey-service] and
[I-D.shaw-openpgp-hkp] both offer certificate discovery
mechanisms.
o "Certificate validation" is the process whereby a user decides o "Certificate validation" is the process whereby a user decides
whether a given user ID in an OpenPGP certificate is acceptable. whether a given user ID in an OpenPGP certificate is acceptable
For example, if the certificate has a user ID of "Alice for use. For example, if the certificate has a user ID of "Alice
<alice@example.org>" and the user wants to send an e-mail to <alice@example.org>" and the user wants to send an e-mail to
"alice@example.org", the mail user agent might want to ensure that "alice@example.org", the mail user agent might want to ensure that
the certificate is valid for this e-mail address before encrypting the certificate is valid for this e-mail address before encrypting
to it. This process can take different forms, and can consider to it. Some clients may rely on specific keystores for
many different factors, some of which are not directly contained certificate validation, but some keystores (e.g., [SKS]) make no
in the certificate itself. For example, certificate validation assertions whatsoever about certificate validity, and others offer
might consider whether the certificate was fetched via DANE only very subtle guarantees. See Section 3.4 for more details.
([RFC7929]) or WKD ([I-D.koch-openpgp-webkey-service]); or whether
it has seen e-mails from that address signed by the certificate in
the past; or how long it has known about certificate.
o "Certificate update" is the process whereby a user fetches new o "Certificate lookup" refers to the retrieval of a set of
information about a certificate, potentially merging those OpnePGP certificates from a keystore based on the user ID or some
packets to change the status of the certificate. Updates might substring match of the user ID. See Section 3.3 for more details.
include adding or revoking user IDs or subkeys, updating
expiration dates, or even revoking the entire certificate by
revoking the primary key directly. A user attempting to update a
certificate typically queries a keystore based on the
certificate's fingerprint.
o A "keyserver" is a particular kind of keystore, typically means of o "Certificate update" refers to retrieval of a certificate from a
publicly distributing OpenPGP certificates or updates to them. keystore based on the fingerprint of the primary key. See
Section 3.1 for more details.
o "Certificate discovery" refers to the retrieval of a set of
certificates from a keystore based on the fingerprint or key ID of
any key in the certificate. See Section 3.2 for more details.
o A "keyserver" is a particular kind of keystore, typically a means
of publicly distributing OpenPGP certificates or updates to them.
Examples of keyserver software include [SKS] and Examples of keyserver software include [SKS] and
[MAILVELOPE-KEYSERVER]. One common HTTP interface for keyservers [MAILVELOPE-KEYSERVER]. One common HTTP interface for keyservers
is [I-D.shaw-openpgp-hkp]. is [I-D.shaw-openpgp-hkp].
o A "synchronizing keyserver" is a keyserver which gossips with o A "synchronizing keyserver" is a keyserver which gossips with
other peers, and typically acts as an append-only log. Such a other peers, and typically acts as an append-only log. Such a
keyserver is typically useful for certificate discovery, keyserver is typically useful for certificate lookup, certificate
certificate updates, and revocation information. They are discovery, and certificate update (including revocation
typically _not_ useful for certificate validation, since they make information). They are typically _not_ useful for certificate
no assertions about whether the identities in the certificates validation, since they make no assertions about whether the
they server are accurate. As of the writing of this document, identities in the certificates they server are accurate. As of
[SKS] is the canonical synchronizing keyserver implementation, the writing of this document, [SKS] is the canonical synchronizing
though other implementations exist. keyserver implementation, though other implementations exist.
o An "e-mail validating keyserver" is a keyserver which attempts to o An "e-mail validating keyserver" is a keyserver which attempts to
verify the identity in an OpenPGP certificate's user ID by verify the identity in an OpenPGP certificate's user ID by
confirming access to the e-mail account, and possibly by confirming access to the e-mail account, and possibly by
confirming access to the secret key. Some implementations permit confirming access to the secret key. Some implementations permit
removal of a certificate by anyone who can prove access to the removal of a certificate by anyone who can prove access to the
e-mail address in question. They are useful for certificate e-mail address in question. They are useful for certificate
discovery based on e-mail address and certificate validation (by lookup based on e-mail address and certificate validation (by
users who trust the operator), but some may not be useful for users who trust the operator), but some may not be useful for
certificate update or revocation, since a certificate could be certificate update or certificate discovery, since a certificate
simply replaced by an adversary who also has access to the e-mail could be simply replaced by an adversary who also has access to
address in question. [MAILVELOPE-KEYSERVER] is an example of such the e-mail address in question. [MAILVELOPE-KEYSERVER] is an
a keyserver. example of such a keyserver.
o "Cryptographic validity" refers to mathematical evidence that a o "Cryptographic validity" refers to mathematical evidence that a
signature came from the secret key associated with the public key signature came from the secret key associated with the public key
it claims to come from. Note that a certification may be it claims to come from. Note that a certification may be
cryptographically valid without the signed data being true (for cryptographically valid without the signed data being true (for
example, a given certificate with the user ID "Alice example, a given certificate with the user ID "Alice
<alice@example.org>" might not belong to the person who controls <alice@example.org>" might not belong to the person who controls
the e-mail address "alice@example.org" even though the self-sig is the e-mail address "alice@example.org" even though the self-sig is
cryptographically valid). In particular, cryptographic validity cryptographically valid). In particular, cryptographic validity
for user ID in a certificate is typically insufficient evidence for user ID in a certificate is typically insufficient evidence
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public key of the issuer is necessary to determine whether any public key of the issuer is necessary to determine whether any
given signature is cryptographically valid. Some keyservers given signature is cryptographically valid. Some keyservers
perform cryptographic validation in some contexts. Other perform cryptographic validation in some contexts. Other
keyservers (like [SKS]) perform no cryptographic validation keyservers (like [SKS]) perform no cryptographic validation
whatsoever. whatsoever.
o OpenPGP revocations can have "Reason for Revocation" (see o OpenPGP revocations can have "Reason for Revocation" (see
[RFC4880]), which can be either "soft" or "hard". The set of [RFC4880]), which can be either "soft" or "hard". The set of
"soft" reasons is: "Key is superseded" and "Key is retired and no "soft" reasons is: "Key is superseded" and "Key is retired and no
longer used". All other reasons (and revocations that do not longer used". All other reasons (and revocations that do not
state a reason) are "hard" revocations. See Section 12.1 for more state a reason) are "hard" revocations. See Section 14.1 for more
detail. detail.
2. Problem Statement 2. Problem Statement
OpenPGP keystores that handle submissions from the public are subject OpenPGP keystores that handle submissions from the public are subject
to a range of attacks by malicious submitters. to a range of attacks by malicious submitters.
This section describes four distinct attacks that public keystores This section describes five distinct attacks that public keystores
should consider. should consider.
The rest of the document describes some mitigations that can be used The rest of the document describes some mitigations that can be used
by keystores that are concerned about these problems but want to by keystores that are concerned about these problems but want to
continue to offer some level of service for certificate discovery, continue to offer some level of service for certificate lookup,
certificate update, or certificate validation. certificate update, certificate discovery, or certificate validation.
2.1. Certificate Flooding 2.1. Certificate Flooding
Many public keystores (including both the [SKS] keyserver network and Many public keystores (including both the [SKS] keyserver network and
[MAILVELOPE-KEYSERVER]) allow anyone to attach arbitrary data (in the [MAILVELOPE-KEYSERVER]) allow anyone to attach arbitrary data (in the
form of third-party certifications) to any certificate, bloating that form of third-party certifications) to any certificate, bloating that
certificate to the point of being impossible to effectively retrieve. certificate to the point of being impossible to effectively retrieve.
For example, some OpenPGP implementations simply refuse to process For example, some OpenPGP implementations simply refuse to process
certificates larger than a certain size. certificates larger than a certain size.
This kind of Denial-of-Service attack makes it possible to make This kind of Denial-of-Service attack makes it possible to make
someone else's certificate unretrievable from the keystore, someone else's certificate unretrievable from the keystore,
preventing certificate discovery. It also makes it possible to swamp preventing certificate lookup, discovery, or update. In the case of
a certificate that has been revoked, preventing certificate update, a revoked certificate that has been flooded, this potentially leaves
potentially leaving the client of the keystore with the compromised the client of the keystore with the compromised certificate in an
certificate in an unrevoked state locally. unrevoked state locally because it was unable to fetch the revocation
information.
Additionally, even without malice, OpenPGP certificates can Additionally, even without malice, OpenPGP certificates can
potentially grow without bound. potentially grow without bound.
2.2. User ID Flooding 2.2. User ID Flooding
Public keystores that are used for certificate discovery may also be Public keystores that are used for certificate lookup may also be
vulnerable to attacks that flood the space of known user IDs. In vulnerable to attacks that flood the space of known user IDs. In
particular, if the keystore accepts arbitrary certificates from the particular, if the keystore accepts arbitrary certificates from the
public and does no verification of the user IDs, then any client public and does no verification of the user IDs, then any client
searching for a given user ID may need to review and process an searching for a given user ID may need to review and process an
effectively unbounded set of maliciously-submitted certificates to effectively unbounded set of maliciously-submitted certificates to
find the non-malicious certificates they are looking for. find the non-malicious certificates they are looking for.
For example, if an attacker knows that a given system consults a For example, if an attacker knows that a given system consults a
keystore looking for certificates which match the e-mail address keystore looking for certificates which match the e-mail address
"alice@example.org", the attacker may upload hundreds or thousands of "alice@example.org", the attacker may upload hundreds or thousands of
certificates containing user IDs that match that address. Even if certificates containing user IDs that match that address. Even if
those certificates would not be accepted by a client (e.g., because those certificates would not be accepted by a client (e.g., because
they were not certified by a known-good authority), the client they were not certified by a known-good authority), the client
typically still has to wade through all of them in order to find the typically still has to wade through all of them in order to find the
non-malicious certificates. non-malicious certificates.
If the keystore does not offer a discovery interface at all (that is, If the keystore does not offer a lookup interface at all (that is, if
if clients cannot search it by user ID), then user ID flooding is of clients cannot search it by user ID), then user ID flooding is of
less consequence. less consequence.
2.3. Keystore Flooding 2.3. Fingerprint Flooding
A malicious actor who wants to render a certificate unavailable for
update may generate an arbitrary number of OpenPGP certificates with
the targeted primary key attached as a subkey. If they can convince
a keystore to accept all of those certificates, and the keystore
returns them by subkey match during certificate update, then the
certificate update client will need to spend an arbitrary amount of
bandwidth and processing power filtering out the irrelevant data, and
may potentially give up before discovering the certificate of
interest.
A malicious actor may also want to confuse a certificate discovery
request that was targeted at a particular subkey, by binding that
subkey to multiple bogus certificates. If these bogus certificates
are ingested and redistributed by the keystore, then a certificate
discovery client may receive a set of certificates that cannot be
adequately distinguished.
2.4. Keystore Flooding
A public keystore that accepts arbitrary OpenPGP material and is A public keystore that accepts arbitrary OpenPGP material and is
append-only is at risk of being overwhelmed by sheer quantity of append-only is at risk of being overwhelmed by sheer quantity of
malicious uploaded packets. This is a risk even if the user ID space malicious uploaded packets. This is a risk even if the user ID space
is not being deliberately flooded, and if individual certificates are is not being deliberately flooded, and if individual certificates are
protected from flooding by any of the mechanisms described later in protected from flooding by any of the mechanisms described later in
this document. this document.
The keystore itself can become difficult to operate if the total The keystore itself can become difficult to operate if the total
quantity of data is too large, and if it is a synchronizing quantity of data is too large, and if it is a synchronizing
keyserver, then the quantities of data may impose unsustainable keyserver, then the quantities of data may impose unsustainable
bandwidth costs on the operator as well. bandwidth costs on the operator as well.
Effectively mitigating against keystore flooding requires either Effectively mitigating against keystore flooding requires either
abandoning the append-only property that some keystores prefer, or abandoning the append-only property that some keystores prefer, or
imposing very strict controls on initial ingestion. imposing very strict controls on initial ingestion.
3. Toxic Data 2.5. Toxic Data
Like any large public dataset, it's possible that a keystore ends up Like any large public dataset, it's possible that a keystore ends up
hosting some content that is legally actionable in some hosting some content that is legally actionable in some
jurisdictions, including libel, child pornography, material under jurisdictions, including libel, child pornography, material under
copyright or other "intellectual property" controls, blasphemy, hate copyright or other "intellectual property" controls, blasphemy, hate
speech, etc. speech, etc.
A public keystore that accepts and redistributes arbitrary content A public keystore that accepts and redistributes arbitrary content
may face risk due to uploads of toxic data. may face risk due to uploads of toxic data.
3. Keystore Interfaces
Some keystores have simple interfaces, like files present in a local
filesystem. But many keystores offer an API for certificate
retrieval of different types. This section documents a set of useful
interactions that a client may have with such a keystore.
They are represented in abstract form, and are not intended to be the
full set of interfaces offered by any keystore, but rather a
convenient way to think about the operations that make the keystore
useful for its clients.
