< draft-dkg-openpgp-abuse-resistant-keystore-00.txt   draft-dkg-openpgp-abuse-resistant-keystore-01.txt >
openpgp D. Gillmor openpgp D. Gillmor
Internet-Draft ACLU Internet-Draft ACLU
Intended status: Informational April 04, 2019 Intended status: Informational April 06, 2019
Expires: October 6, 2019 Expires: October 8, 2019
Abuse-Resistant OpenPGP Keystores Abuse-Resistant OpenPGP Keystores
draft-dkg-openpgp-abuse-resistant-keystore-00 draft-dkg-openpgp-abuse-resistant-keystore-01
Abstract Abstract
OpenPGP transferable public keys are composite certificates, made up OpenPGP transferable public keys are composite certificates, made up
of primary keys, user IDs, identity certifications ("signature of primary keys, direct key signatures, user IDs, identity
packets"), subkeys, and so on. They are often assembled by merging certifications ("signature packets"), subkeys, and so on. They are
multiple certificates that all share the same primary key, and often assembled by merging multiple certificates that all share the
distributed in public keystores. same primary key, and are distributed in public keystores.
Unfortunately, since any third-party can add certifications with any Unfortunately, since any third-party can add certifications with any
content to any OpenPGP certificate, the assembled/merged form of a content to any OpenPGP certificate, the assembled/merged form of a
certificate can become unwieldy or undistributable. certificate can become unwieldy or undistributable.
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 third-party certificates can use to mitigate the impact of these various forms of
certificate flooding attacks. flooding attacks.
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.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 6, 2019. This Internet-Draft will expire on October 8, 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
3. Simple Mitigations . . . . . . . . . . . . . . . . . . . . . 6 2.1. Certificate Flooding . . . . . . . . . . . . . . . . . . 6
3.1. Limited Packet Sizes . . . . . . . . . . . . . . . . . . 6 2.2. User ID Flooding . . . . . . . . . . . . . . . . . . . . 6
3.2. Strict User IDs . . . . . . . . . . . . . . . . . . . . . 6 2.3. Keystore Flooding . . . . . . . . . . . . . . . . . . . . 7
3.3. Drop or Standardize Unhashed Subpackets . . . . . . . . . 6 3. Simple Mitigations . . . . . . . . . . . . . . . . . . . . . 7
3.4. Drop User Attributes . . . . . . . . . . . . . . . . . . 7 3.1. Decline Large Packets . . . . . . . . . . . . . . . . . . 7
3.5. Drop Non-exportable Certifications . . . . . . . . . . . 7 3.2. Enforce Strict User IDs . . . . . . . . . . . . . . . . . 8
3.6. Accept Only Cryptographically-verifiable Certifications . 7 3.3. Scoped User IDs . . . . . . . . . . . . . . . . . . . . . 8
3.7. Accept Only Profiled Certifications . . . . . . . . . . . 7 3.4. Strip or Standardize Unhashed Subpackets . . . . . . . . 8
4. Contextual Mitigations . . . . . . . . . . . . . . . . . . . 8 3.5. Decline User Attributes . . . . . . . . . . . . . . . . . 9
4.1. Drop Superseded Signatures . . . . . . . . . . . . . . . 8 3.6. Decline Non-exportable Certifications . . . . . . . . . . 9
4.2. Drop Expired Signatures . . . . . . . . . . . . . . . . . 8 3.7. Decline Data From the Future . . . . . . . . . . . . . . 9
4.3. Drop Dangling User IDs, User Attributes, and Subkeys . . 9 3.8. Accept Only Profiled Certifications . . . . . . . . . . . 9
4.4. Drop All Other Elements of a Directly-Revoked Certificate 9 3.9. Accept Only Certificates Issued by Designated Authorities 10
4.5. Implicit Expiration Date . . . . . . . . . . . . . . . . 10 3.10. Decline Packets by Blocklist . . . . . . . . . . . . . . 10
5. First-party-only Keystores . . . . . . . . . . . . . . . . . 10 4. Contextual Mitigations . . . . . . . . . . . . . . . . . . . 11
6. First-party-attested Third-party Certifications . . . . . . . 11 4.1. Accept Only Cryptographically-verifiable Certifications . 11
6.1. Key Server Preferences "No-modify" . . . . . . . . . . . 12 4.2. Accept Only Certificates Issued by Known Certificates . . 11
6.2. Client Interactions . . . . . . . . . . . . . . . . . . . 12 4.3. Rate-limit Submissions by IP Address . . . . . . . . . . 12
7. Side Effects and Ecosystem Impacts . . . . . . . . . . . . . 12 4.4. Accept Certiifcates Based on Exterior Process . . . . . . 12
7.1. Designated Revoker . . . . . . . . . . . . . . . . . . . 12 4.5. Accept Certificates by E-mail Validation . . . . . . . . 12
7.2. Certification-capable Subkeys . . . . . . . . . . . . . . 12 5. Non-append-only mitigations . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 5.1. Drop Superseded Signatures . . . . . . . . . . . . . . . 13
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13 5.2. Drop Expired Signatures . . . . . . . . . . . . . . . . . 14
10. User Considerations . . . . . . . . . . . . . . . . . . . . . 14 5.3. Drop Dangling User IDs, User Attributes, and Subkeys . . 14
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 5.4. Drop All Other Elements of a Directly-Revoked Certificate 14
12. Document Considerations . . . . . . . . . . . . . . . . . . . 14 5.5. Implicit Expiration Date . . . . . . . . . . . . . . . . 15
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 6. Updates-only Keystores . . . . . . . . . . . . . . . . . . . 15
13.1. Normative References . . . . . . . . . . . . . . . . . . 14 7. First-party-only Keystores . . . . . . . . . . . . . . . . . 16
13.2. Informative References . . . . . . . . . . . . . . . . . 15 8. First-party-attested Third-party Certifications . . . . . . . 16
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8.1. Key Server Preferences "No-modify" . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 8.2. Client Interactions . . . . . . . . . . . . . . . . . . . 18
9. Side Effects and Ecosystem Impacts . . . . . . . . . . . . . 18
9.1. Designated Revoker . . . . . . . . . . . . . . . . . . . 18
9.2. Certification-capable Subkeys . . . . . . . . . . . . . . 18
9.3. Assessing Certificates in the Past . . . . . . . . . . . 19
10. OpenPGP details . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Revocations . . . . . . . . . . . . . . . . . . . . . . 19
10.2. User ID Conventions . . . . . . . . . . . . . . . . . . 20
11. Security Considerations . . . . . . . . . . . . . . . . . . . 21
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
12.1. Publishing Identity Information . . . . . . . . . . . . 22
12.2. Social Graph . . . . . . . . . . . . . . . . . . . . . . 22
12.3. Tracking Clients by Queries . . . . . . . . . . . . . . 22
12.4. Cleartext Queries . . . . . . . . . . . . . . . . . . . 23
12.5. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 23
13. User Considerations . . . . . . . . . . . . . . . . . . . . . 24
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
15. Document Considerations . . . . . . . . . . . . . . . . . . . 24
15.1. Document History . . . . . . . . . . . . . . . . . . . . 24
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
17.1. Normative References . . . . . . . . . . . . . . . . . . 25
17.2. Informative References . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 27
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.
