< draft-ietf-dane-openpgpkey-07.txt   draft-ietf-dane-openpgpkey-08.txt >
Network Working Group P. Wouters Network Working Group P. Wouters
Internet-Draft Red Hat Internet-Draft Red Hat
Intended status: Experimental January 27, 2016 Intended status: Experimental March 08, 2016
Expires: July 30, 2016 Expires: September 09, 2016
Using DANE to Associate OpenPGP public keys with email addresses Using DANE to Associate OpenPGP public keys with email addresses
draft-ietf-dane-openpgpkey-07 draft-ietf-dane-openpgpkey-08
Abstract Abstract
OpenPGP is a message format for email (and file) encryption that OpenPGP is a message format for email (and file) encryption that
lacks a standardized lookup mechanism to securely obtain OpenPGP lacks a standardized lookup mechanism to securely obtain OpenPGP
public keys. DNS-Based Authentication of Named Entities ("DANE") is public keys. DNS-Based Authentication of Named Entities ("DANE") is
a method for publishing public keys in DNS. This document specifies a method for publishing public keys in DNS. This document specifies
a DANE method for publishing and locating OpenPGP public keys in DNS a DANE method for publishing and locating OpenPGP public keys in DNS
for a specific email address using a new OPENPGPKEY DNS Resource for a specific email address using a new OPENPGPKEY DNS Resource
Record. Security is provided via Secure DNS, however the OPENPGPKEY Record. Security is provided via Secure DNS, however the OPENPGPKEY
record is not a replacement for verification of authenticity via the record is not a replacement for verification of authenticity via the
"Web of Trust" or manual verification. The OPENPGPKEY record can be "Web Of Trust" or manual verification. The OPENPGPKEY record can be
used to encrypt an email that would otherwise have to be send used to encrypt an email that would otherwise have to be sent
unencrypted. unencrypted.
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
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on July 30, 2016. This Internet-Draft will expire on September 09, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Experiment goal . . . . . . . . . . . . . . . . . . . . . 3 1.1. Experiment goal . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4
2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4 2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 5
2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 4 2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 5
2.1.2. Reducing the Transferable Public Key size . . . . . . 5 2.1.2. Reducing the Transferable Public Key size . . . . . . 6
2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 6 2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 6
2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 6 2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 7
3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 6 3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 7
4. Email address variants . . . . . . . . . . . . . . . . . . . 7 4. Email address variants and internationalization
5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 7 considerations . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Obtaining an OpenPGP key for a specific email address . . 7 5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 8
5.2. Confirming that an OpenPGP key is current . . . . . . . . 8 5.1. Obtaining an OpenPGP key for a specific email address . . 9
5.3. Public Key UIDs and query names . . . . . . . . . . . . . 8 5.2. Confirming that an OpenPGP key is current . . . . . . . . 9
6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 9 5.3. Public Key UIDs and query names . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 10
7.1. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 10 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7.2. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 10 7.1. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 11
7.3. Email client behaviour . . . . . . . . . . . . . . . . . 11 7.2. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 11
7.4. Response size . . . . . . . . . . . . . . . . . . . . . . 11 7.3. Email client behaviour . . . . . . . . . . . . . . . . . 12
7.5. Email address information leak . . . . . . . . . . . . . 11 7.4. Response size . . . . . . . . . . . . . . . . . . . . . . 12
7.6. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 12 7.5. Email address information leak . . . . . . . . . . . . . 12
7.7. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 12 7.6. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 13
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 12 7.7. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 13
8.1. The GNU Privacy Guard (GNUpg) . . . . . . . . . . . . . . 13 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 13
8.2. hash-slinger . . . . . . . . . . . . . . . . . . . . . . 13 8.1. The GNU Privacy Guard (GNUpg) . . . . . . . . . . . . . . 14
8.3. openpgpkey-milter . . . . . . . . . . . . . . . . . . . . 14 8.2. hash-slinger . . . . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 8.3. openpgpkey-milter . . . . . . . . . . . . . . . . . . . . 15
9.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 14 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 9.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . 15 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . 16 11.1. Normative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 17 11.2. Informative References . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18 Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
OpenPGP [RFC4880] public keys are used to encrypt or sign email OpenPGP [RFC4880] public keys are used to encrypt or sign email
messages and files. To encrypt an email message, or verify a messages and files. To encrypt an email message, or verify a
sender's OpenPGP signature, the email client or MTA needs to locate sender's OpenPGP signature, the email client or MTA needs to locate
the recipient's OpenPGP public key. the recipient's OpenPGP public key.