Not all keystores may offer all of these interfaces, or they may
offer them in subtly different forms, but clients will nevertheless
try to perform something like these operations with keystores that
they interact with.
3.1. Certificate Update
This is the simplest keystore operation. The client sends the
keystore the full fingerprint of the certificate's primary key, and
the keystore sends the client the corresponding certificate (or
nothing, if the keystore does not contain a certificate with a
matching primary key).
keystore.cert_update(primary_fpr) -> certificate?
A client uses certificate update to retrieve the full details of a
certificate that it already knows about. For example, it might be
interested in updates to the certificate known to the keystore,
including revocations, expiration updates, new third-party
certifications, etc.
Upon successful update, the client SHOULD merge the retrieved
certificate with its local copy.
Not all keystores offer this operation. For example, clients cannot
use WKD ([I-D.koch-openpgp-webkey-service]) or OPENPGPKEY ([RFC7929]
for certificate update.
3.2. Certificate Discovery
If a client is aware of an OpenPGP signature or certification that it
cannot verify because it does not know the issuing certificate, it
may consult a keystore to try to discover the certificate based on
the Issuer or Issuer Fingerprint subpacket in the signature or
certification it is trying to validate.
keystore.cert_discovery(keyid|fpr) -> certificate_list
This is subtly different from certificate update (Section 3.1) in
three ways:
o it may return more than one certificate (e.g., when multiple
certificates share a subkey, or when a primary key on one
certificate is a subkey on another)
o it is willing to accept searches by short key ID, not just
fingerprint
o it is willing to match against a subkey, not just a primary key
While a certificate discovery client does not initially know the
certificate it is looking for, it's possible that the returned
certificate is one that the client already knows about. For example,
a new subkey may have been added to a certificate.
Upon successful discovery, the client SHOULD merge any retrieved
certificates with discovered local copies (as determined by primary
key), and then evaluate the original signature against any retrieved
certificate that appears to be valid and reasonable for use in the
signing context.
It is unclear what a client should do if multiple certificates do
appear to be valid for a given signature, because of ambiguity this
represents about the identity of the signer. However, this ambiguity
is similar to the ambiguity of a certificate with multiple valid user
IDs, which the client already needs to deal with.
Not all keystores offer this operation. For example, clients cannot
use WKD ([I-D.koch-openpgp-webkey-service]) or OPENPGPKEY ([RFC7929]
for certificate discovery.
3.3. Certificate Lookup
If a client wants to encrypt a message to a particular e-mail
address, or wants to encrypt a backup to some identity that it knows
of but does not have a certificate for, it may consult a keystore to
discover certificates that claim that identity in their user ID
packets. Both [I-D.koch-openpgp-webkey-service] and
[I-D.shaw-openpgp-hkp] offer certificate lookup mechanisms.
[RFC4880] User IDs are constrained only in that they are a UTF-8
string, but some conventions govern their practical use. See
Section 14.2 for more discussion of some common conventions around
user ID structure.
Note that lookup does not necessarily imply user ID or certificate
validation. It is entirely possible for a keystore to return a
certificate during lookup that the client cannot validate.
Abuse-resistant keystores that offer a lookup interface SHOULD
distinguish interfaces that perform full-string-match lookup from
interfaces that perform e-mail address based lookup.
3.3.1. Full User ID Lookup
The most straightforward form of certificate lookup asks for the set
of all certificates that contain a user ID that exactly and
completely matches the query parameter supplied by the client.
keystore.cert_lookup(uid) -> certificate_list
In its simplest form, this match is done by a simple bytestring
comparison. More sophisticated keystores MAY perform the comparison
after applying [UNICODE-NORMALIZATION] form NFC to both the "uid"
query and the user IDs from the stored certificates.
3.3.2. E-mail Address Lookup
However, some common use cases look for specific patterns in the user
ID rather than the entire user ID. Most useful to many existing
OpenPGP clients is a lookup by e-mail address.
keystore.cert_lookup(addr) -> certificate_list
For certificates with a user ID that matches the structure of an
[RFC5322] "name-addr" or "addr-spec", a keystore SHOULD extract the
"addr-spec" from the user ID, canonicalize it (see Section 14.3), and
compare it to the canonicalized form of of the "addr" query
parameter.
3.3.3. Other Lookup Mechanisms
Some keystores offer other forms of substring or regular expression
matching against the stored user IDs. These other forms of lookup
may be useful in some contexts (e.g., Section 13.7), but they may
also represent privacy concerns (e.g., Section 16.1), and they may
impose additional computational or indexing burdens on the keystore.
3.4. Certificate Validation
An OpenPGP client may assess certificate and user ID validity based
on many factors, some of which are directly contained in the
certificate itself (e.g., third-party certifications), and some of
which are based on the context known to the client, including:
o Whether it has seen e-mails from that address signed by that
certificate in the past,
o How long it has known about the certificate,
o Whether the certificate was fetched from a keystore that asserts
validity of the user ID or some part of it (such as the e-mail
address).
A keystore MAY facilitate clients pursuing this last point of
contextual corroboration via a direct interface:
keystore.cert_validate(primary_fpr, uid) -> boolean
In an e-mail-specific context, the client might only care about the
keystore's opinion about the validity of the certificate for the
e-mail address portion of the user ID only:
keystore.cert_validate(primary_fpr, addr) -> boolean
For some keystores, the presence of a certificate in the keystore
alone implies that the keystore asserts the validity of all user IDs
in the certificate retrieved. For others, the presence in the
keystore applies only to some part of the user ID. For example,
[PGP-GLOBAL-DIRECTORY] will only return user IDs that have completed
an e-mail validation step, so presence in that keystore implies an
assertion of validity of the e-mail address part of the user IDs
returned, but makes no claim about the "display-name" portion of any
returned user IDs. Note that a client retrieving a certificate from
such a keystore may merge the certificate with a local copy - but the
validity asserted by the keystore of course has no bearing on the
packets that the keystore did not return.
In a more subtle example, the retrieval of a certificate looked up
via WKD ([I-D.koch-openpgp-webkey-service]) or DANE ([RFC7929])
should only be interpreted as a claim of validity about any user ID
which matches the e-mail address by which the certificate was looked
up, with no claims made about any "display-name" portions, or about
any user ID that doesn't match the queried e-mail address at all.
A keystore that offers some sort of validation interface may also
change its opinion about the validity of a given certificate or user
ID over time; the interface described above only allows the client to
ask about the keystore's current opinion, but a more complex
interface might be capable of describing the keystore's assertion
over time. See also Section 13.5.
An abuse-resistant keystore that clients rely on for any part of
their certificate validation process SHOULD offer a distinct
interface for making assertions about certificate and user ID
validity to help clients avoid some of the subtleties involved with
inference based on presence described above.
Note that the certificate validation operation as described above has
a boolean response. While a "true" response indicates that keystore
believes the user ID or e-mail address is acceptable for use with the
certificate referred to by the public key fingerprint, a "false"
response doesn't necessarily mean that the keystore actively thinks
that the certificate is actively bad, or must not be used for the
referenced identity. Rather, "false" is the default state: no
opinion is expressed by the keystore, and the client is left to make
their own inference about validity based on other factors. A
keystore MAY offer a more nuanced validity interface; if it does, it
SHOULD explicitly document the semantics of the different response
types so that clients can make appropriate judgement.
3.5. Certificate Submission
Different keystores have different ways to submit a certificate for
consideration for ingestion, including:
o a simple upload of a certificate via http
o round-trip e-mail verification
o proof of presence in some other service
o vouching, or other forms of multi-party attestation
Because these schemes vary so widely, this document does not attempt
to describe the keystore certificate submission process in detail.
However, guidance can be found for implementations that generate,
manage, and submit certificates in Section 12.
4. Simple Mitigations 4. Simple Mitigations
These steps can be taken by any keystore that wants to avoid These steps can be taken by any keystore that wants to avoid
obviously malicious abuse. They can be implemented on receipt of any obviously malicious abuse. They can be implemented on receipt of any
new packet, and are based strictly on the structure of the packet new packet, and are based strictly on the structure of the packet
itself. itself.
4.1. Decline Large Packets 4.1. Decline Large Packets
While [RFC4880] permits OpenPGP packet sizes of arbitrary length, While [RFC4880] permits OpenPGP packet sizes of arbitrary length,
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packets specifically, but SHOULD NOT allow even user attributes packets specifically, but SHOULD NOT allow even user attributes
packets larger than 65536 octets. packets larger than 65536 octets.
4.2. Enforce Strict User IDs 4.2. Enforce Strict User IDs
[RFC4880] indicates that User IDs are expected to be UTF-8 strings. [RFC4880] indicates that User IDs are expected to be UTF-8 strings.
An abuse-resistant keystore MUST reject any user ID that is not valid An abuse-resistant keystore MUST reject any user ID that is not valid
UTF-8. UTF-8.
Some abuse-resistant keystores MAY only accept User IDs that meet Some abuse-resistant keystores MAY only accept User IDs that meet
even stricter conventions, such as an [RFC5322] name-addr or addr- even stricter conventions, such as an [RFC5322] "name-addr" or "addr-
spec, or a URL like "ssh://host.example.org". spec", or a URL like "ssh://host.example.org" (see Section 14.2).
As simple text strings, User IDs don't need to be nearly as long as As simple text strings, User IDs don't need to be nearly as long as
any other packets. An abuse-resistant keystore SHOULD reject any any other packets. An abuse-resistant keystore SHOULD reject any
user ID packet larger than 1024 octets. user ID packet larger than 1024 octets.
4.3. Scoped User IDs 4.3. Scoped User IDs
Some abuse-resistant keystores may restrict themselves to publishing Some abuse-resistant keystores may restrict themselves to publishing
only certificates with User IDs that match a specific pattern. For only certificates with User IDs that match a specific pattern. For
example, [RFC7929] encourages publication in the DNS of only example, [RFC7929] encourages publication in the DNS of only
certificates whose user IDs refer to e-mail addresses within the DNS certificates whose user IDs refer to e-mail addresses within the DNS
zone. [I-D.koch-openpgp-webkey-service] similarly aims to restrict zone. [I-D.koch-openpgp-webkey-service] similarly aims to restrict
publication to certificates relevant to the specific e-mail domain. publication to certificates relevant to the specific e-mail domain.
4.4. Strip or Standardize Unhashed Subpackets 4.4. Strip or Standardize Unhashed Subpackets
[RFC4880] signature packets contain an "unhashed" block of [RFC4880] signature packets contain an "unhashed" block of
subpackets. These subpackets are not covered by any cryptographic subpackets. These subpackets are not covered by any cryptographic
signature, so they are ripe for abuse. signature, so they are ripe for abuse.
An abuse-resistant keysetore SHOULD strip out all unhashed An abuse-resistant keystore SHOULD strip out all unhashed subpackets
subpackets. but the following exceptions:
Note that some certifications only identify the issuer of the 4.4.1. Issuer Fingerprint
certification by an unhashed Issuer ID subpacket. If a
certification's hashed subpacket section has no Issuer ID or Issuer
Fingerprint (see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an
abuse-resistant keystore that has cryptographically validated the
certification SHOULD make the unhashed subpackets contain only a
single subpacket. That subpacket should be of type Issuer
Fingerprint, and should contain the fingerprint of the issuer.
A special exception may be made for unhashed subpackets in a third- Some certifications only identify the issuer of the certification by
party certification that contain attestations from the certificate's an unhashed Issuer or Issuer Fingerprint subpacket. If a
primary key as described in Section 10. certification's hashed subpacket section has no Issuer Fingerprint
(see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an abuse-
resistant keystore that has cryptographically validated the
certification SHOULD synthesize an appropriate Issuer Fingerprint
subpacket and include it in the certification's unhashed subpackets.
4.4.2. Cross-sigs
Some Primary Key Binding Signatures ("cross-sigs") are distributed as
unhashed subpackets in a Subkey Binding Signature. A
cryptographically-validating abuse-resistant keystore SHOULD be
willing to redistribute a valid cross-sig as an unhashed subpacket.
The redistributed unhashed cross-sig itself should be stripped of all
unhashed subpackets.
4.4.3. First-party Attestations
Some third-party certifications are attested to by the certificate
primary key itself in an unhashed subpacket, as described in
Section 10. A cryptographically-validating abuse-resistant keystore
SHOULD be willing to redistribute a valid first-party attestation as
an unhashed subpacket.
The redistributed first-party attestation itself should be stripped
of all unhashed subpackets.
4.5. Decline User Attributes 4.5. Decline User Attributes
Due to size concerns, some abuse-resistant keystores MAY choose to Due to size concerns, some abuse-resistant keystores MAY choose to
ignore user attribute packets entirely, as well as any certifications ignore user attribute packets entirely, as well as any certifications
that cover them. that cover them.