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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 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 o "Certificate discovery" is the process whereby a user retrieves an
OpenPGP certificate based on user ID. A user attempting to OpenPGP certificate based on user ID (see Section 10.2). A user
discover a certificate from a keystore will search for a substring attempting to discover a certificate from a keystore will search
of the known user IDs, most typically an e-mail address if the for a substring of the known user IDs, most typically an e-mail
user ID is an [RFC5322] name-addr or addr-spec. Some certificate address if the user ID is an [RFC5322] name-addr or addr-spec.
discovery mechanisms look for an exact match on the known user Some certificate discovery mechanisms look for an exact match on
IDs. [I-D.koch-openpgp-webkey-service] and [I-D.shaw-openpgp-hkp] the known user IDs. [I-D.koch-openpgp-webkey-service] and
are both certificate discovery mechanisms. [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 example, if the certificate has a user ID of "Alice
alice@example.org [1]" 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. This process can take different forms, and can consider
many different factors, some of which are not directly contained many different factors, some of which are not directly contained
in the certificate itself. For example, certificate validation in the certificate itself. For example, certificate validation
might consider whether the certificate was fetched via DANE might consider whether the certificate was fetched via DANE
([RFC7929]) or WKD ([I-D.koch-openpgp-webkey-service]); or whether ([RFC7929]) or WKD ([I-D.koch-openpgp-webkey-service]); or whether
it has seen e-mails from that address signed by the certificate in it has seen e-mails from that address signed by the certificate in
the past; or how long it has known about certificate. the past; or how long it has known about certificate.
o "Certificate update" is the process whereby a user fetches new o "Certificate update" is the process whereby a user fetches new
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certificate update or revocation, since a certificate could be certificate update or revocation, since a certificate could be
simply replaced by an adversary who also has access to the e-mail simply replaced by an adversary who also has access to the e-mail
address in question. [MAILVELOPE-KEYSERVER] is an example of such address in question. [MAILVELOPE-KEYSERVER] is an example of such
a keyserver. 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 [2]" 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
for certificate validation. Also note that knowledge of the for certificate validation. Also note that knowledge of the
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
[RFC4880]), which can be either "soft" or "hard". The set of
"soft" reasons is: "Key is superseded" and "Key is retired and no
longer used". All other reasons (and revocations that do not
state a reason) are "hard" revocations. See Section 10.1 for more
detail.
2. Problem Statement 2. Problem Statement
OpenPGP keystores that handle submissions from the public are subject
to a range of flooding attacks by malicious submitters.
This section describes three distinct flooding attacks that public
keystores should consider.
The rest of the document describes some mitigations that can be used
by keystores that are concerned about these problems but want to
continue to offer some level of service for certificate discovery,
certificate update, or certificate validation.
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 discovery. It also makes it possible to swamp
a certificate that has been revoked, preventing certificate update, a certificate that has been revoked, preventing certificate update,
potentially leaving the client of the keystore with the compromised potentially leaving the client of the keystore with the compromised
certificate in an unrevoked state locally. certificate in an unrevoked state locally.
Additionally, even without malice, OpenPGP certificates can Additionally, even without malice, OpenPGP certificates can
potentially grow without bound. potentially grow without bound.
The rest of this document describes some mitigations that can be used 2.2. User ID Flooding
by keystores that are concerned about these problems but want to
continue to offer some level of service for certificate discovery, Public keystores that are used for certificate discovery may also be
certificate update, or certificate validation. vulnerable to attacks that flood the space of known user IDs. In
particular, if the keystore accepts arbitrary certificates from the
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
effectively unbounded set of maliciously-submitted certificates to
find the non-malicious certificates they are looking for.
For example, if an attacker knows that a given system consults a
keystore looking for certificates which match the e-mail address
"alice@example.org", the attacker may upload hundreds or thousands of
certificates containing user IDs that match that address. Even if
those certificates would not be accepted by a client (e.g., because
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
non-malicious certificates.
If the keystore does not offer a discovery interface at all (that is,
if clients cannot search it by user ID), then user ID flooding is of
less consequence.