OpenPGP clients have relied on centralized "well-known" key servers OpenPGP clients have relied on centralized "well-known" key servers
that are accessed using either the HTTP Keyserver Protocol [HKP] that are accessed using the HTTP Keyserver Protocol [HKP].
Alternatively, users need to manually browse a variety of different Alternatively, users need to manually browse a variety of different
front-end websites. These key servers do not require a confirmation front-end websites. These key servers do not require a confirmation
of the email address used in the User ID of the uploaded OpenPGP of the email address used in the User ID of the uploaded OpenPGP
public key. Attackers can - and have - uploaded rogue public keys public key. Attackers can - and have - uploaded rogue public keys
with other people's email addresses to these key servers. with other people's email addresses to these key servers.
Once uploaded, public keys cannot be deleted. People who did not Once uploaded, public keys cannot be deleted. People who did not
pre-sign a key revocation can never remove their OpenPGP public key pre-sign a key revocation can never remove their OpenPGP public key
from these key servers once they have lost access to their private from these key servers once they have lost access to their private
key. This results in receiving encrypted email that cannot be key. This results in receiving encrypted email that cannot be
decrypted. decrypted.
Therefor, these keyservers are not well suited to support email Therefore, these keyservers are not well suited to support email
clients and MTA's to automatically encrypt email - especially in the clients and MTA's to automatically encrypt email - especially in the
absence of an interactive user. absence of an interactive user.
This document describes a mechanism to associate a user's OpenPGP This document describes a mechanism to associate a user's OpenPGP
public key with their email address, using the OPENPGPKEY DNS RRtype. public key with their email address, using the OPENPGPKEY DNS RRtype.
These records are published in the DNS zone of the user's email These records are published in the DNS zone of the user's email
address. If the user loses their private key, the OPENPGPKEY DNS address. If the user loses their private key, the OPENPGPKEY DNS
record can simply be updated or removed from the zone. record can simply be updated or removed from the zone.
The OPENPGPKEY data is secured using Secure DNS [RFC4035] The OPENPGPKEY data is secured using Secure DNS [RFC4035]
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The main goal of the OPENPGPKEY resource record is to stop passive The main goal of the OPENPGPKEY resource record is to stop passive
attacks against plaintext emails. While it can also thwart some attacks against plaintext emails. While it can also thwart some
active attacks (such as people uploading rogue keys to keyservers in active attacks (such as people uploading rogue keys to keyservers in
the hopes that others will encrypt to these rogue keys), this the hopes that others will encrypt to these rogue keys), this
resource record is not a replacement for verifying OpenPGP public resource record is not a replacement for verifying OpenPGP public
keys via the web of trust signatures, or manually via a fingerprint keys via the web of trust signatures, or manually via a fingerprint
verification. verification.
1.1. Experiment goal 1.1. Experiment goal
This specification is one experiment in improving access to public
keys for end-to-end email security. There are a range of ways in
which this can reasonably be done, for OpenPGP or S/MIME, for example
using the DNS, or SMTP, or HTTP. Proposals for each of these have
been made with various levels of support in terms of implementation
and deployment. For each such experiment, specifications such as
this will enable experiments to be carried out that may succeed or
that may uncover technical or other impediments to large- or small-
scale deployments. The IETF encourages those implementing and
deploying such experiments to publicly document their experiences so
that future specifications in this space can benefit.
This document defines an RRtype whose use is Experimental. The goal This document defines an RRtype whose use is Experimental. The goal
of the experiment is to see whether encrypted email usage will of the experiment is to see whether encrypted email usage will
increase if an automated discovery method is available to MTA's and increase if an automated discovery method is available to MTA's and
MUA's to help the enduser with email encryption key management. MUA's to help the enduser with email encryption key management.
It is unclear if this RRtype will scale to some of the larger email It is unclear if this RRtype will scale to some of the larger email
service deployments. Concerns have been raised about the size of the service deployments. Concerns have been raised about the size of the
OPENPGPKEY record and the size of the resulting DNS zone files. This OPENPGPKEY record and the size of the resulting DNS zone files. This
experiment hopefully will give the working group some insight into experiment hopefully will give the working group some insight into
whether this is a problem or not. whether this is a problem or not.