4.6. Decline Non-exportable Certifications 4.6. Decline Non-exportable Certifications
An abuse-resistant keystore MUST NOT accept any certification that An abuse-resistant keystore MUST NOT accept any certification that
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Some keystores may choose to apply a blocklist only at retrieval time Some keystores may choose to apply a blocklist only at retrieval time
and not apply it at ingestion time. This allows the keystore to be and not apply it at ingestion time. This allows the keystore to be
append-only, and permits synchronization between keystores that don't append-only, and permits synchronization between keystores that don't
share a blocklist, and somewhat reduces the attacker's incentive for share a blocklist, and somewhat reduces the attacker's incentive for
flooding the keystore (see Section 5 for more discussion). flooding the keystore (see Section 5 for more discussion).
Note that development and maintenance of a blocklist is not without Note that development and maintenance of a blocklist is not without
its own potentials for abuse. For one thing, the blocklist may its own potentials for abuse. For one thing, the blocklist may
itself grow without bound. Additionally, a blocklist may be socially itself grow without bound. Additionally, a blocklist may be socially
or politically contentious as it may describe data that is toxic or politically contentious as it may describe data that is toxic
(Section 3) in one community or jurisdiction but not another. There (Section 2.5) in one community or jurisdiction but not another.
needs to be a clear policy about how it is managed, whether by There needs to be a clear policy about how it is managed, whether by
delegation to specific decision-makers, or explicit tests. delegation to specific decision-makers, or explicit tests.
Furthermore, the existence of even a well-intentioned blocklist may Furthermore, the existence of even a well-intentioned blocklist may
be an "attractive nuisance," drawing the interest of would-be censors be an "attractive nuisance," drawing the interest of would-be censors
or other attacker interested in controlling the ecosystem reliant on or other attacker interested in controlling the ecosystem reliant on
the keystore in question. the keystore in question.
5. Retrieval-time Mitigations 5. Retrieval-time Mitigations
Most of the abuse mitigations described in this document are Most of the abuse mitigations described in this document are
described as being applied at certificate ingestion time. It's also described as being applied at certificate ingestion time. It's also
possible to apply the same mitigations when a certificate is possible to apply the same mitigations when a certificate is
retrieved from the keystore (that is, during certificate update or retrieved from the keystore (that is, during certificate lookup,
certificate discovery). Applying an abuse mitigation at retrieval update, or discovery). Applying an abuse mitigation at retrieval
time may help a client defend against a user ID flooding time may help a client defend against a user ID flooding
(Section 2.2) or certificate flooding (Section 2.1) attack. However, (Section 2.2), certificate flooding (Section 2.1), or fingerprint
only mitigations applied at ingestion time are able to mitigate flooding (Section 2.3) attack. It may also help a keystore limit its
keystore flooding attacks (Section 2.3). liability for redistributing toxic data (Section 2.5). However, only
mitigations applied at ingestion time are able to mitigate keystore
flooding attacks (Section 2.4).
Some mitigations (like the non-append-only mitigations described in Some mitigations (like the non-append-only mitigations described in
Section 7) may be applied as filters at retrieval time, while still Section 7) may be applied as filters at retrieval time, while still
allowing access to the (potentially much larger) unfiltered dataset allowing access to the (potentially much larger) unfiltered dataset
associated given certificate or user ID via a distinct interface. associated given certificate or user ID via a distinct interface.
The rest of this section documents a specific mitigation that is The rest of this section documents specific mitigations that are only
applied only at retrieval time. relevant at retrieval time (certificate discovery, lookup, or
update).
5.1. Redacting User IDs 5.1. Redacting User IDs
Some abuse-resistant keystores may accept and store user IDs but Some abuse-resistant keystores may accept and store user IDs but
decline to redistribute some or all of them, while still distributing decline to redistribute some or all of them, while still distributing
the certifications that cover those redacted user IDs. This draft the certifications that cover those redacted user IDs. This draft
refers to such a keystore as a "user ID redacting" keystore. refers to such a keystore as a "user ID redacting" keystore.
The certificates distributed by such a keystore are technically The certificates distributed by such a keystore are technically
invalid [RFC4880] "transferable public keys", because they lack a invalid [RFC4880] "transferable public keys", because they lack a
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A user ID redacting keystore is useful for certificate update by a A user ID redacting keystore is useful for certificate update by a
client that already knows the user ID it expects to see associated client that already knows the user ID it expects to see associated
with the certificate. For example, a client that knows a given with the certificate. For example, a client that knows a given
certificate currently has two specific user IDs could access the certificate currently has two specific user IDs could access the
keystore to learn that one of the user IDs has been revoked, without keystore to learn that one of the user IDs has been revoked, without
any other client learning the user IDs directly from the keystore. any other client learning the user IDs directly from the keystore.
5.1.2. Certificate Discovery with Redacted User IDs 5.1.2. Certificate Discovery with Redacted User IDs
A user ID redacting keystore is somewhat less useful for clients
doing certificate discovery. Consider the circumstance of receiving
a signed e-mail without access to the signing certificate. If the
verifier retrieves the certificate from a user ID redacting keystore
by via the Issuer Fingerprint from the signature, and the signature
validates, the received certificate might not be a valid
"transferable public key" unless the client can synthesize the proper
user ID.
A reasonable client that wants to validate a certification in the
user ID redacted certificate SHOULD try to synthesize possible user
IDs based on the value of the [RFC5322] From: header in the message:
o Decode any [RFC2047] encodings present in the raw header value,
converting into UTF-8 [UNICODE-NORMALIZATION] form C (NFC),
trimming all whitespace from the beginning and the end of the
string.
o The resulting string should be an [RFC5322] "name-addr" or "addr-
spec".
o If it is a "name-addr", convert the UTF-8 string into an OpenPGP
user ID and check whether the certification validates, terminating
on success.
* If the test fails, extract the "addr-spec" from the "name-addr"
and continue.
o Canonicalize the "addr-spec" according to Section 14.3, and check
whether the certification validates, terminating on success.
o If it doesn't validate wrap the canonicalized "addr-spec" in
angle-brackets ("<" and ">") and test the resulting minimalist
"name-addr" against the certification, terminating on success.
o If all of the above fails, synthesis has failed.
5.1.3. Certificate Lookup with Redacted User IDs
It's possible (though non-intuitive) to use a user ID redacting It's possible (though non-intuitive) to use a user ID redacting
keystore for certificate discovery. Since the keystore retains (but keystore for certificate lookup. Since the keystore retains (but
does not distribute) the user IDs, they can be used to select does not distribute) the user IDs, they can be used to select
certificates in response to a search. The OpenPGP certificates sent certificates in response to a search. The OpenPGP certificates sent
back in response to the search will not contain the user IDs, but a back in response to the search will not contain the user IDs, but a
client that knows the full user ID they are searching for will be client that knows the full user ID they are searching for will be
able to verify the returned certifications. able to verify the returned certifications.
Certificate discovery from a user ID redacting keystore works better Certificate lookup from a user ID redacting keystore works better for
for certificate discovery by exact user ID match than it does for certificate lookup by exact user ID match than it does for substring
substring match, because a client that discovers a substring match match, because a client that retrieves a certificate via a substring
may not be able to reconstruct the redacted user ID. match may not be able to reconstruct the redacted user ID.
However, without some additional restrictions on which certifications However, without some additional restrictions on which certifications
are redistributed (whether the user ID is redacted or not), are redistributed (whether the user ID is redacted or not),
certificate discovery can be flooded (see Section 13.1). certificate lookup can be flooded (see Section 15.1).
5.1.3. Hinting Redacted User IDs 5.1.4. Hinting Redacted User IDs
To ensure that the distributed certificate is at least structurally a To ensure that the distributed certificate is at least structurally a
valid [RFC4880] transferable public key, a user ID redacting keystore valid [RFC4880] transferable public key, a user ID redacting keystore
MAY distribute an empty user ID (an OpenPGP packet of tag 13 whose MAY distribute an empty user ID (an OpenPGP packet of tag 13 whose
contents are a zero-octet string) in place of the omitted user ID. contents are a zero-octet string) in place of the omitted user ID.
This two-octet replacement user ID packet ("\xb4\x00") is called the This two-octet replacement user ID packet ("\xb4\x00") is called the
"unstated user ID". "unstated user ID".
To facilitate clients that match certifications with specific user To facilitate clients that match certifications with specific user
IDs, a user ID redacting keystore MAY insert a non-hashed notation IDs, a user ID redacting keystore MAY insert a non-hashed notation
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"uidhash", with 0x80 ("human-readable") flag unset. The value of "uidhash", with 0x80 ("human-readable") flag unset. The value of
such a notation MUST be 32 octets long, and contains the SHA-256 such a notation MUST be 32 octets long, and contains the SHA-256
cryptographic digest of the UTF-8 string of the redacted user ID. cryptographic digest of the UTF-8 string of the redacted user ID.
A certificate update client which receives such a certification after A certificate update client which receives such a certification after
the "unstated user ID" SHOULD compute the SHA-256 digest of all user the "unstated user ID" SHOULD compute the SHA-256 digest of all user
IDs it knows about on the certificate, and compare the result with IDs it knows about on the certificate, and compare the result with
the contents of the "uidhash" notation to decide which user ID to try the contents of the "uidhash" notation to decide which user ID to try
to validate the certification against. to validate the certification against.
5.1.4. User ID Recovery by Client Brute Force 5.1.5. User ID Recovery by Client Brute Force
User ID redaction is at best an imperfect process. Even if a User ID redaction is at best an imperfect process. Even if a
keystore redacts a User ID, if it ships a certification over that keystore redacts a User ID, if it ships a certification over that
user ID, an interested client can guess user IDs until it finds one user ID, an interested client can guess user IDs until it finds one
that causes the signature to verify. This is even easier when the that causes the signature to verify. This is even easier when the
space of legitimate user IDs is relatively small, such as the set of space of legitimate user IDs is relatively small, such as the set of
commonly-used hostnames commonly-used hostnames
5.2. Primary-key Only Certificate Update
Abuse-resistant keystores can defend against a fingerprint flooding
Section 2.3 attack during certificate update by implementing a
narrowly-constrained certificate update interface.
Such a keystore MUST accept only a full fingerprint as the search
parameter from the certificate update client, and it MUST return at
most a single certificate whose primary key matches the requested
fingerprint. It MUST NOT return more than one certificate, and it
MUST NOT return any certificate whose primary key does not match the
fingerprint. In particular, it MUST NOT return certificates where
only the subkey fingerprint matches.
Note that [I-D.shaw-openpgp-hkp] does not offer the primitive
described in Section 3.1 exactly. In that specification, the set of
keys returned by a "get" operation with a "search" parameter that
appears to be a full fingerprint is ambiguous. Some popular
implementations (e.g., [SKS]) do not currently implement this
mitigation, because they return certificates with subkeys that match
the fingerprint.
5.3. Require Valid Cross-Sigs for Certificate Discovery
By definition, certificate discovery needs to be able to match
subkeys, not just primary keys. This means that the mitigation in
Section 5.2 is ineffective for a keystore that offers a certificate
discovery interface.
An abuse-resistant keystore that aims to defend its certificate
discovery interface from a fingerprint flooding (Section 2.3) attack
can follow the following procedure.
Such a keystore MUST accept only a full fingerprint or a 64-bit key
ID as the search parameter from the certificate discovery client. It
MUST only match that fingerprint against the following:
o the fingerprint or key ID of a primary key associated with a valid
certificate
o the fingerprint or key ID of a cryptographically-valid subkey that
also has a cross-sig.
This defends against the fingerprint flooding attack because a
certificate will only be returned by subkey if the subkey has agreed
to be associated with the primary key (and vice versa).
Note that this mitigation means that certificate discovery will fail
if used for subkeys that lack cross-sigs. In particular, this means
that a client that tries to use the certificate discovery interface
to retrieve a certificate based on its encryption-capable subkey
(e.g., taking the key ID from a Public Key Encrypted Session Key
(PKESK) packet) will have no success.
This is an acceptable loss, since the key ID in a PKESK is typically
unverifiable anyway.
6. Contextual Mitigations 6. Contextual Mitigations
Some mitigations make the acceptance or rejection of packets Some mitigations make the acceptance or rejection of packets
contingent on data that is already in the keystore or the keystore's contingent on data that is already in the keystore or the keystore's
developing knowledge about the world. This means that, depending on developing knowledge about the world. This means that, depending on
the order that the keystore encounters the various material, or how the order that the keystore encounters the various material, or how
it discovers the material, the final set of material retained and it accesses or finds the material, the final set of material retained
distributed by the keystore might be different. and distributed by the keystore might be different.
While this isn't necessarily bad, it may be a surprising property for While this isn't necessarily bad, it may be a surprising property for
some users of keystores. some users of keystores.
6.1. Accept Only Cryptographically-verifiable Certifications 6.1. Accept Only Cryptographically-verifiable Certifications
An abuse-resistant keystore that is capable of doing cryptographic An abuse-resistant keystore that is capable of doing cryptographic
validation MAY decide to reject certifications that it cannot validation MAY decide to reject certifications that it cannot
cryptographically validate. cryptographically validate.