2.3. Keystore Flooding
A public keystore that accepts arbitrary OpenPGP material and is
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
is not being deliberately flooded, and if individual certificates are
protected from flooding by any of the mechanisms described later in
this document.
The keystore itself can become difficult to operate if the total
quantity of data is too large, and if it is a synchronizing
keyserver, then the quantities of data may impose unsustainable
bandwidth costs on the operator as well.
Effectively mitigating against keystore flooding requires either
abandoning the append-only property that some keystores prefer, or
imposing very strict controls on initial ingestion.
3. Simple Mitigations 3. 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.
3.1. Limited Packet Sizes 3.1. Decline Large Packets
While [RFC4880] permits OpenPGP packet sizes of arbitrary length, While [RFC4880] permits OpenPGP packet sizes of arbitrary length,
OpenPGP certificates rarely need to be so large. An abuse-resistant OpenPGP certificates rarely need to be so large. An abuse-resistant
keystore SHOULD reject any OpenPGP packet larger than 8383 octets. keystore SHOULD reject any OpenPGP packet larger than 8383 octets.
(This cutoff is chosen because it guarantees that the packet size can (This cutoff is chosen because it guarantees that the packet size can
be represented as a one- or two-octet [RFC4880] "New Format Packet be represented as a one- or two-octet [RFC4880] "New Format Packet
Length", but it could be reduced further) Length", but it could be reduced further)
This may cause problems for user attribute packets that contain large This may cause problems for user attribute packets that contain large
images, but it's not clear that these images are concretely useful in images, but it's not clear that these images are concretely useful in
any context. Some keystores MAY extend this limit for user attribute any context. Some keystores MAY extend this limit for user attribute
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.
3.2. Strict User IDs 3.2. 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".
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.
3.3. Drop or Standardize Unhashed Subpackets 3.3. Scoped User IDs
Some abuse-resistant keystores may restrict themselves to publishing
only certificates with User IDs that match a specific pattern. For
example, [RFC7929] encourages publication in the DNS of only
certificates whose user IDs refer to e-mail addresses within the DNS
zone. [I-D.koch-openpgp-webkey-service] similarly aims to restrict
publication to certificates relevant to the specific e-mail domain.
3.4. 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 keysetore SHOULD strip out all unhashed
subpackets. subpackets.
Note that some certifications only identify the issuer of the Note that some certifications only identify the issuer of the
certification by an unhashed Issuer ID subpacket. If a certification by an unhashed Issuer ID subpacket. If a
certification's hashed subpacket section has no Issuer ID or Issuer certification's hashed subpacket section has no Issuer ID or Issuer
Fingerprint (see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an Fingerprint (see [I-D.ietf-openpgp-rfc4880bis]) subpacket, then an
abuse-resistant keystore that has cryptographically validated the abuse-resistant keystore that has cryptographically validated the
certification SHOULD make the unhashed subpackets contain only a certification SHOULD make the unhashed subpackets contain only a
single subpacket. That subpacket should be of type Issuer single subpacket. That subpacket should be of type Issuer
Fingerprint, and should contain the fingerprint of the issuer. Fingerprint, and should contain the fingerprint of the issuer.
A special exception may be made for unhashed subpackets in a third- A special exception may be made for unhashed subpackets in a third-
party certification that contain attestations from the certificate's party certification that contain attestations from the certificate's
primary key as described in Section 6. primary key as described in Section 8.
3.4. Drop User Attributes 3.5. 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.
3.5. Drop Non-exportable Certifications 3.6. Decline Non-exportable Certifications
An abuse-resistant keystore MUST NOT accept any certification that An abuse-resistant keystore MUST NOT accept any certification that
has the "Exportable Certification" subpacket present and set to 0. has the "Exportable Certification" subpacket present and set to 0.
While most keystore clients will not upload these "local" While most keystore clients will not upload these "local"
certifications anyway, a reasonable public keystore that wants to certifications anyway, a reasonable public keystore that wants to
minimize data has no business storing or distributing these minimize data has no business storing or distributing these
certifications. certifications.
3.6. Accept Only Cryptographically-verifiable Certifications 3.7. Decline Data From the Future
An abuse-resistant keystore that is capable of doing cryptographic Many OpenPGP packets have time-of-creation timestamps in them. An
validation MAY decide to reject certifications that it cannot abuse-resistant keystore with a functional real-time clock MAY decide
cryptographically validate. to only accept packets whose time-of-creation is in the future.
This may mean that the keystore rejects some packets while it is Note that some OpenPGP implementations may pre-generate OpenPGP
unaware of the public key of the issuer of the packet. material intended for use only in some future window (e.g. "Here is
the certificate we plan to use to sign our software next year; do not
accept signatures from it until then."), and may use modified time-
of-creation timestamps to try to acheive that purpose. This material
would not be distributable ahead of time by an abuse-resistant
keystore that adopts this mitigation.
3.7. Accept Only Profiled Certifications 3.8. Accept Only Profiled Certifications
An aggressively abuse-resistant keystore MAY decide to only accept An aggressively abuse-resistant keystore MAY decide to only accept
certifications that meet a specific profile. For example, it MAY certifications that meet a specific profile. For example, it MAY
reject certifications with unknown subpacket types, unknown reject certifications with unknown subpacket types, unknown
notations, or certain combinations of subpackets. This can help to notations, or certain combinations of subpackets. This can help to
minimize the amount of room for garbage data uploads. minimize the amount of room for garbage data uploads.
Any abuse-resistant keystore that adopts such a strict posture should Any abuse-resistant keystore that adopts such a strict posture should
clearly document what its expected certificate profile is, and should clearly document what its expected certificate profile is, and should
have a plan for how to extend the profile if new types of have a plan for how to extend the profile if new types of
certification appear that it wants to be able to distribute. certification appear that it wants to be able to distribute.