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o [ other subkeys if relevant ... ] o [ other subkeys if relevant ... ]
2.1.2. Reducing the Transferable Public Key size 2.1.2. Reducing the Transferable Public Key size
When preparing a Transferable Public Key for a specific OPENPGPKEY When preparing a Transferable Public Key for a specific OPENPGPKEY
RDATA format with the goal of minimizing certificate size, a user RDATA format with the goal of minimizing certificate size, a user
would typically want to: would typically want to:
o Where one User ID from the certifications matches the looked-up o Where one User ID from the certifications matches the looked-up
address, strip away non-matching User IDs and any associated address, strip away non-matching User IDs and any associated
certifications (self-signatures or third-party certifications) certifications (self-signatures or third-party certifications).
o Strip away all User Attribute packets and associated o Strip away all User Attribute packets and associated
certifications. certifications.
o Strip away all expired subkeys. The user may want to keep revoked o Strip away all expired subkeys. The user may want to keep revoked
subkeys if these were revoked prior to their preferred expiration subkeys if these were revoked prior to their preferred expiration
time to ensure that correspondents know about these earlier then time to ensure that correspondents know about these earlier than
expected revocations. expected revocations.
o Strip away all but the most recent self-sig for the remaining user o Strip away all but the most recent self-signature for the
IDs and subkeys remaining user IDs and subkeys.
o Optionally strip away any uninteresting or unimportant third-party o Optionally strip away any uninteresting or unimportant third-party
User ID certifications. This is a value judgment by the user that User ID certifications. This is a value judgment by the user that
is difficult to automate. At the very least, expired and is difficult to automate. At the very least, expired and
superseded third-party certifcations should be stripped out. The superseded third-party certifcations should be stripped out. The
user should attempt to keep the most recent and most well user should attempt to keep the most recent and most well
connected certifications in the Web Of Trust in their Transferable connected certifications in the Web Of Trust in their Transferable
Public Key. Public Key.
2.2. The OPENPGPKEY RDATA wire format 2.2. The OPENPGPKEY RDATA wire format
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For example, to request an OPENPGPKEY resource record for a user For example, to request an OPENPGPKEY resource record for a user
whose email address is "hugh@example.com", an OPENPGPKEY query would whose email address is "hugh@example.com", an OPENPGPKEY query would
be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35 be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35
eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding
RR in the example.com zone might look like (key shortened for RR in the example.com zone might look like (key shortened for
formatting): formatting):
c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key> c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key>
4. Email address variants 4. Email address variants and internationalization considerations
Mail systems usually handle variant forms of local-parts. The most Mail systems usually handle variant forms of local-parts. The most
common variants are upper and lower case, often automatically common variants are upper and lower case, often automatically
corrected when a name is recognized as such. Other variants include corrected when a name is recognized as such. Other variants include
systems that ignore "noise" characters such as dots, so that local systems that ignore "noise" characters such as dots, so that local
parts johnsmith and John.Smith would be equivalent. Many systems parts johnsmith and John.Smith would be equivalent. Many systems
allow "extensions" such as john-ext or mary+ext where john or mary is allow "extensions" such as john-ext or mary+ext where john or mary is
treated as the effective local-part, and the ext is passed to the treated as the effective local-part, and the ext is passed to the
recipient for further handling. This can complicate finding the recipient for further handling. This can complicate finding the
OPENPGPKEY record associated with the dynamically created email OPENPGPKEY record associated with the dynamically created email
address. address.
[RFC5321] and its predecessors have always made it clear that only [RFC5321] and its predecessors have always made it clear that only
the recipient MTA is allowed to interpret the local-part of an the recipient MTA is allowed to interpret the local-part of an
address. A client supporting OPENPGPKEY therefor MUST NOT perform address. A client supporting OPENPGPKEY therefore MUST NOT perform
any kind of mapping rules based on the email address. any kind of mapping rules based on the email address.
Section 3 above defines how the local-part is used to determine the
location in which one looks for an OPENPGPKEY record. Given the
variety of local-parts seen in email, designing a good experiment for
this is difficult as: a) some current implementations are known to
lowercase at least US-ASCII local-parts, b) we know from (many) other
situations that any strategy based on guessing and making multiple
DNS queries is not going to achieve consensus for good reasons, and
c) the underlying issues are just hard - see Section 10.1 of
[RFC6530] for discussion of just some of the issues that would need
to be tackled to fully address this problem.