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The following mitigations may cause some previously-retained packets The following mitigations may cause some previously-retained packets
to be dropped after the keystore receives new information, or as time to be dropped after the keystore receives new information, or as time
passes. This is entirely reasonable for some keystores, but it may passes. This is entirely reasonable for some keystores, but it may
be surprising for any keystore that expects to be append-only (for be surprising for any keystore that expects to be append-only (for
example, some keyserver synchronization techniques may expect this example, some keyserver synchronization techniques may expect this
property to hold). property to hold).
Furthermore, keystores that drop old data (e.g., superseded Furthermore, keystores that drop old data (e.g., superseded
certifications) may make it difficult or impossible for their users certifications) may make it difficult or impossible for their users
to reason about the validity of signatures that were made in the to reason about the validity of signatures that were made in the
past. See Section 11.3 for more considerations. past. See Section 13.5 for more considerations.
Note also that many of these mitigations depend on cryptographic Note also that many of these mitigations depend on cryptographic
validation, so they're typically contextual as well. validation, so they're typically contextual as well.
A keystore that needs to be append-only, or which cannot perform A keystore that needs to be append-only, or which cannot perform
cryptographic validation MAY omit these mitigations. Alternately, a cryptographic validation MAY omit these mitigations. Alternately, a
keystore may omit these mitigations at certificate ingestion time, keystore may omit these mitigations at certificate ingestion time,
but apply these mitigations at retrieval time (during certificate but apply these mitigations at retrieval time (during certificate
update or discovery), and offer a more verbose (non-mitigated) update, discovery, or lookup), and offer a more verbose (non-
interface for auditors, as described in Section 5. mitigated) interface for auditors, as described in Section 5.
Note that [GnuPG] anticipates some of these suggestions with its Note that [GnuPG] anticipates some of these suggestions with its
"clean" subcommand, which is documented as: "clean" subcommand, which is documented as:
Compact (by removing all signatures except the selfsig) Compact (by removing all signatures except the selfsig)
any user ID that is no longer usable (e.g. revoked, or any user ID that is no longer usable (e.g. revoked, or
expired). Then, remove any signatures that are not usable expired). Then, remove any signatures that are not usable
by the trust calculations. Specifically, this removes by the trust calculations. Specifically, this removes
any signature that does not validate, any signature that any signature that does not validate, any signature that
is superseded by a later signature, revoked signatures, is superseded by a later signature, revoked signatures,
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subkeys, as both of these kinds of signature packets may be subkeys, as both of these kinds of signature packets may be
superseded. superseded.
Getting this right requires a nuanced understanding of subtleties in Getting this right requires a nuanced understanding of subtleties in
[RFC4880] related to timing and revocation. [RFC4880] related to timing and revocation.
7.2. Drop Expired Signatures 7.2. Drop Expired Signatures
If a signature packet is known to only be valid in the past, there is If a signature packet is known to only be valid in the past, there is
no reason to distribute it further. An abuse-resistant keystore with no reason to distribute it further. An abuse-resistant keystore with
access to a functionally real-time clock SHOULD drop all access to a functional real-time clock SHOULD drop all certifications
certifications and subkey signature packets with an expiration date and subkey signature packets with an expiration date in the past.
in the past.
Note that this assumes that the keystore and its clients all have Note that this assumes that the keystore and its clients all have
roughly-synchronized clocks. If that is not the case, then there roughly-synchronized clocks. If that is not the case, then there
will be many other problems! will be many other problems!
7.3. Drop Dangling User IDs, User Attributes, and Subkeys 7.3. Drop Dangling User IDs, User Attributes, and Subkeys
If enough signature packets are dropped, it's possible that some of If enough signature packets are dropped, it's possible that some of
the things that those signature packets cover are no longer valid. the things that those signature packets cover are no longer valid.
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signature, an abuse-resistant keystore SHOULD drop all the rest of signature, an abuse-resistant keystore SHOULD drop all the rest of
the associated data (user IDs, user attributes, and subkeys, and all the associated data (user IDs, user attributes, and subkeys, and all
attendant certifications and subkey signatures). This defends attendant certifications and subkey signatures). This defends
against an adversary who compromises a primary key and tries to flood against an adversary who compromises a primary key and tries to flood
the certificate to hide the revocation. the certificate to hide the revocation.
Note that the direct key revocation signature MUST NOT be dropped. Note that the direct key revocation signature MUST NOT be dropped.
In the event that an abuse-resistant keystore is flooded with direct In the event that an abuse-resistant keystore is flooded with direct
key revocation signatures, it should retain the hardest, earliest key revocation signatures, it should retain the hardest, earliest
revocation (see also Section 12.1). revocation (see also Section 14.1).
In particular, if any of the direct key revocation signatures is a In particular, if any of the direct key revocation signatures is a
"hard" revocation, the abuse-resistant keystore SHOULD retain the "hard" revocation, the abuse-resistant keystore SHOULD retain the
earliest such revocation signature (by signature creation date). earliest such revocation signature (by signature creation date).
Otherwise, the abuse-resistant keystore SHOULD retain the earliest Otherwise, the abuse-resistant keystore SHOULD retain the earliest
"soft" direct key revocation signature it has seen. "soft" direct key revocation signature it has seen.
If either of the above date comparisons results in a tie between two If either of the above date comparisons results in a tie between two
revocation signatures of the same "hardness", an abuse-resistant revocation signatures of the same "hardness", an abuse-resistant
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7.5. Implicit Expiration Date 7.5. Implicit Expiration Date
In combination with some of the dropping mitigations above, a In combination with some of the dropping mitigations above, a
particularly aggressive abuse-resistant keystore MAY choose an particularly aggressive abuse-resistant keystore MAY choose an
implicit expiration date for all signature packets. For example, a implicit expiration date for all signature packets. For example, a
signature packet that claims no expiration could be treated by the signature packet that claims no expiration could be treated by the
keystore as expiring 3 years after issuance. This would permit the keystore as expiring 3 years after issuance. This would permit the
keystore to eject old packets on a rolling basis. keystore to eject old packets on a rolling basis.
FIXME: it's not clear what should happen with signature packets An abuse-resistant keystore that adopts this mitigation needs a
marked with an explicit expiration that is longer than implicit policy for handling signature packets marked with an explicit
maximum. Should it be capped to the implicit date, or accepted? expiration that is longer than implicit maximum. The two obvious
strategies are:
Warning: This idea is pretty radical, and it's not clear what it o cap the packet's expiration to the system's implicit expiration
would do to an ecosystem that depends on such a keystore. It date, or
probably needs more thinking.
o accept the explicit expiration date.
Warning: Any implementation of this idea is pretty radical, and it's
not clear what it would do to an ecosystem that depends on such a
keystore. It probably needs more thinking.
8. Updates-only Keystores 8. Updates-only Keystores
In addition to the mitigations above, some keystores may resist abuse In addition to the mitigations above, some keystores may resist abuse
by declining to accept any user IDs or certifications whatsoever. by declining to accept any user IDs or certifications whatsoever.
Such a keystore MUST be capable of cryptographic validation. It Such a keystore MUST be capable of cryptographic validation. It
accepts primary key packets, cryptographically-valid direct-key accepts primary key packets, cryptographically-valid direct-key
signatures from a primary key over itself, subkeys and their signatures from a primary key over itself, subkeys and their
cryptographically-validated binding signatures (and cross signatures, cryptographically-validated binding signatures (and cross-sigs, where
where necessary). necessary).
Clients of an updates-only keystore cannot possibly use the keystore A client of an updates-only keystore cannot possibly use the keystore
for certificate discovery, because there are no user IDs to match. for certificate lookup, because there are no user IDs to match. And
However, they can use it for certificate update, as it's possible to it is not particularly useful for certificate discovery, because the
ship revocations (which are direct key signatures), new subkeys, returned certificate would have no identity information. However,
such a keystore can be used for certificate update, as it's possible
to ship revocations (which are direct key signatures), new subkeys,
updates to subkey expiration, subkey revocation, and direct key updates to subkey expiration, subkey revocation, and direct key
signature-based certificate expiration updates. signature-based certificate expiration updates.
Note that many popular OpenPGP implementations do not implement Note that many popular OpenPGP implementations do not implement
direct primary key expiration mechanisms, relying instead on user ID direct primary key expiration mechanisms, relying instead on user ID
expirations. These user ID expiration dates or other metadata expirations. These user ID expiration dates or other metadata
associated with a self-certification will not be distributed by an associated with a self-certification will not be distributed by an
updates-only keystore. updates-only keystore.
Certificates shipped by an updates-only keystore are technically Certificates shipped by an updates-only keystore are technically
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Section 8 is a keystore that also permits user IDs and their self- Section 8 is a keystore that also permits user IDs and their self-
sigs. sigs.
A first-party-only keystore only accepts and distributes A first-party-only keystore only accepts and distributes
cryptographically-valid first-party certifications. Given a primary cryptographically-valid first-party certifications. Given a primary
key that the keystore understands, it will only attach user IDs that key that the keystore understands, it will only attach user IDs that
have a valid self-sig, and will only accept and re-distribute subkeys have a valid self-sig, and will only accept and re-distribute subkeys
that are also cryptographically valid (including requiring cross-sigs that are also cryptographically valid (including requiring cross-sigs
for signing-capable subkeys as recommended in [RFC4880]). for signing-capable subkeys as recommended in [RFC4880]).
This effectively solves the problem of abusive bloating attacks on This effectively avoids certificate flooding attacks, because the
any certificate, because the only party who can make a certificate only party who can make a certificate overly large is the holder of
overly large is the holder of the secret corresponding to the primary the secret corresponding to the primary key itself.
key itself.
Note that a first-party-only keystore is still problematic for those Note that a first-party-only keystore is still problematic for those
people who rely on the keystore for discovery of third-party people who rely on the keystore for retrieval of third-party
certifications. Section 10 attempts to address this lack. certifications. Section 10 attempts to address this lack.
9.1. First-party-only Without User IDs 9.1. First-party-only Without User IDs
It is possible to operate an updates-only keystore that also declines It is possible to operate an first-party-only keystore that
to redistribute user IDs (Section 5.1). This defends against redistributes certifications while declining to redistribute user IDs
concerns about publishing identifiable information, while enabling (see Section 5.1). This defends against concerns about publishing
full certificate update for those keystore clients that already know identifiable information, while enabling full certificate update for
the associated user IDs for a given certificate. those keystore clients that already know the associated user IDs for
a given certificate.
10. First-party-attested Third-party Certifications 10. First-party-attested Third-party Certifications
We can augment a first-party-only keystore to allow it to distribute We can augment a first-party-only keystore to allow it to distribute
third-party certifications as long as the first-party has signed off third-party certifications as long as the first-party has signed off
on the specific third-party certification. on the specific third-party certification.
An abuse-resistant keystore SHOULD only accept a third-party An abuse-resistant keystore SHOULD only accept a third-party
certification if it meets the following criteria: certification if it meets the following criteria:
o The third-party certification MUST be cryptographically valid. o The third-party certification MUST be cryptographically valid.
Note that this means that the keystore needs to know the primary Note that this means that the keystore needs to know the primary
key for the issuer of the third-party certification. key for the issuer of the third-party certification.
o The third-party certification MUST have an unhashed subpacket of o The third-party certification MUST have an unhashed subpacket of
type Embedded Signature, the contents of which we'll call the type Embedded Signature, the contents of which we'll call the
"attestation". This attestation is from the certificate's primary "attestation". This attestation is from the certificate's primary
key over the third-party certification itself, as detailed in the key over the third-party certification itself, as detailed in the
steps below: steps below:
o The attestation MUST be an OpenPGP signature packet of type 0x50 * The attestation MUST be an OpenPGP signature packet of type
(Third-Party Confirmation signature) 0x50 (Third-Party Confirmation signature)
o The attestation MUST contain a hashed "Issuer Fingerprint" * The attestation MUST contain a hashed "Issuer Fingerprint"
subpacket with the fingerprint of the primary key of the subpacket with the fingerprint of the primary key of the
certificate in question. certificate in question.
o The attestation MUST NOT be marked as non-exportable. * The attestation MUST NOT be marked as non-exportable.
o The attestation MUST contain a hashed Notation subpacket with the * The attestation MUST contain a hashed Notation subpacket with
name "ksok", and an empty (0-octet) value. the name "ksok", and an empty (0-octet) value.
o The attestation MUST contain a hashed "Signature Target" subpacket * The attestation MUST contain a hashed "Signature Target"
with "public-key algorithm" that matches the public-key algorithm subpacket with "public-key algorithm" that matches the public-
of the third-party certification. key algorithm of the third-party certification.
o The attestation's hashed "Signature Target" subpacket MUST use a * The attestation's hashed "Signature Target" subpacket MUST use
reasonably strong hash algorithm (as of this writing, any a reasonably strong hash algorithm (as of this writing, any
[RFC4880] hash algorithm except MD5, SHA1, or RIPEMD160), and MUST [RFC4880] hash algorithm except MD5, SHA1, or RIPEMD160), and
have a hash value equal to the hash over the third-party MUST have a hash value equal to the hash over the third-party
certification with all unhashed subpackets removed. certification with all unhashed subpackets removed.
o The attestation MUST be cryptographically valid, verifiable by the * The attestation MUST be cryptographically valid, verifiable by
primary key of the certificate in question. the primary key of the certificate in question.