Note that if the profile is ever restricted (rather than extended),
and the restriction is applied to the material already present, such
a keystore is no longer append-only (please see Section 5).
3.9. Accept Only Certificates Issued by Designated Authorities
An abuse-resistant keystore capable of cryptographic validation MAY
retain a list of designated authorities, typically in the form of a
set of known public keys. Upon receipt of a new OpenPGP certificate,
the keystore can decide whether to accept or decline each user ID of
the certificate based whether that user ID has a certification that
was issued by one or more of the designated authorities.
If no user IDs are certified by designated authority, such a keystore
SHOULD decline the certificate and its primary key entirely. Such a
keystore SHOULD decline to retain or propagate all certifications
associated with each accepted user ID except for first-party
certifications and certifications by the designated authorities.
The operator of such a keystore SHOULD have a clear policy about its
set of designated authorities.
Given the ambiguities about expiration and revocation, such a
keyserver SHOULD ignore expiration and revocation of authority
certifications, and simply accept and retain as long as the
cryptographic signature is valid.
Note that if any key is removed from the set of designated
authorities, and that change is applied to the existing keystore,
such a keystore may no longer be append-only (please see Section 5).
3.10. Decline Packets by Blocklist
The maintainer of the keystore may keep a specific list of "known-
bad" material, and decline to accept or redistribute items matching
that blocklist. The material so identified could be anything, but
most usefully, specific public keys or User IDs could be blocked.
Note that if a blocklist grows to include an element already present
in the keystore, it will no longer be append-only (please see
Section 5).
Some keystores may choose to apply a blocklist only at distribution
time and not apply it at input time. This allows the keystore to be
append-only, and permits synchronization between keystores that don't
share a blocklist, and somewhat reduces the attacker's incentive for
flooding the keystore.
Note that development and maintenance of a blocklist is not without
its own potentials for abuse. For one thing, the blocklist may
itself grow without bound. Additionally, a blocklist may be socially
or politically contentious. There needs to be a clear policy about
how it is managed, whether by delegation to specific decision-makers,
or explicit tests. Furthermore, the existence of even a well-
intentioned blocklist may be an "attractive nuisance," drawing the
interest of would-be censors or other attacker interested in
controlling the ecosystem reliant on the keystore in question.
4. Contextual Mitigations 4. Contextual Mitigations
The following mitigations may cause some packets to be dropped after Some mitigations make the acceptance or rejection of packets
the keystore receives new information, or as time passes. This is contingent on data that is already in the keystore or the keystore's
entirely reasonable for some keystores, but it may be surprising for developing knowledge about the world. This means that, depending on
any keystore that expects to be append-only (for example, some the order that the keystore encounters the various material, or how
keyserver synchronization techniques may expect this property to it discovers the material, the final set of material retained and
hold). distributed by the keystore might be different.
While this isn't necessarily bad, it may be a surprising property for
some users of keystores.
4.1. Accept Only Cryptographically-verifiable Certifications
An abuse-resistant keystore that is capable of doing cryptographic
validation MAY decide to reject certifications that it cannot
cryptographically validate.
This may mean that the keystore rejects some packets while it is
unaware of the public key of the issuer of the packet.
4.2. Accept Only Certificates Issued by Known Certificates
This is an extension of Section 3.9, but where the set of authorities
is just the set of certificates already known to the keystore. An
abuse-resistant keystore that adopts this strategy is effectively
only crawling the reachable graph of OpenPGP certificates from some
starting core.
A keystore adopting the mitigation SHOULD have a clear documentation
of the core of initial certificates it starts with, as this is
effectively a policy decision.
This mitigation measure may fail due to a compromise of any secret
key that is associated with a primary key of a certificate already
present in the keystore. Such a compromise permits an attacker to
flood the rest of the network. In the event that such a compromised
key is identified, it might be placed on a blocklist (see
Section 3.10). In particular, if a public key is added to a
blocklist for a keystore implementing this mitigation, and it is
removed from the keystore, then all certificates that were only
"reachable" from the blocklisted certificate should also be
simultaneously removed.
4.3. Rate-limit Submissions by IP Address
Some OpenPGP keystores accept material from the general public over
the Internet. If an abuse-resistant keystore observes a flood of
material submitted to the keystore from a given Internet address, it
MAY choose to throttle submissions from that address. When receiving
submissions over IPv6, such a keystore MAY choose to throttle entire
nearby subnets, as a malicious IPv6 host is more likely to have
multiple addresses.
This requires that the keystore maintain state about recent
submissions over time and address. It may also be problematic for
users who appear to share an IP address from the vantage of the
keystore, including those beind a NAT, using a VPN, or accessing the
keystore via Tor.
4.4. Accept Certiifcates Based on Exterior Process
Some public keystores resist abuse by explicitly filtering OpenPGP
material based on a set of external processes. For example,
[DEBIAN-KEYRING] adjudicates the contents of the "Debian keyring"
keystore based on organizational procedure and manual inspection.
4.5. Accept Certificates by E-mail Validation
Some keystores resist abuse by declining any certificate until the
user IDs have been verified by e-mail. When these "e-mail-
validating" keystores review a new certificate that has a user ID
with an e-mail address in it, they send an e-mail to the associated
address with a confirmation mechanism (e.g., a high-entropy HTTPS URL
link) in it. In some cases, the e-mail itself is encrypted to an
encryption-capable key found in the proposed certificate. If the
keyholder triggers the confirmation mechanism, then the keystore
accepts the certificate.
[PGP-GLOBAL-DIRECTORY] describes some concerns held by a keystore
operator using this approach. [MAILVELOPE-KEYSERVER] is another
example.