However, while this specification is not the place to try to address
these issues with local-parts, doing so is also not required to
determine the outcome of this experiment. If this experiment
succeeds then further work on email addresses with non-ASCII local-
parts will be needed and that would be better based on the findings
from this experiment, rather than doing nothing or starting this
experiment based on a speculative approach to what is a very complex
topic.
5. Application use of OPENPGPKEY 5. Application use of OPENPGPKEY
The OPENPGPKEY record allows an application or service to obtain an The OPENPGPKEY record allows an application or service to obtain an
OpenPGP public key and use it for verifying a digital signature or OpenPGP public key and use it for verifying a digital signature or
encrypting a message to the public key. The DNS answer MUST pass encrypting a message to the public key. The DNS answer MUST pass
DNSSEC validation; if DNSSEC validation reaches any state other than DNSSEC validation; if DNSSEC validation reaches any state other than
"Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be "Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be
treated as a failure. treated as a failure.
5.1. Obtaining an OpenPGP key for a specific email address 5.1. Obtaining an OpenPGP key for a specific email address
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5.2. Confirming that an OpenPGP key is current 5.2. Confirming that an OpenPGP key is current
Locally stored OpenPGP public keys are not automatically refreshed. Locally stored OpenPGP public keys are not automatically refreshed.
If the owner of that key creates a new OpenPGP public key, that owner If the owner of that key creates a new OpenPGP public key, that owner
is unable to securely notify all users and applications that have its is unable to securely notify all users and applications that have its
old OpenPGP public key. Applications and users can perform an old OpenPGP public key. Applications and users can perform an
OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is
still the correct key to use. If the locally stored OpenPGP public still the correct key to use. If the locally stored OpenPGP public
key is different from the DNSSEC validated OpenPGP public key key is different from the DNSSEC validated OpenPGP public key
currently published in DNS, the verification MUST be treated as a currently published in DNS, the confirmation MUST be treated as a
failure unless the locally stored OpenPGP key signed the newly failure unless the locally stored OpenPGP key signed the newly
published OpenPGP public key found in DNS. An application that can published OpenPGP public key found in DNS. An application that can
interact with the user MAY ask the user for guidance. For privacy interact with the user MAY ask the user for guidance, otherwise the
reasons, an application MUST NOT attempt to lookup an OpenPGP key application will have to apply local policy. For privacy reasons, an
from DNSSEC at every use of that key. application MUST NOT attempt to lookup an OpenPGP key from DNSSEC at
every use of that key.
5.3. Public Key UIDs and query names 5.3. Public Key UIDs and query names
An OpenPGP public key can be associated with multiple email addresses An OpenPGP public key can be associated with multiple email addresses
by specifying multiple key uids. The OpenPGP public key obtained by specifying multiple key uids. The OpenPGP public key obtained
from a OPENPGPKEY RR can be used as long as the query and resulting from a OPENPGPKEY RR can be used as long as the query and resulting
data form a proper email to uid identity association. data form a proper email to uid identity association.
CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed
to obtain an OPENPGPKEY RR, as long as the original recipient's email to obtain an OPENPGPKEY RR, as long as the original recipient's email
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Anyone who can obtain a DNSSEC private key of a domain name via Anyone who can obtain a DNSSEC private key of a domain name via
coercion, theft or brute force calculations, can replace any coercion, theft or brute force calculations, can replace any
OPENPGPKEY record in that zone and all of the delegated child zones. OPENPGPKEY record in that zone and all of the delegated child zones.
Any future messages encrypted with the malicious OpenPGP key could Any future messages encrypted with the malicious OpenPGP key could
then be read. then be read.
Therefore, an OpenPGP key obtained via a DNSSEC validated OPENPGPKEY Therefore, an OpenPGP key obtained via a DNSSEC validated OPENPGPKEY
record can only be trusted as much as the DNS domain can be trusted, record can only be trusted as much as the DNS domain can be trusted,
and is no substitute for in-person OpenPGP key verification or and is no substitute for in-person OpenPGP key verification or
additional Openpgp verification via "Web of Trust" signatures present additional Openpgp verification via "Web Of Trust" signatures present
on the OpenPGP in question. on the OpenPGP in question.
8. Implementation Status 8. Implementation Status
[RFC Editor Note: Please remove this entire seciton prior to [RFC Editor Note: Please remove this entire seciton prior to
publication as an RFC.] publication as an RFC.]
This section records the status of known implementations of the This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982]. Internet-Draft, and is based on a proposal described in [RFC6982].
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