What this means is that a third-party certificate will only be This means that a third-party certificate will only be accepted/
accepted/distributed by the keystore if: distributed by the keystore if:
o the keystore knows about both the first- and third-parties. o the keystore knows about both the first- and third-parties.
o the third-party has made the identity assertion o the third-party has made the identity assertion
o the first-party has confirmed that they're OK with the third-party o the first-party has confirmed that they're OK with the third-party
certification being distributed by any keystore. certification being distributed by any keystore.
FIXME: it's not clear whether the "ksok" notification is necessary - The "ksok" notification is not strictly necessary for this
it's in place to avoid some accidental confusion with any other use mitigation, but it is intended to avoid potential accidental
of the Third-Party Confirmation signature packet type, but the author confusion with any other use of the Third-Party Confirmation
does not know of any such use that might collide. signature packet type. The author does not know of any current use
that might collide.
10.1. Key Server Preferences "No-modify" 10.1. Key Server Preferences "No-modify"
[RFC4880] defines "Key Server Preferences" with a "No-modify" bit. [RFC4880] defines "Key Server Preferences" with a "No-modify" bit.
That bit has never been respected by any keyserver implementation That bit has never been respected by any keyserver implementation
that the author is aware of. An abuse-resistant keystore following that the author is aware of. An abuse-resistant keystore following
Section 10 effectively treats that bit as always set, whether it is Section 10 effectively treats that bit as always set, whether it is
present in the certificate or not. present in the certificate or not.
10.2. Client Interactions 10.2. Client Interactions
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o The first party attests to the third party's certification. o The first party attests to the third party's certification.
o Finally, the first party then transfers the compound certificate o Finally, the first party then transfers the compound certificate
to the keystore. to the keystore.
The complexity and length of such a sequence may represent a The complexity and length of such a sequence may represent a
usability obstacle to a user who needs a third-party-certified usability obstacle to a user who needs a third-party-certified
OpenPGP certificate. OpenPGP certificate.
No current OpenPGP client can easily create the attestions described No current OpenPGP client can easily create the attestations
in this section. More implementation work needs to be done to make described in this section. More implementation work needs to be done
it easy (and understandable) for a user to perform this kind of to make it easy (and understandable) for a user to perform this kind
attestation. of attestation.
11. Side Effects and Ecosystem Impacts 11. Keystore Client Best Practices
11.1. Designated Revoker An OpenPGP client that needs to interact with an abuse-resistant
keystore can take steps to minimize the extent that its interactions
with a keystore can be abused by other parties via the attacks
described in Section 2. This section describes steps that an abuse-
resistant client can take.
11.1. Use Constrained Keystores for Lookup
When performing certificate lookup, an abuse-resistant client SHOULD
prefer to query constrained keystores to avoid the risks described in
Section 15.1.
11.2. Normalize Addresses and User IDs for Lookup
When performing lookup by e-mail address, an abuse-resistant client
SHOULD consider canonicalizing the e-mail address before searching
(see Section 14.3).
When searching by full User ID, unless there is a strong reason to
believe that a specific non-normalized form is preferable, an abuse-
resistant client SHOULD normalize the entire user ID into
[UNICODE-NORMALIZATION] Form C (NFC) before performing certificate
lookup.
11.3. Avoid Fuzzy Lookups
Certificate lookup by arbitrary substring matching, or regular
expression is prone to abuse. An abuse-resistant client SHOULD
prefer exact-userid or exact-email match lookups where possible.
In particular, an abuse-resistant client should avoid trying to offer
reliable functionality that performs these sort of fuzzy lookups, and
SHOULD warn the user about risks of abuse if the user triggers a
codepath that unavoidably performs such a fuzzy lookup.
11.4. Prefer Full Fingerprint for Discovery and Update
Key IDs are inherently weaker and easier to spoof or collide than
full fingerprints. Where possible, an abuse-resistant keystore
client SHOULD use the full fingerprint when interacting with the
keystore.
11.5. Use Caution with Keystore-provided Validation
When an abuse-resistant client relies on a keystore for certificate
validation, many things can go subtly wrong if the client fails to
closely track the specific semantics of the keystore's validation
claims.
For example, a certificate published by WKD
([I-D.koch-openpgp-webkey-service]) at
"https://openpgpkey.example.org/.well-known/openpgpkey/hu/
iy9q119eutrkn8s1mk4r39qejnbu3n5q?l=joe.doe" offers a validation claim
only for the e-mail address "joe.doe@example.org". If the
certificate retrieved at that address contains other user IDs, or if
the user ID containing that e-mail address contains an [RFC5322]
"display-name", none of that information should be considered
"validated" by the fact that the certificate was retrieved via
certificate lookup by WKD.
When certificate validation is represented more generally by a
keystore via certificate retrieval (e.g. from an e-mail validating
keyserver that has no distinct certificate validation interface), the
thing validated is the certificate received from the keystore, and
not the result of the merge into any local copy of the certificate
already possessed by the client.
Consider also timing and duration of these assertions of validity,
which represent a subtle tradeoff between privacy and risk as
described in Section 16.4.
12. Certificate Generation and Management Best Practices
An OpenPGP implementation that generates or manages certificates and
expects to distribute them via abuse-resistant keystores can take
steps to ensure that the certificates generated are more likely to be
accessible when needed. This section describes steps such an abuse-
sensitive implementation can take.
12.1. Canonicalized E-Mail Addresses
E-mail addresses can be written in many different ways. An abuse-
sensitive implementation considering attaching a user ID containing
an e-mail address on a certificate SHOULD ensure that the e-mail
address is structured as simply as possible. See Section 14.3 for
details about e-mail address canonicalization.
For example, if the e-mail domain considers the local part to be
case-insensitive (as most e-mail domains do today), if a proposed
user ID contains the "addr-spec": "Alice@EXAMPLE.org", the
implementation SHOULD warn the user and, if possible, propose
replacing the "addr-spec" part of the user ID with
"alice@example.org".
12.2. Normalized User IDs
User IDs are arbitrary UTF-8 strings, but UTF-8 offers several ways
to represent the same string. An abuse-sensitive implementation
considering attaching a user ID to a certificate SHOULD normalize the
string using [UNICODE-NORMALIZATION] Form C (NFC) before creating the
self-sig.
At the same time, the implementation MAY also warn the user if the
"compatibility" normalized form (NFKC) differs from the candidate
user ID and, if appropriate, offer to convert the user ID to
compatibility normalized form at the user's discretion.
12.3. Avoid Large User Attributes
An abuse-sensitive implementation SHOULD warn the user when attaching
a user attribute larger than 8383 octets, and SHOULD refuse to attach
user attributes entirely larger than 65536 octets. (See Section 4.1)
12.4. Provide Cross-Sigs
[RFC4880] requires cross-sigs for all signing-capable subkeys, but is
agnostic about the use of cross-sigs for subkeys of other
capabilities.
An abuse-sensitive implementation that wants a certificate to be
discoverable by subkey SHOULD provide cross-sigs for any subkey
capable of making a cross-sig.
12.5. Provide Issuer Fingerprint Subpackets
Issuer subpackets contain only 64-bit key IDs. Issuer Fingerprint
subpackets contain an unambiguous designator of the issuing key,
avoiding the ambiguities described in Section 13.2. Abuse-sensitive
implementations SHOULD providue Issuer Fingerprint subpackets.
12.6. Put Cross-Sigs and Issuer Fingerprint in Hashed Subpackets
Unhashed subpackets may be stripped or mangled. Placing cross-sigs
and issuer fingerprint subpackets in the hashed subpackets will
ensure that they are propagated by any cryptographically-validating
keystore, even if that keystore fails to observe the exceptions in
Section 4.4.
12.7. Submit Certificates to Restricted, Lookup-Capable Keystores
If an abuse-sensitive implementation wants other peers to be able to
to retrieve the managed certificate by certificate lookup (that is,
by searching based on user ID or e-mail address), it needs to be
aware that submission to an unrestricted keystore is not reliable
(see Section 15.1 for more details).
Consequently, such an implementation SHOULD submit the managed
certificate to restricted, lookup-capable keystores where possible,
as those keystores are more likely to be able to offer reliable
lookup.
13. Side Effects and Ecosystem Impacts
13.1. Designated Revoker
A first-party-only keystore as described in Section 9 might decline A first-party-only keystore as described in Section 9 might decline
to distribute revocations made by the designated revoker. This is a to distribute revocations made by the designated revoker. This is a
risk to certificate-holder who depend on this mechanism, because an risk to certificate-holder who depend on this mechanism, because an
important revocation might be missed by clients depending on the important revocation might be missed by clients depending on the
keystore. keystore.
FIXME: adjust this document to point out where revocations from a FIXME: adjust this document to point out where revocations from a
designated revoker SHOULD be propagated, maybe even in first-party- designated revoker SHOULD be propagated, maybe even in first-party-
only keystores. only keystores.
11.2. Certification-capable Subkeys 13.2. Key IDs vs. Fingerprints in Certificate Discovery
During signature verification, a user performing certificate
discovery against a keystore SHOULD prefer to use the full
fingerprint as an index, rather than the 64-bit key ID. Using a
64-bit key ID is more likely to run into collision attacks; and if
the retrieved certificate has a matching key ID but the signature
cannot be validated with it, the client is in an ambiguous state -
did it retrieve the wrong certificate, or is the signature incorrect?
Using the fingerprint resolves the ambiguity: the signature is
incorrect, because the a fingerprint match is overwhelmingly stronger
than a key ID match.
Unfortunately, many OpenPGP implementations distribute signatures
with only an Issuer subpacket, so a client attempting to find such a
certificate MAY perform certificate discovery based on only the key
ID.
A keystore that offers certificate discovery MAY choose to require
full fingerprint, but such a keystore will not be useful for a client
attempting to verify a bare signature from an unknown party if that
signature only has an Issuer subpacket (and no Issuer Fingerprint
subpacket).
13.3. In-band Certificates
There are contexts where it is expected and acceptable that the
signature appears without its certificate: for example, if the set of
valid signers is already known (as in some OpenPGP-signed operating
system updates), shipping the certificate alongside the signature
would be pointless bloat.
However, OpenPGP signatures are often found in contexts where the
certificate is not yet known by the verifier. For example, many
OpenPGP-signed e-mails are not accompanied by the signing
certificate.
In another example, the use of authentication-capable OpenPGP keys in
standard SSH connections do not contain the full OpenPGP
certificates, which means that the SSH clients and servers need to
resort to out-of-band processes if evaluation of the OpenPGP
certificates is necessary.
The certificate discovery interface offered by keystores is an
attempt to accommodate these situations. But in the event that a
keystore is unavailable, does not know the certificate, or suffers
from a flooding attack, signature validation may fail unnecessarily.
In an encrypted e-mail context specifically, such a failure may also
limit the client's ability to reply with an encrypted e-mail.
Certificate discovery also presents a potential privacy concern for
the signature verifier, as noted in Section 16.5.
These problematic situations can be mitigated by shipping the
certificate in-band, alongside the signature. Signers SHOULD adopt
this practice where possible to reduce the dependence of the verifier
on the keystores for certificate discovery. [AUTOCRYPT] is an
example of e-mail recommendations that include in-band certificates.
13.3.1. In-band Certificate Minimization and Validity
OpenPGP certificates are potenitally large. When distributing an in-
band certificate alongside a signature in a context where size is a
concern (e.g. bandwidth, latency, or storage costs are a factor), the
distributor SHOULD reduce the size of the in-band certificate by
stripping unnecessary packets. For example, the distributor may:
o Strip certification and signature packets that (due to creation
and expiration time) are not relevant to the time of the signature
itself. This ensures that the reduced certificate is
contemporaneously valid with the signature.
o Strip irrelevant subkeys (and associated Subkey Binding Signature
packets and cross-sigs). If the signature was issued by a
signing-capable subkey, that subkey (and its binding signature and
cross-sig) are clearly relevant. Other signing-capable subkeys
are likely to be irrelevant. But determining which other subkeys
are relevant may be context-specific. For example, in the e-mail
context, an encryption-capable subkey is likely to be contextually
relevant, because it enables the recipient to reply encrypted, and
therefore should not be stripped.
o Strip user IDs (and associated certifications) that are unlikely
to be relevant to the signature in question. For example, in the
e-mail context, strip any user IDs that do not match the declared
sender of the message.
o Strip third-party certifications that are unlikely to be relevant
to the verifier. Doing this successfully requires some knowledge
about what the third-parties the recipient is likely to care
about. Stripping all third-party certifications is a simple means
of certificate reduction. The verifier of such a certificate may
need to do certificate update against their preferred keystore to
learn about third-party certifications useful to them.