5. Non-append-only mitigations
The following mitigations may cause some previously-retained packets
to be dropped after the keystore receives new information, or as time
passes. This is entirely reasonable for some keystores, but it may
be surprising for any keystore that expects to be append-only (for
example, some keyserver synchronization techniques may expect this
property to hold).
Furthermore, keystores that drop old data, or certifications that are
superseded may make it difficult or impossible for their users to
reason about the validity of signatures that were made in the past.
See Section 9.3 for more considerations.
Note also that many of these mitigations depend on cryptographic Note also that many of these mitigations depend on cryptographic
validation. 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. cryptographic validation MAY omit these mitigations.
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,
and signatures issued by keys that are not present on the and signatures issued by keys that are not present on the
keyring. keyring.
4.1. Drop Superseded Signatures 5.1. Drop Superseded Signatures
An abuse-resistant keystore SHOULD drop all signature packets that An abuse-resistant keystore SHOULD drop all signature packets that
are explicitly superseded. For example, there's no reason to retain are explicitly superseded. For example, there's no reason to retain
or distribute a self-sig by key K over User ID U from 2017 if the or distribute a self-sig by key K over User ID U from 2017 if the
keystore have a cryptographically-valid self-sig over <K,U> from keystore have a cryptographically-valid self-sig over <K,U> from
2019. 2019.
Note that this covers both certifications and signatures over Note that this covers both certifications and signatures over
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.
4.2. Drop Expired Signatures 5.2. 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 functionally real-time clock SHOULD drop all
certifications and subkey signature packets with an expiration date certifications 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!
4.3. Drop Dangling User IDs, User Attributes, and Subkeys 5.3. 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.
An abuse-resistant keystore which has dropped all certifications that An abuse-resistant keystore which has dropped all certifications that
cover a User ID SHOULD also drop the User ID packet. cover a User ID SHOULD also drop the User ID packet.
Note that a User ID that becomes invalid due to revocation MUST NOT Note that a User ID that becomes invalid due to revocation MUST NOT
be dropped, because the User ID's revocation signature itself remains be dropped, because the User ID's revocation signature itself remains
valid, and needs to be distributed. valid, and needs to be distributed.
A primary key with no User IDs and no subkeys and no revocations MAY A primary key with no User IDs and no subkeys and no revocations MAY
itself also be removed from distribution, though note that the itself also be removed from distribution, though note that the
removal of a primary key may make it impossible to cryptographically removal of a primary key may make it impossible to cryptographically
validate other certifications held by the keystore. validate other certifications held by the keystore.
4.4. Drop All Other Elements of a Directly-Revoked Certificate 5.4. Drop All Other Elements of a Directly-Revoked Certificate
If the primary key of a certiifcate is revoked via a direct key If the primary key of a certiifcate is revoked via a direct key
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 strongest, earliest key revocation signatures, it should retain the hardest, earliest
revocation. revocation (see also Section 10.1).
In particular, if any of the revocation signatures has a "Reason for
Revocation" of "Key material has been compromised", the keystore
SHOULD retain the earliest such revocation signature (by signature
creation date).
If none have "Key material has been compromised", but some have "No In particular, if any of the direct key revocation signatures is a
reason specified", or lack a "Reason for Revocation" entirely, then "hard" revocation, the abuse-resistant keystore SHOULD retain the
the keystore SHOULD retain the earliest such revocation signature. 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
direct key revocation signature it has seen. "soft" direct key revocation signature it has seen.
If any of the 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 severity, an abuse-resistant revocation signatures of the same "hardness", an abuse-resistant
keystore SHOULD retain the signature that sorts earliest based on a keystore SHOULD retain the signature that sorts earliest based on a
binary string comparison of the signature packet itself. binary string comparison of the direct key revocation signature
packet itself.
4.5. Implicit Expiration Date 5.5. Implicit Expiration Date
A particularly aggressive abuse-resistant keystore MAY choose an In combination with some of the dropping mitigations above, a
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. keystore as expiring 3 years after issuance. This would permit the
keystore to eject old packets on a rolling basis.
FIXME: it's not clear what should happen with signature packets FIXME: it's not clear what should happen with signature packets
marked with an explicit expiration that is longer than implicit marked with an explicit expiration that is longer than implicit
maximum. Should it be capped to the implicit date, or accepted? maximum. Should it be capped to the implicit date, or accepted?
Warning: This idea is pretty radical, and it's not clear what it Warning: This idea is pretty radical, and it's not clear what it
would do to an ecosystem that depends on such a keystore. It would do to an ecosystem that depends on such a keystore. It
probably needs more thinking. probably needs more thinking.
5. First-party-only Keystores 6. Updates-only Keystores
In addition to all of the mitigations above, some keystores may In addition to the mitigations above, some keystores may resist abuse
resist abuse by declining to carry third-party certifications by declining to accept any user IDs or certifications whatsoever.
entirely.
A first-party-only keystore _only_ accepts and distributes Such a keystore MUST be capable of cryptographic validation. It
accepts primary key packets, cryptographically-valid direct-key
signatures from a primary key over itself, subkeys and their
cryptographically-validated binding signatures (and cross signatures,
where necessary).
Clients of an updates-only keystore cannot possibly use the keystore
for certificate discovery, because there are no user IDs to match.
However, they can use it 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
signature-based certificate expiration updates.
Note that many popular OpenPGP implemenations do not implement direct
primary key expiration mechanisms, relying instead on user ID
expirations. These user ID expiration dates or other metadata
associated with a self-certification will not be distributed by an
updates-only keystore.
Certificates shipped by an updates-only keystore are technically
invalid [RFC4880] "transferable public keys," because they lack a
user ID packet. However many OpenPGP implementations will accept
such a certificate if they already know of a user ID for the
certificate, because the composite certificate resulting from a merge
will be a standards-compliant transferable public key.
7. First-party-only Keystores
Slightly more permissive than the updates-only keystore described in
Section 6 is a keystore that also permits user IDs and their self-
sigs.
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 solves the problem of abusive bloating attacks on
any certificate, because the only party who can make a certificate any certificate, because the only party who can make a certificate
overly large is the holder of the secret corresponding to the primary overly large is the holder of the secret corresponding to the primary
key itself. key itself.
However, first-party-only keystores also introduce new problems, for However, a first-party-only keystore is still problematic for those
those people who rely on the keystore for discovery of third-party people who rely on the keystore for discovery of third-party
certifications. Section 6 attempts to address this lack. certifications. Section 8 attempts to address this lack.
6. First-party-attested Third-party Certifications 8. 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
skipping to change at page 11, line 27 skipping to change at page 17, line 10
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 o The attestation MUST be an OpenPGP signature packet of type 0x50
(Third-Party Confirmation signature) (Third-Party Confirmation signature)
o The attestation MUST contain a notation subpacket
o The attestation MUST contain a hashed "Issuer Fingerprint" o 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. o The attestation MUST NOT be marked as non-exportable.
o The attestation MUST contain a hashed Notation subpacket with the o The attestation MUST contain a hashed Notation subpacket with the
name "ksok", and an empty (0-octet) value. name "ksok", and an empty (0-octet) value.
o The attestation MUST contain a hashed "Signature Target" subpacket o The attestation MUST contain a hashed "Signature Target" subpacket
skipping to change at page 12, line 17 skipping to change at page 17, line 47
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 - FIXME: it's not clear whether the "ksok" notification is necessary -
it's in place to avoid some accidental confusion with any other use it's in place to avoid some accidental confusion with any other use
of the Third-Party Confirmation signature packet type, but the author of the Third-Party Confirmation signature packet type, but the author
does not know of any such use that might collide. does not know of any such use that might collide.
6.1. Key Server Preferences "No-modify" 8.1. Key Server Preferences "No-modify"
[RFC4880] section 5.2.3.17 ("Key Server Preferences") defines a "No- [RFC4880] defines "Key Server Preferences" with a "No-modify" bit.
modify" bit. That bit has never been respected by any keyserver That bit has never been respected by any keyserver implementation
implementation that the author is aware of. This section effectively that the author is aware of. This section effectively asks an abuse-
asks an abuse-resistant keystore to treat that bit as always set, resistant keystore to treat that bit as always set, whether it is
whether it is present in the certificate or not. present in the certificate or not.
6.2. Client Interactions 8.2. Client Interactions
The multi-stage layer of creating such an attestation (certificate The multi-stage layer of creating such an attestation (certificate
creation by the first-party, certification by the third-party, creation by the first-party, certification by the third-party,
attestation by the first-party, then handoff to the keystore) may attestation by the first-party, then handoff to the keystore) may
represent a usability obstacle to a user who needs a third-party- represent a usability obstacle to a user who needs a third-party-
certified OpenPGP certificate. certified OpenPGP certificate.
No current OpenPGP client can easily create the attestions described No current OpenPGP client can easily create the attestions described
in this section. More implementation work needs to be done to make in this section. More implementation work needs to be done to make
it easy (and understandable) for a user to perform this kind of it easy (and understandable) for a user to perform this kind of
attestation. attestation.
7. Side Effects and Ecosystem Impacts 9. Side Effects and Ecosystem Impacts
7.1. Designated Revoker 9.1. Designated Revoker
A first-party-only keystore might decline to distribute revocations A first-party-only keystore as described in Section 7 might decline
made by the designated revoker. This is a risk to certificate-holder to distribute revocations made by the designated revoker. This is a
who depend on this mechanism. Perhaps this document should be risk to certificate-holder who depend on this mechanism, because an
amended to include these important revocation might be missed by clients depending on the
keystore.
7.2. Certification-capable Subkeys FIXME: adjust this document to point out where revocations from a
designated revoker SHOULD be propagated, maybe even in first-party-
only keystores.
9.2. 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 are much fuzzier. time. So questions about what data can be dropped (e.g. in
Section 5) are much fuzzier, and the underlying assumptions may need
to be reviewed.
The author of this draft recommends _not_ considering any subkeys to If some OpenPGP implementations accept certification-capable subkeys,
be certification-capable to avoid this headache. but an abuse-resistant keystore does not accept certifications from
subkeys in general, then interactions between that keystore and those
implementations may be surprising.
8. Security Considerations 9.3. Assessing Certificates in the Past
These mitigations defend individual OpenPGP certificates against Online protocols like TLS perform signature and certificate
bloating attacks. They collectively reduce the amount of data that evaluation based entirely on the present time. If a certificate that
such a keystore needs to track over time, but given the near-infinite signs a TLS handshake message is invalid now, it doesn't matter
space of possible OpenPGP keys that can be generated, the keystore in whether it was valid a week ago, because the present TLS session is
aggregate can still be made to grow without bound. This document the context of the evaluation.
proposes no clear measures to defend against such a denial of service
attack against the keystore itself.
Section 7.1 describes a potentially But OpenPGP signatures are often evaluated at some temporal remove
from when the signature was made. For example, software packages are
signed at release time, but those signatures are validated at
download time.
Further complicating matters, the composable nature of an OpenPGP
certificate means that the certificate associated with any particular
signing key (primary key or subkey) can transform over time. So when
evaluating a signature that appears to have been made by a given
certificate, it may be better to try to evaluate the certificate at
the time the signature was made, rather than the present time.