13.4. Certification-capable Subkeys
Much of this discussion assumes that primary keys are the only Much of this discussion assumes that primary keys are the only
certification-capable keys in the OpenPGP ecosystem. Some proposals certification-capable keys in the OpenPGP ecosystem. Some proposals
have been put forward that assume that subkeys can be marked as have been put forward that assume that subkeys can be marked as
certification-capable. If subkeys are certification-capable, then certification-capable. If subkeys are certification-capable, then
much of the reasoning in this draft becomes much more complex, as much of the reasoning in this draft becomes much more complex, as
subkeys themselves can be revoked by their primary key without subkeys themselves can be revoked by their primary key without
invalidating the key material itself. That is, a subkey can be both invalidating the key material itself. That is, a subkey can be both
valid (in one context) and invalid (in another context) at the same valid (in one context) and invalid (in another context) at the same
time. So questions about what data can be dropped (e.g. in time. So questions about what data can be dropped (e.g. in
Section 7) are much fuzzier, and the underlying assumptions may need Section 7) are much fuzzier, and the underlying assumptions may need
to be reviewed. to be reviewed.
If some OpenPGP implementations accept certification-capable subkeys, If some OpenPGP implementations accept certification-capable subkeys,
but an abuse-resistant keystore does not accept certifications from but an abuse-resistant keystore does not accept certifications from
subkeys in general, then interactions between that keystore and those subkeys in general, then interactions between that keystore and those
implementations may be surprising. implementations may be surprising.
11.3. Assessing Certificates in the Past 13.5. Assessing Certificates in the Past
Online protocols like TLS perform signature and certificate Online protocols like TLS perform signature and certificate
evaluation based entirely on the present time. If a certificate that evaluation based entirely on the present time. If a certificate that
signs a TLS handshake message is invalid now, it doesn't matter signs a TLS handshake message is invalid now, it doesn't matter
whether it was valid a week ago, because the present TLS session is whether it was valid a week ago, because the present TLS session is
the context of the evaluation. the context of the evaluation.
But OpenPGP signatures are often evaluated at some temporal remove But OpenPGP signatures are often evaluated at some temporal remove
from when the signature was made. For example, software packages are from when the signature was made. For example, software packages are
signed at release time, but those signatures are validated at signed at release time, but those signatures are validated at
download time. download time. A verifier that does not already know the certificate
that made the signature will need to perform certificate discovery
against some set of keystores to find a certificate with which to
check the signature.
Further complicating matters, the composable nature of an OpenPGP Further complicating matters, the composable nature of an OpenPGP
certificate means that the certificate associated with any particular certificate means that the certificate associated with any particular
signing key (primary key or subkey) can transform over time. So when signing key (primary key or subkey) can transform over time. So when
evaluating a signature that appears to have been made by a given evaluating a signature that appears to have been made by a given
certificate, it may be better to try to evaluate the certificate at certificate, it may be better to try to evaluate the certificate at
the time the signature was made, rather than the present time. the time the signature was made, rather than the present time.
11.3.1. Point-in-time Certificate Evaluation 13.5.1. Point-in-time Certificate Evaluation
When evaluating a certificate at a time T in the past (for example, When evaluating a certificate at a time T in the past (for example,
when trying to validate a data signature by that certificate that was when trying to validate a data signature by that certificate that was
created at time T), one approach is to discard all packets from the created at time T), one approach is to discard all packets from the
certificate if the packet has a creation time later than T. Then certificate if the packet has a creation time later than T. Then
evaluate the resulting certificate from the remaining packets in the evaluate the resulting certificate from the remaining packets in the
context of time T. context of time T.
However, any such evaluation MUST NOT ignore "hard" OpenPGP key However, any such evaluation MUST NOT ignore "hard" OpenPGP key
revocations, regardless of their creation date. (see Section 12.1). revocations, regardless of their creation date. (see Section 14.1).
11.3.2. Signature Verification and Non-append-only Keystores 13.5.2. Signature Verification and Non-append-only Keystores
If a non-append-only keystore (Section 7) has dropped superseded If a non-append-only keystore (Section 7) has dropped superseded
(Section 7.1) or expired (Section 7.2) certifications, it's possible (Section 7.1) or expired (Section 7.2) certifications, it's possible
for the certificate composed of the remaining packets to have no for the certificate composed of the remaining packets to have no
valid first-party certification at the time that a given signature valid first-party certification at the time that a given signature
was made. Such a certificate would be invalid according to was made. If a user performs certificate discovery against such a
[RFC4880], and consequently verification of any signature . keystore, the certificate it retrieves would be invalid according to
[RFC4880], and consequently verification of any signature by that
certificate would fail.
However, there is a simple mitigation: anyone distributing a One simple mitigation to this problem is to ship a contemporaneously-
signature (e.g. a software archive) should ship the contemporary valid certificate in-band alongside the signature (see Section 13.3).
signing certificate alongside the signature. If the distributor does
this, then the verifier can perform a certificate update (to learn If the distributor does this, then the verifier need only learn about
about revocations) against any preferred keystore, including non- revocations. If knowledge about revocation is needed, the verifier
append-only keystores, merging what it learns into the distributed might perform a certificate update (not "certificate discovery")
contemporary certificate. against any preferred keystore, including non-append-only keystores,
merging what it learns into the in-band contemporary certificate.
Then the signature verifier can follow the certificate evaluation Then the signature verifier can follow the certificate evaluation
process outlined in Section 11.3.1, using the merged certificate. process outlined in Section 13.5.1, using the merged certificate.
11.4. Global Append-only Ledgers ("Blockchain") 13.6. Global Append-only Ledgers ("Blockchain")
The append-only aspect of some OpenPGP keystores encourages a user of The append-only aspect of some OpenPGP keystores encourages a user of
the keystore to rely on that keystore as a faithful reporter of the keystore to rely on that keystore as a faithful reporter of
history, and one that will not misrepresent or hide the history that history, and one that will not misrepresent or hide the history that
they know about. An unfaithful "append-only" keystore could abuse they know about. An unfaithful "append-only" keystore could abuse
the trust in a number of ways, including withholding revocation the trust in a number of ways, including withholding revocation
certificates, offering different sets of certificates to different certificates, offering different sets of certificates to different
clients doing key discovery, and so on. clients doing certificate lookup, and so on.
However, the most widely used append-only OpenPGP keystore, the [SKS] However, the most widely used append-only OpenPGP keystore, the [SKS]
keyserver pool, offers no cryptographically verifiable guarantees keyserver pool, offers no cryptographically verifiable guarantees
that it will actually remain append-only. Users of the pool have that it will actually remain append-only. Users of the pool have
traditionally relied on its distributed nature, and the presumption traditionally relied on its distributed nature, and the presumption
that coordination across a wide range of administrators would make it that coordination across a wide range of administrators would make it
difficult for the pool to reliably lie or omit data. However, the difficult for the pool to reliably lie or omit data. However, the
endpoint most commonly used by clients to access the network is endpoint most commonly used by clients to access the network is
"hkps://hkps.pool.sks-keyservers.net", the default for [GnuPG]. That "hkps://hkps.pool.sks-keyservers.net", the default for [GnuPG]. That
endpoint is increasingly consolidated, and currently consists of endpoint is increasingly consolidated, and currently consists of
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by identity providers, however. And both offer the ability to detect by identity providers, however. And both offer the ability to detect
a misbehaving identity provider, but no specific enforcement or a misbehaving identity provider, but no specific enforcement or
recovery strategies against such an actor. recovery strategies against such an actor.
It's conceivable that a keystore could piggyback on the CT logs or It's conceivable that a keystore could piggyback on the CT logs or
other blockchain/ledger mechanisms like [BITCOIN] to store other blockchain/ledger mechanisms like [BITCOIN] to store
irrevocable pieces of data (such as revocation certificates). irrevocable pieces of data (such as revocation certificates).
Further work is needed to describe how to do this in an effective and Further work is needed to describe how to do this in an effective and
performant way. performant way.
11.5. Certificate Discovery for Identity Monitoring 13.7. Certificate Lookup for Identity Monitoring
A typical use case for certificate discovery is a user looking for a A typical use case for certificate lookup is a user looking for a
certificate in order to be able to encrypt an outbound message certificate in order to be able to encrypt an outbound message
intended for a given e-mail address, but this is not the only use intended for a given e-mail address, but this is not the only use
case. case.
Another use caes is when the party in control of a particular Another use caes is when the party in control of a particular
identity wants to determine whether anyone else is claiming that identity wants to determine whether anyone else is claiming that
identity. That is, a client in control of the secret key material identity. That is, a client in control of the secret key material
associated with a particular certificate with user ID X might search associated with a particular certificate with user ID X might search
a keystore for all certificates matching X in order to discover a keystore for all certificates matching X in order to find out
whether any other certificates claim it. whether any other certificates claim it.
This is an important safeguard as part of the ledger-based detection This is an important safeguard as part of the ledger-based detection
mechanisms described in Section 11.4, but may also be useful for mechanisms described in Section 13.6, but may also be useful for
keystores in general. keystores in general.
However, identity monitoring against a keystore that does not defend However, identity monitoring against a keystore that does not defend
against user ID flooding (Section 2.2) is expensive and potentially against user ID flooding (Section 2.2) is expensive and potentially
of limited value. In particular, a malicious actor with a of limited value. In particular, a malicious actor with a
certificate which duplicates a given User ID could flood the keystore certificate which duplicates a given User ID could flood the keystore
with similar certificates, hiding whichever one is in malicious use. with similar certificates, hiding whichever one is in malicious use.
Since such a keystore is not considered authoritative by any Since such a keystore is not considered authoritative by any
reasonable client for the user ID in question, this attack forces the reasonable client for the user ID in question, this attack forces the
identity-monitoring defender to spend arbitrary resources fetching identity-monitoring defender to spend arbitrary resources fetching
and evaluating each certificate in the flood, without knowing which and evaluating each certificate in the flood, without knowing which
certificate other clients might be evaluating. certificate other clients might be evaluating.
12. OpenPGP details 14. OpenPGP details
This section collects details about common OpenPGP implementation This section collects details about common OpenPGP implementation
behavior that are useful in evaluating and reasoning about OpenPGP behavior that are useful in evaluating and reasoning about OpenPGP
certificates. certificates.
12.1. Revocations 14.1. Revocations
It's useful to classify OpenPGP revocations of key material into two It's useful to classify OpenPGP revocations of key material into two
categories: "soft" and "hard". categories: "soft" and "hard".
If the "Reason for Revocation" of an OpenPGP key is either "Key is If the "Reason for Revocation" of an OpenPGP key is either "Key is
superseded" or "Key is retired and no longer used", it is a "soft" superseded" or "Key is retired and no longer used", it is a "soft"
revocation. revocation.
An implementation that interprets a "soft" revocation will typically An implementation that interprets a "soft" revocation will typically
not invalidate signatures made by the associated key with a creation not invalidate signatures made by the associated key with a creation
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these two categories. these two categories.
A defensive OpenPGP implementation that does not distinguish between A defensive OpenPGP implementation that does not distinguish between
these two categories SHOULD treat all revocations as "hard". these two categories SHOULD treat all revocations as "hard".
An implementation aware of a "soft" revocation or of key or An implementation aware of a "soft" revocation or of key or
certificate expiry at time T SHOULD accept and process a "hard" certificate expiry at time T SHOULD accept and process a "hard"
revocation even if it appears to have been issued at a time later revocation even if it appears to have been issued at a time later
than T. than T.
12.2. User ID Conventions 14.2. User ID Conventions
[RFC4880] requires a user ID to be a UTF-8 string, but does not [RFC4880] requires a user ID to be a UTF-8 string, but does not
constrain it beyond that. In practice, a handful of conventions constrain it beyond that. In practice, a handful of conventions
predominate in how User IDs are formed. predominate in how User IDs are formed.
The most widespread convention is a name-addr as defined in The most widespread convention is a "name-addr" as defined in
[RFC5322]. For example: [RFC5322]. For example:
Alice Jones <alice@example.org> Alice Jones <alice@example.org>
But a growing number of OpenPGP certificates contain user IDs that But a growing number of OpenPGP certificates contain user IDs that
are instead a raw [RFC5322] addr-spec, omitting the display-name and are instead a raw [RFC5322] "addr-spec", omitting the "display-name"
the angle brackets entirely, like so: and the angle brackets entirely, like so:
alice@example.org alice@example.org
Some certificates have user IDs that are simply "normal" human names Some certificates have user IDs that are simply normal human names
(perhaps display-name in [RFC5322] jargon, though not necessarily (perhaps "display-name" in [RFC5322] jargon, though not necessarily
conforming to a specific ABNF). For example: conforming to a specific ABNF). For example:
Alice Jones Alice Jones
Still other certificates identify a particular network service by Still other certificates identify a particular network service by
scheme and hostname. For example, the administrator of an ssh host scheme and hostname. For example, the administrator of an ssh host
participating in the [MONKEYSPHERE] might choose a user ID for the participating in the [MONKEYSPHERE] might choose a user ID for the
OpenPGP representing the host like so: OpenPGP representing the host like so:
ssh://foo.example.net ssh://foo.example.net
13. Security Considerations 14.3. E-mail Address Canonicalization
When an OpenPGP user IDs includes an "addr-spec", there still may be
multiple ways of representing the addr-spec that refer to the same
underlying mailbox. Having a truly canonical form of an "addr-spec"
is probably impossible because of legacy deployments of mailservers
that do odd things with the local part, but e-mail addresses used in
an abuse-resistant ecosystem SHOULD be constrained enough to admit to
some sensible form of canonicalization.