When evaluating a certificate at a time T in the past, one approach
is to discard all packets with a creation time later than T, and then
evaluate the resulting certificate from the remaining packets in the
context of time T.
However, any such evaluator SHOULD NOT ignore "hard" OpenPGP key
revocations, regardless of their creation date. (see Section 10.1).
If a non-append-only keystore (Section 5) has dropped superseded
(Section 5.1) or expired (Section 5.2) certifications, it's possible
for the certificate composed of the remaining packets to have no
valid first-party certification at the time that a given signature
was made. Such a certificate would be invalid according to
[RFC4880].
10. OpenPGP details
This section collects details about common OpenPGP implementation
behavior that are useful in evaluating and reasoning about OpenPGP
certificates.
10.1. Revocations
It's useful to classify OpenPGP revocations of key material into two
categories: "soft" and "hard".
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"
revocation.
An implementation that interprets a "soft" revocation will typically
not invalidate signatures made by the associated key with a creation
date that predates the date of the soft revocation. A "soft"
revocation in some ways behaves like a non-overridable expiration
date.
All other revocations of OpenPGP keys (with any other Reason for
Revocation, or with no Reason for Revocation at all) should be
considered "hard".
The presence of a "hard" revocation of an OpenPGP key indicates that
the user should reject all signatures and certifications made by that
key, regardless of the creation date of the signature.
Note that some OpenPGP implementations do not distinguish between
these two categories.
A defensive OpenPGP implementation that does not distinguish between
these two categories SHOULD treat all revocations as "hard".
An implementation aware of a "soft" revocation or of key or
certificate expiry at time T SHOULD accept and process a "hard"
revocation even if it appears to have been issued at a time later
than T.
10.2. User ID Conventions
[RFC4880] requires a user ID to be a UTF-8 string, but does not
constrain it beyond that. In practice, a handful of conventions
predominate in how User IDs are formed.
The most widespread convention is a name-addr as defined in
[RFC5322]. For example:
Alice Jones <alice@example.org>
But a growing number of OpenPGP certificates contain user IDs that
are instead a raw [RFC5322] addr-spec, omitting the display-name and
the angle brackets entirely, like so:
alice@example.org
Some certificates have user IDs that are simply "normal" human names
(perhaps display-name in [RFC5322] jargon, though not necessarily
conforming to a specific ABNF). For example:
Alice Jones
Still other certificates identify a particular network service by
scheme and hostname. For example, the administrator of an ssh host
participating in the [MONKEYSPHERE] might choose a user ID for the
OpenPGP representing the host like so:
ssh://foo.example.net
11. Security Considerations
This document offers guidance on mitigating a range of denial-of-
service attacks on public keystores, so the entire document is in
effect about security considerations.
Many of the mitigations described here defend individual OpenPGP
certificates against flooding attacks (see Section 2.1). But only
some of these mitigations defend against flooding attacks against the
keystore itself (see Section 2.3), or against flooding attacks on the
space of possible user IDs (see Section 2.2). Thoughtful threat
modeling and monitoring of the keystore and its defenses are probably
necessary to maintain the long-term health of the keystore.
Section 9.1 describes a potentially scary security problem for
designated revokers.
Note that there is an inherent tension between accepting arbitrary
certificate uploads and permitting effective certificate discovery.
If a keystore accepts arbitrary certificate uploads for
redistribution, it appears to be vulnerable to user ID flooding
(Section 2.2), which makes it difficult or impossible to rely on for
certificate discovery.
TODO (more security considerations) TODO (more security considerations)
9. Privacy Considerations 12. Privacy Considerations
Keystores themselves raise a host of potential privacy concerns.
Additional privacy concerns are raised by traffic to and from the
keystores. This section tries to outline some of the risks to the
privacy of people whose certificates are stored and redistributed in
public keystores, as well as risks to the privacy of people who make
use of the key stores for certificate discovery or certificate
update.
TODO (more privacy considerations)
12.1. 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 4.4 can help such users strip that regard. The mitigation in Section 5.4 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.
An updates-only keyserver (Section 6) avoids this particular privacy
concern because it distributes no user IDs at all.
12.2. Social Graph
Third-party certifications effectively map out some sort of social Third-party certifications effectively map out some sort of social
graph. While the certifications basically only assert a binding graph. A certification asserts a statement of belief by the issuer
between user IDs, the parties those user IDs represent in the real that the real-world party identified by the user ID is in control of
world, and cryptographic key material, those connections may be the subject cryptographic key material. But those connections may be
potentially sensitive, and users may not want to see these maps potentially sensitive, and some people may not want these maps built.
built.
TODO (more privacy considerations) A first-party-only keyserver (Section 7) avoids this privacy concern
because it distribues no third-party privacy concern.
10. User Considerations First-party attested third-party certifications described in
Section 8 are even more relevant edges in the social graph, because
their bidirectional nature suggests that both parties are aware of
each other, and see some value in mutual association.
Section 6.2 describes some outstanding work that needs to be done to 12.3. Tracking Clients by Queries
Even without third-party certifications, the acts of certificate
discovery and certificate update represent a potential privacy risk,
because the keystore queried gets to learn which user IDs (in the
case of discovery) or which certificates (in the case of update) the
client is interested in. In the case of certificate update, if a
client attempts to update all of its known certificates from the same
keystore, that set is likely to be a unique set, and therefore
identifies the client. A keystore that monitors the set of queries
it receives might be able to profile or track those clients who use
it repeatedly.
Clients which want to to avoid such a tracking attack MAY try to
perform certificate update from multiple different keystores. To
hide network location, a client making a network query to a keystore
SHOULD use an anonymity network like [TOR]. Tools like [PARCIMONIE]
are designed to facilitate this type of certificate update.
Keystores which permit public access and want to protect the privacy
of their clients SHOULD NOT reject access from clients using [TOR] or
comparable anonymity networks. Additionally, they SHOULD minimize
access logs they retain.
Alternately, some keystores may distribute their entire contents to
any interested client, in what can be seen as the most trivial form
of private information retrieval. [DEBIAN-KEYRING] is one such
example; its contents are distributed as an operating system package.
Clients can interrogate their local copy of such a keystore without
exposing their queries to a third-party.
12.4. Cleartext Queries
If access to the keystore happens over observable channels (e.g.,
cleartext connections over the Internet), then a passive network
monitor could perform the same type profiling or tracking attack
against clients of the keystore described in Section 12.3. Keystores
which offer network access SHOULD provide encrypted transport.
12.5. Traffic Analysis
Even if a keystore offers encrypted transport, the size of queries
and responses may provide effective identification of the specific
certificates fetched during discovery or update, leaving open the
types of tracking attacks described in Section 12.3. Clients of
keystores SHOULD pad their queries to increase the size of the
anonymity set. And keystores SHOULD pad their responses.
The appropriate size of padding to effectively anonymize traffic to
and from keystores is likely to be mechanism- and cohort-specific.
For example, padding for keystores accessed via the DNS ([RFC7929]
may use different padding strategies that padding for keystores
accessed over WKD ([I-D.koch-openpgp-webkey-service]), which may in
turn be different from keystores accessed over HKPS
([I-D.shaw-openpgp-hkp]). A keystore which only accepts user IDs
within a specific domain (e.g., Section 3.3) or which uses custom
process (Section 4.4) for verification might have different padding
criteria than a keystore that serves the general public.
Specific padding policies or mechanisms are out of scope for this
document.
13. User Considerations
Section 8.2 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.
11. IANA Considerations 14. IANA Considerations
This document asks IANA to register the "ksok" notation name in the This document asks IANA to register the "ksok" notation name in the
OpenPGP Notation IETF namespace, with a reference to this document, OpenPGP Notation IETF namespace, with a reference to this document,
as defined in Section 6. as defined in Section 8.
12. Document Considerations 15. 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
13. References 15.1. Document History
13.1. Normative References substantive changes between -00 and -01:
o split out Contextual and Non-Append-Only mitigations
o documented several other mitigations, including:
* Decline Data From the Future
* Blocklist
* Exterior Process
* Designated Authorities
* Known Certificates
* Rate-Limiting
* Scoped User IDs
o documented Updates-Only Keystores
o consider three different kinds of flooding
o deeper discussion of privacy considerations
o better documentation of Reason for Revocation
o document user ID conventions
16. Acknowledgements
This document is the result of years of operational experience and
observation, as well as conversations with many different people -
users, implementors, keystore operators, etc. A non-exhaustive list
of people who have contriubuted ideas or nuance to this document
specifically includes:
o Antoine Beaupre
o Jamie McClelland
o Jonathan McDowell
o Justus Winter
o Neal Walfield
o vedaal
o Vincent Breitmoser
o Wiktor Kwapisiewicz
17. References
17.1. 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.
[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>.
13.2. Informative References 17.2. Informative References
[DEBIAN-KEYRING]
McDowell, J., "Debian Keyring", n.d.,
<https://keyring.debian.org/>.
[GnuPG] Koch, W., "Using the GNU Privacy Guard", n.d., [GnuPG] Koch, W., "Using the GNU Privacy Guard", n.d.,
<https://www.gnupg.org/documentation/manuals/gnupg.pdf>. <https://www.gnupg.org/documentation/manuals/gnupg.pdf>.
[I-D.koch-openpgp-webkey-service] [I-D.koch-openpgp-webkey-service]
Koch, W., "OpenPGP Web Key Directory", draft-koch-openpgp- Koch, W., "OpenPGP Web Key Directory", draft-koch-openpgp-
webkey-service-07 (work in progress), November 2018. webkey-service-07 (work in progress), November 2018.
[I-D.shaw-openpgp-hkp] [I-D.shaw-openpgp-hkp]
Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)", Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
draft-shaw-openpgp-hkp-00 (work in progress), March 2003. draft-shaw-openpgp-hkp-00 (work in progress), March 2003.
[MAILVELOPE-KEYSERVER] [MAILVELOPE-KEYSERVER]
Oberndoerfer, T., "Mailvelope Keyserver", n.d., Oberndoerfer, T., "Mailvelope Keyserver", n.d.,
<https://github.com/mailvelope/keyserver/>. <https://github.com/mailvelope/keyserver/>.
[MONKEYSPHERE]
Gillmor, D. and J. Rollins, "Monkeysphere", n.d.,
<https://web.monkeysphere.info/>.
[PARCIMONIE]
Intrigeri, ., "Parcimonie", n.d.,
<https://gaffer.ptitcanardnoir.org/intrigeri/code/
parcimonie/>.
[PGP-GLOBAL-DIRECTORY]
Symantec Corporation, "PGP Global Directory Key
Verification Policy", 2011,
<https://keyserver.pgp.com/vkd/
VKDVerificationPGPCom.html>.
[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>.
[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] Pennock, P., "SKS Keyserver Documentation", March 2018, [SKS] Pennock, P., "SKS Keyserver Documentation", March 2018,
<https://bitbucket.org/skskeyserver/sks-keyserver/wiki/ <https://bitbucket.org/skskeyserver/sks-keyserver/wiki/
Home>. Home>.
13.3. URIs [TOR] "The Tor Project", n.d., <https://www.torproject.org/>.
[1] mailto:alice@example.org
[2] mailto:alice@example.org
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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