14.3.1. Disallowing Non-UTF-8 Local Parts
In [RFC5322], the "local-part" can be any "dot-atom". But if this is
[RFC2047] decoded, it could be any arbitrary charset, not necessarily
UTF-8. FIXME: Do we convert from the arbitrary charset to UTF-8?
14.3.2. Domain Canonicalization
FIXME: should domain name be canonicalized into punycode form? User
IDs are typically user-facing, so i think the canonicalized form
should be the [UNICODE-NORMALIZATION] Form C (NFC) of the domain
name. Can we punt to some other draft here?
14.3.3. Local Part Canonicalization
FIXME: [I-D.koch-openpgp-webkey-service] recommends downcasing all
ASCII characters in the left-hand side, but leaves all
15. Security Considerations
This document offers guidance on mitigating a range of denial-of- This document offers guidance on mitigating a range of denial-of-
service attacks on public keystores, so the entire document is in service attacks on public keystores, so the entire document is in
effect about security considerations. effect about security considerations.
Many of the mitigations described here defend individual OpenPGP Many of the mitigations described here defend individual OpenPGP
certificates against flooding attacks (see Section 2.1). But only certificates against flooding attacks (see Section 2.1). But only
some of these mitigations defend against flooding attacks against the some of these mitigations defend against flooding attacks against the
keystore itself (see Section 2.3), or against flooding attacks on the keystore itself (see Section 2.4), or against flooding attacks on the
space of possible user IDs (see Section 2.2). Thoughtful threat space of possible user IDs (see Section 2.2). Thoughtful threat
modeling and monitoring of the keystore and its defenses are probably modeling and monitoring of the keystore and its defenses are probably
necessary to maintain the long-term health of the keystore. necessary to maintain the long-term health of the keystore.
Section 11.1 describes a potentially scary security problem for Section 13.1 describes a potentially scary security problem for
designated revokers. designated revokers.
TODO (more security considerations) TODO (more security considerations)
13.1. Tension Between Unrestricted Uploads and Certificate Discovery 15.1. Tension Between Unrestricted Uploads and Certificate Lookup
Note that there is an inherent tension between accepting arbitrary Note that there is an inherent tension between accepting arbitrary
certificate uploads and permitting effective certificate discovery. certificate uploads and permitting effective certificate lookup. If
If a keystore accepts arbitrary certificate uploads for a keystore accepts arbitrary certificate uploads for redistribution,
redistribution, it appears to be vulnerable to user ID flooding it appears to be vulnerable to user ID flooding (Section 2.2), which
(Section 2.2), which makes it difficult or impossible to rely on for makes it difficult or impossible to rely on for certificate lookup.
certificate discovery.
In the broader ecosystem, it may be necessary to use gated/controlled In the broader ecosystem, it may be necessary to use gated/controlled
certificate discovery mechanisms. For example, both certificate lookup mechanisms. For example, both
[I-D.koch-openpgp-webkey-service] and [RFC7929] enable the [I-D.koch-openpgp-webkey-service] and [RFC7929] enable the
administrator of a DNS domain to distribute certificates associated administrator of a DNS domain to distribute certificates associated
with e-mail addresses within that domain, while excluding other with e-mail addresses within that domain, while excluding other
parties. As a rather different example, [I-D.mccain-keylist] offers parties. As a rather different example, [I-D.mccain-keylist] offers
certificate discovery on the basis of interest - a client interested certificate lookup on the basis of interest - a client interested in
in an organization can use that mechanism to learn what certificates an organization can use that mechanism to learn what certificates
that organization thinks are worth knowing about, regardless of the that organization thinks are worth knowing about, associated with a
particular DNS domain. Note that this [I-D.mccain-keylist] does not range of identities regardless of the particular DNS domain. Note
provide the certificates directly, but instead expects the client to that [I-D.mccain-keylist] does not provide the certificates directly,
be able to retrieve them by certificate fingerprint through some but instead expects the client to be able to retrieve them by primary
other keystore capable of (and responsible for) certificate update. key fingerprint through some other keystore capable of (and
responsible for) certificate update.
14. Privacy Considerations 16. Privacy Considerations
Keystores themselves raise a host of potential privacy concerns. Keystores themselves raise a host of potential privacy concerns.
Additional privacy concerns are raised by traffic to and from the Additional privacy concerns are raised by traffic to and from the
keystores. This section tries to outline some of the risks to the keystores. This section tries to outline some of the risks to the
privacy of people whose certificates are stored and redistributed in privacy of people whose certificates are stored and redistributed in
public keystores, as well as risks to the privacy of people who make public keystores, as well as risks to the privacy of people who make
use of the key stores for certificate discovery or certificate use of the key stores for certificate lookup, certificate discovery,
update. or certificate update.
TODO (more privacy considerations)
14.1. Publishing Identity Information 16.1. Publishing Identity Information
Public OpenPGP keystores often distribute names or e-mail addresses Public OpenPGP keystores often distribute names or e-mail addresses
of people. Some people do not want their names or e-mail addresses of people. Some people do not want their names or e-mail addresses
distributed in a public keystore, or may change their minds about it distributed in a public keystore, or may change their minds about it
at some point. Append-only keystores are particularly problematic in at some point. Append-only keystores are particularly problematic in
that regard. The mitigation in Section 7.4 can help such users strip that regard. The mitigation in Section 7.4 can help such users strip
their details from keys that they control. However, if an OpenPGP their details from keys that they control. However, if an OpenPGP
certificate with their details is uploaded to a keystore, but is not certificate with their details is uploaded to a keystore, but is not
under their control, it's unclear what mechanisms can be used to under their control, it's unclear what mechanisms can be used to
remove the certificate that couldn't also be exploited to take down remove the certificate that couldn't also be exploited to take down
an otherwise valid certificate. an otherwise valid certificate.
Some jurisdictions may present additional legal risk for keystore Some jurisdictions may present additional legal risk for keystore
operators that distribute names or e-mail addresses of non-consenting operators that distribute names or e-mail addresses of non-consenting
parties. parties.
Updates-only keystores (Section 8) and user ID redacting keystores Updates-only keystores (Section 8) and user ID redacting keystores
(Section 5.1) may reduce this particular privacy concern because they (Section 5.1) may reduce this particular privacy concern because they
distribute no user IDs at all. distribute no user IDs at all.
14.2. Social Graph 16.2. Social Graph
Third-party certifications effectively map out some sort of social Third-party certifications effectively map out some sort of social
graph. A certification asserts a statement of belief by the issuer graph. A certification asserts a statement of belief by the issuer
that the real-world party identified by the user ID is in control of that the real-world party identified by the user ID is in control of
the subject cryptographic key material. But those connections may be the subject cryptographic key material. But those connections may be
potentially sensitive, and some people may not want these maps built. potentially sensitive, and some people may not want these maps built.
A first-party-only keyserver (Section 9) avoids this privacy concern A first-party-only keyserver (Section 9) avoids this privacy concern
because it distribues no third-party privacy concern. because it distributes no third-party privacy concern.
First-party attested third-party certifications described in First-party attested third-party certifications described in
Section 10 are even more relevant edges in the social graph, because Section 10 are even more relevant edges in the social graph, because
their bidirectional nature suggests that both parties are aware of their bidirectional nature suggests that both parties are aware of
each other, and see some value in mutual association. each other, and see some value in mutual association.
14.3. Tracking Clients by Queries 16.3. Tracking Clients by Queries
Even without third-party certifications, the acts of certificate Even without third-party certifications, the acts of certificate
discovery and certificate update represent a potential privacy risk, lookup, certificate discovery, and certificate update represent a
because the keystore queried gets to learn which user IDs (in the potential privacy risk, because the keystore queried gets to learn
case of discovery) or which certificates (in the case of update) the which user IDs (in the case of lookup) or which certificates (in the
client is interested in. In the case of certificate update, if a case of update or discovery) the client is interested in. In the
client attempts to update all of its known certificates from the same case of certificate update, if a client attempts to update all of its
keystore, that set is likely to be a unique set, and therefore known certificates from the same keystore, that set is likely to be a
identifies the client. A keystore that monitors the set of queries unique set, and therefore identifies the client. A keystore that
it receives might be able to profile or track those clients who use monitors the set of queries it receives might be able to profile or
it repeatedly. track those clients who use it repeatedly.
Clients which want to to avoid such a tracking attack MAY try to A privacy-aware client which wants to to avoid such a tracking attack
perform certificate update from multiple different keystores. To MAY try to perform certificate update from multiple different
hide network location, a client making a network query to a keystore keystores. To hide network location, a client making a network query
SHOULD use an anonymity network like [TOR]. Tools like [PARCIMONIE] to a keystore SHOULD use an anonymity network like [TOR]. Tools like
are designed to facilitate this type of certificate update. [PARCIMONIE] are designed to facilitate this type of certificate
update. Such a client SHOULD also decline to use protocol features
that permit linkability across interactions with the same keystore,
such as TLS session resumption, HTTP cookies, and so on.
Keystores which permit public access and want to protect the privacy Keystores which permit public access and want to protect the privacy
of their clients SHOULD NOT reject access from clients using [TOR] or of their clients SHOULD NOT reject access from clients using [TOR] or
comparable anonymity networks. Additionally, they SHOULD minimize comparable anonymity networks. Additionally, they SHOULD minimize
access logs they retain. access logs they retain.
Alternately, some keystores may distribute their entire contents to Alternately, some keystores may distribute their entire contents to
any interested client, in what can be seen as the most trivial form any interested client, in what can be seen as the most trivial form
of private information retrieval. [DEBIAN-KEYRING] is one such of private information retrieval. [DEBIAN-KEYRING] is one such
example; its contents are distributed as an operating system package. example; its contents are distributed as an operating system package.
Clients can interrogate their local copy of such a keystore without Clients can interrogate their local copy of such a keystore without
exposing their queries to a third-party. exposing their queries to a third-party.
14.4. Cleartext Queries 16.4. "Live" Certificate Validation Leaks Client Activity
If a client relies on a keystore's certificate validation interface,
or on the presence of a certificate in a keystore as a part of its
validation calculations, it's unclear how long the assertion from the
keystore is or should be considered to hold. One seemingly simple
approach is to simply query the keystore's validation interface each
time that the client needs to validate the certificate.
This "live" validation approach poses a quandary to the client in the
event that the keystore is unavailable. How should in interpret the
"unknown" result? In addition, live validation reveals the client's
activity to the keystore with fine precision.
A privacy-aware client that depends on keystores for certificate
validation SHOULD NOT perform "live" certificate validation on each
use it makes of the certificate. Rather, it SHOULD cache the
validation information for some period of time and make use of the
cached copy where possible. If such a client does a regular
certificate update from the same keystore, it SHOULD also pre-
emptively query the keystore for certificate validation at the same
time.
Choosing the appropriate time intervals for this kind of caching has
implications for the windows of risk for the client that might use a
revoked certificate. Defining an appropriate schedule to make this
tradeoff is beyond the scope of this document.
16.5. Certificate Discovery Leaks Client Activity
The act of doing certificate discovery on unknown signatures offers a
colluding keystore and remote peer a chance to track a client's
consumption of a given signature.
An attacker with access to keystore logs could sign a message with a
unique key, and then watch the keystore activity to determine when a
client consumes the signature. This is potentially a tracking or
"phone-home" situation.
A signer that has no interest in this particular form of tracking can
mitigate this concern by shipping their certificate in-band,
alongside the signature, as recommended in Section 13.3.
A privacy-aware client MAY insist on in-band certificates by
declining to use any certificate discovery interface at all, and
treat a bare signature by an unknown certificate as an invalid
signature.
16.6. Certificate Update Leaks Client Activity
The act of doing certificate update itself reveals some information
that the client is interested in a given certificate and how it may
have changed since the last time the client updated it, or since it
was first received by the client.
This is essentially the same privacy problem presented by OCSP
[RFC6960] in the X.509 world. In the online world of TLS, this
privacy leak is mitigated by the CertificateStatus TLS handshake
extension ([RFC4366]), a.k.a. "OCSP stapling". There is no
comparable solution for the store-and-forward or non-online scenarios
where OpenPGP is often found.
Privacy-aware clients MAY prefer to access update interfaces from
anonymity-preserving networks like [TOR] to obscure where they are on
the network, but if the certificate being updated is known to be used
only by a single client that may not help.
Privacy-aware clients MAY prefer to stage their certificate updates
over time, but longer delays imply greater windows of vulnerability
for use of an already-revoked certificate. This strategy also does
not help when a previously-unknown certificate is encountered in-band
(see Section 13.3), and the OpenPGP client wants to evaluate it for
use in the immediate context.
16.7. Distinct Keystore Interfaces Leak Client Context and Intent
The distinct keystore interfaces documented in Section 3 offer subtly
different semantics, and are used by a reasonable keystore client at
different times. A keystore that offers distinct discovery and
update interfaces may infer that a client visiting the update
interface already knows about the certificate in question, or that a
client visiting the discovery interface is in the process of
verifying a signature from a certificate it has not seen before.
HKP itself ([I-D.shaw-openpgp-hkp]) conflates these two interfaces -
the same HKP query is be used to perform both discovery and update
(though implementations like [SKS] are not at all abuse-resistant for
either use), which may obscure the context and intent of the client
from the keystore somewhat.
A privacy-aware client that can afford the additional bandwidth and
complexity MAY use the keystore's discovery interface for both update
and discovery, since the discovery interface is a proper superset of
the update interface.
16.8. Cleartext Queries
If access to the keystore happens over observable channels (e.g., If access to the keystore happens over observable channels (e.g.,
cleartext connections over the Internet), then a passive network cleartext connections over the Internet), then a passive network
monitor could perform the same type profiling or tracking attack monitor could perform the same type profiling or tracking attack
against clients of the keystore described in Section 14.3. Keystores against clients of the keystore described in Section 16.3. Keystores
which offer network access SHOULD provide encrypted transport. which offer network access SHOULD provide encrypted transport.
14.5. Traffic Analysis 16.9. Traffic Analysis
Even if a keystore offers encrypted transport, the size of queries Even if a keystore offers encrypted transport, the size of queries
and responses may provide effective identification of the specific and responses may provide effective identification of the specific
certificates fetched during discovery or update, leaving open the certificates fetched during lookup, discovery, or update, leaving
types of tracking attacks described in Section 14.3. Clients of open the types of tracking attacks described in Section 16.3.
keystores SHOULD pad their queries to increase the size of the Clients of keystores SHOULD pad their queries to increase the size of
anonymity set. And keystores SHOULD pad their responses. the anonymity set. And keystores SHOULD pad their responses.
The appropriate size of padding to effectively anonymize traffic to The appropriate size of padding to effectively anonymize traffic to
and from keystores is likely to be mechanism- and cohort-specific. and from keystores is likely to be mechanism- and cohort-specific.
For example, padding for keystores accessed via the DNS ([RFC7929] For example, padding for keystores accessed via the DNS ([RFC7929]
may use different padding strategies that padding for keystores may use different padding strategies that padding for keystores
accessed over WKD ([I-D.koch-openpgp-webkey-service]), which may in accessed over WKD ([I-D.koch-openpgp-webkey-service]), which may in
turn be different from keystores accessed over HKPS turn be different from keystores accessed over HKPS
([I-D.shaw-openpgp-hkp]). A keystore which only accepts user IDs ([I-D.shaw-openpgp-hkp]). A keystore which only accepts user IDs
within a specific domain (e.g., Section 4.3) or which uses custom within a specific domain (e.g., Section 4.3) or which uses custom
process (Section 6.4) for verification might have different padding process (Section 6.4) for verification might have different padding
criteria than a keystore that serves the general public. criteria than a keystore that serves the general public.
Specific padding policies or mechanisms are out of scope for this Specific padding policies or mechanisms are out of scope for this
document. document.
15. User Considerations 17. User Considerations
Section 10.2 describes some outstanding work that needs to be done to Section 10.2 describes some outstanding work that needs to be done to
help users understand how to produce and distribute a third-party- help users understand how to produce and distribute a third-party-
certified OpenPGP certificate to an abuse-resistant keystore. certified OpenPGP certificate to an abuse-resistant keystore.
Additionally, some keystores present directly user-facing Additionally, some keystores present directly user-facing
affordances. For example, [SKS] keyservers typically offer forms and affordances. For example, [SKS] keyservers typically offer forms and
listings that can be viewed directly in a web browser. Such a listings that can be viewed directly in a web browser. Such a
keystore SHOULD be as clear as possible about what abuse mitigations keystore SHOULD be as clear as possible about what abuse mitigations
it takes (or does not take), to avoid user confusion. it takes (or does not take), to avoid user confusion.
Keystores which do not expect to be used directly as part of a
certificate validation calculation SHOULD advise clients as
explicitly as possible that they offer no assertions of validity.
Experience with the [SKS] keyserver network shows that many users Experience with the [SKS] keyserver network shows that many users
treat the keyserver web interfaces as authoritative, even though the treat the keyserver web interfaces as authoritative. That is, users
developer and implementor communities explicitly disavow any act as though the keyserver network offers some type of certificate
authoritative role in the ecosystem, and the implementations attempt validation. Unfortunately, The developer and implementor communities
very few mitigations against abuse, permitting republication of even explicitly disavow any authoritative role in the ecosystem, and the
cryptographically invalid OpenPGP packets. Clearer warnings to end implementations attempt very few mitigations against abuse,
users might reduce this kind of misperception. Or the community permitting redistribution of even cryptographically invalid OpenPGP
could encourage the removal of frequently misinterpreted user packets. Clearer warnings to end users might reduce this kind of
interfaces. misperception. Or the community could encourage the removal of
frequently misinterpreted user interfaces entirely.
16. IANA Considerations 18. IANA Considerations
This document asks IANA to register two entries in the OpenPGP This document asks IANA to register two entries in the OpenPGP
Notation IETF namespace, both with a reference to this document: Notation IETF namespace, both with a reference to this document:
o the "ksok" notation is defined in Section 10. o the "ksok" notation is defined in Section 10.
o the "uidhash" notation is defined in Section 5.1.3. o the "uidhash" notation is defined in Section 5.1.4.
17. Document Considerations 19. Document Considerations
[ RFC Editor: please remove this section before publication ] [ RFC Editor: please remove this section before publication ]
This document is currently edited as markdown. Minor editorial This document is currently edited as markdown. Minor editorial
changes can be suggested via merge requests at changes can be suggested via merge requests at
https://gitlab.com/dkg/draft-openpgp-abuse-resistant-keystore or by https://gitlab.com/dkg/draft-openpgp-abuse-resistant-keystore or by
e-mail to the author. Please direct all significant commentary to e-mail to the author. Please direct all significant commentary to
the public IETF OpenPGP mailing list: openpgp@ietf.org the public IETF OpenPGP mailing list: openpgp@ietf.org
17.1. Document History 19.1. Document History
substantive changes between -02 and -03:
o new sections:
* Keystore Interfaces
* Keystore Client Best Practices
* Certificate Generation and Management Best Practices
o rename "certificate discovery" to "certificate lookup"
o redefine "certificate discovery" to refer to lookup by signing
(sub)key
o new attack: fingerprint flooding
o new retrieval-time mitigations - tighter filters on discovery and
update
o recommend in-band certificates where possible to avoid discovery
and lookup
o new privacy considerations:
* distinct keystore interfaces
* certificate update
* certificate discovery
* certificate validation
o more nuance about unhashed subpacket filtering
substantive changes between -01 and -02: substantive changes between -01 and -02:
o distinguish different forms of flooding attack o distinguish different forms of flooding attack
o distinguish toxic data as distinct from flooding o distinguish toxic data as distinct from flooding
o retrieval-time mitigations o retrieval-time mitigations
o user ID redaction o user ID redaction
skipping to change at page 32, line 4 skipping to change at page 49, line 27
* Scoped User IDs * Scoped User IDs
o documented Updates-Only Keystores o documented Updates-Only Keystores
o consider three different kinds of flooding o consider three different kinds of flooding
o deeper discussion of privacy considerations o deeper discussion of privacy considerations
o better documentation of Reason for Revocation o better documentation of Reason for Revocation
o document user ID conventions o document user ID conventions
18. Acknowledgements 20. Acknowledgements
This document is the result of years of operational experience and This document is the result of years of operational experience and
observation, as well as conversations with many different people - observation, as well as conversations with many different people -
users, implementors, keystore operators, etc. A non-exhaustive list users, implementors, keystore operators, etc. A non-exhaustive list
of people who have contriubuted ideas or nuance to this document of people who have contributed ideas or nuance to this document
specifically includes: specifically includes:
o Antoine Beaupre o Antoine Beaupre
o ilf o ilf
o Jamie McClelland o Jamie McClelland
o Jonathan McDowell o Jonathan McDowell
skipping to change at page 32, line 27 skipping to change at page 50, line 4
o Jamie McClelland o Jamie McClelland
o Jonathan McDowell o Jonathan McDowell
o Justus Winter o Justus Winter
o Marcus Brinkmann o Marcus Brinkmann
o Micah Lee o Micah Lee
o Neal Walfield o Neal Walfield
o Phil Pennock o Phil Pennock
o Philihp Busby
o vedaal o vedaal
o Vincent Breitmoser o Vincent Breitmoser
o Wiktor Kwapisiewicz o Wiktor Kwapisiewicz
19. References 21. References
19.1. Normative References 21.1. Normative References
[I-D.ietf-openpgp-rfc4880bis] [I-D.ietf-openpgp-rfc4880bis]
Koch, W., carlson, b., Tse, R., and D. Atkins, "OpenPGP Koch, W., carlson, b., Tse, R., and D. Atkins, "OpenPGP
Message Format", draft-ietf-openpgp-rfc4880bis-06 (work in Message Format", draft-ietf-openpgp-rfc4880bis-06 (work in
progress), November 2018. progress), November 2018.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC2047, November 1996,
<https://www.rfc-editor.org/info/rfc2047>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007, DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>. <https://www.rfc-editor.org/info/rfc4880>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
19.2. Informative References 21.2. Informative References
[AUTOCRYPT]
Breitmoser, V., Krekel, H., and D. Gillmor, "Autocrypt -
Convenient End-to-End Encryption for E-Mail", n.d.,
<https://autocrypt.org/>.
[BITCOIN] "Bitcoin", n.d., <https://bitcoin.org/>. [BITCOIN] "Bitcoin", n.d., <https://bitcoin.org/>.
[CONIKS] Felten, E., Freedman, M., Melara, M., Blankstein, A., and [CONIKS] Felten, E., Freedman, M., Melara, M., Blankstein, A., and
J. Bonneau, "CONIKS Key Management System", n.d., J. Bonneau, "CONIKS Key Management System", n.d.,
<https://coniks.cs.princeton.edu/>. <https://coniks.cs.princeton.edu/>.
[DEBIAN-KEYRING] [DEBIAN-KEYRING]
McDowell, J., "Debian Keyring", n.d., McDowell, J., "Debian Keyring", n.d.,
<https://keyring.debian.org/>. <https://keyring.debian.org/>.
skipping to change at page 34, line 20 skipping to change at page 52, line 5
Intrigeri, ., "Parcimonie", n.d., Intrigeri, ., "Parcimonie", n.d.,
<https://gaffer.ptitcanardnoir.org/intrigeri/code/ <https://gaffer.ptitcanardnoir.org/intrigeri/code/
parcimonie/>. parcimonie/>.
[PGP-GLOBAL-DIRECTORY] [PGP-GLOBAL-DIRECTORY]
Symantec Corporation, "PGP Global Directory Key Symantec Corporation, "PGP Global Directory Key
Verification Policy", 2011, Verification Policy", 2011,
<https://keyserver.pgp.com/vkd/ <https://keyserver.pgp.com/vkd/
VKDVerificationPGPCom.html>. VKDVerificationPGPCom.html>.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
<https://www.rfc-editor.org/info/rfc4366>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008, DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>. <https://www.rfc-editor.org/info/rfc5322>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<https://www.rfc-editor.org/info/rfc6960>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate [RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>. <https://www.rfc-editor.org/info/rfc6962>.
[RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities [RFC7929] Wouters, P., "DNS-Based Authentication of Named Entities
(DANE) Bindings for OpenPGP", RFC 7929, (DANE) Bindings for OpenPGP", RFC 7929,
DOI 10.17487/RFC7929, August 2016, DOI 10.17487/RFC7929, August 2016,
<https://www.rfc-editor.org/info/rfc7929>. <https://www.rfc-editor.org/info/rfc7929>.
[SKS] Minsky, Y., Fiskerstrand, K., and P. Pennock, "SKS [SKS] Minsky, Y., Fiskerstrand, K., and P. Pennock, "SKS
Keyserver Documentation", March 2018, Keyserver Documentation", March 2018,
<https://bitbucket.org/skskeyserver/sks-keyserver/wiki/ <https://bitbucket.org/skskeyserver/sks-keyserver/wiki/
Home>. Home>.
[TOR] "The Tor Project", n.d., <https://www.torproject.org/>. [TOR] "The Tor Project", n.d., <https://www.torproject.org/>.
[UNICODE-NORMALIZATION]
Whistler, K., "Unicode Normalization Forms", February
2019, <https://unicode.org/reports/tr15/>.
Author's Address Author's Address
Daniel Kahn Gillmor Daniel Kahn Gillmor
American Civil Liberties Union American Civil Liberties Union
125 Broad St. 125 Broad St.
New York, NY 10004 New York, NY 10004
USA USA
Email: dkg@fifthhorseman.net Email: dkg@fifthhorseman.net
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