< draft-wouters-dane-openpgp-00.txt   draft-wouters-dane-openpgp-01.txt >
Network Working Group P. Wouters Network Working Group P. Wouters
Internet-Draft Red Hat Internet-Draft Red Hat
Intended status: Standards Track July 15, 2013 Intended status: Standards Track October 21, 2013
Expires: January 16, 2014 Expires: April 24, 2014
Using DANE to Associate OpenPGP public keys with email addresses Using DANE to Associate OpenPGP public keys with email addresses
draft-wouters-dane-openpgp-00 draft-wouters-dane-openpgp-01
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 standarized secure lookup mechanism to obtain OpenPGP public lacks a standarized lookup mechanism to obtain OpenPGP public keys.
keys. This document specifies a standarized method for securely This document specifies a standarized method for securely publishing
publishing and locating OpenPGP public keys in DNS using a new and locating OpenPGP public keys in DNS using a new OPENPGPKEY DNS
OPENPGPKEY DNS Resource Record. Resource Record.
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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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 January 16, 2014. This Internet-Draft will expire on April 24, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . . 4 2. The OPENPGPKEY record presence . . . . . . . . . . . . . . . 3
2.1. Location of the OpenPGPKEY record . . . . . . . . . . . . 4 3. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4
2.2. The OPENPGPKEY RDATA Format . . . . . . . . . . . . . . . 5 3.1. Location of the OpenPGPKEY record . . . . . . . . . . . . 4
3. OpenPGP public key considerations . . . . . . . . . . . . . . 5 3.2. The OPENPGPKEY RDATA Format . . . . . . . . . . . . . . . 5
3.1. Public Key UIDs and email addresses . . . . . . . . . . . 5 4. OpenPGP public key considerations . . . . . . . . . . . . . . 5
3.2. Public Key UIDs and IDNA . . . . . . . . . . . . . . . . . 5 4.1. Public Key UIDs and email addresses . . . . . . . . . . . 5
3.3. Public Key UIDs and synthesized DNS records . . . . . . . 5 4.2. Public Key UIDs and IDNA . . . . . . . . . . . . . . . . 5
3.4. Public Key size and DNS record size . . . . . . . . . . . 6 4.3. Public Key UIDs and synthesized DNS records . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4.4. Public Key size and DNS record size . . . . . . . . . . . 6
4.1. Email address information leak . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4.2. OpenPGP security and DNSSEC . . . . . . . . . . . . . . . 7 5.1. Email address information leak . . . . . . . . . . . . . 7
4.3. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 7 5.2. OpenPGP security and DNSSEC . . . . . . . . . . . . . . . 7
4.4. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 8 5.3. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 7
4.5. Email client behaviour . . . . . . . . . . . . . . . . . . 8 5.4. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 8
4.6. Subject: line encryption . . . . . . . . . . . . . . . . . 9 5.5. Email client behaviour . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 9 6.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 7. Generating OPENPGPKEY RRdata . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 10 9.1. Normative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 9.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction 1. Introduction
To encrypt a message to a target recipient using OpenPGP [RFC4880], To encrypt a message to a target recipient using OpenPGP [RFC4880],
possession of the recipient's OpenPGP public key is required. To possession of the recipient's OpenPGP public key is required. To
obtain that public key, two problems need to be solved by the obtain that public key, two problems need to be solved by the
sender's email client, MUA or MTA. Where does one find the sender's email client, MUA or MTA. Where does one find the
recipient's public key and how does one trust that the found key recipient's public key and how does one trust that the found key
actually belongs to the intended recipient. actually belongs to the intended recipient.
Obtaining a public key is not a straightforward process as there are Obtaining a public key is not a straightforward process as there are
no standarized locations for publishing OpenPGP public keys indexed no trusted standarized locations for publishing OpenPGP public keys
by email address. Instead, OpenPGP clients rely on "well known key indexed by email address. Instead, OpenPGP clients rely on "well-
servers" that are accessed using the web based HKP protocol or known key servers" that are accessed using the web based HKP protocol
manually by users using a variety of different front-end web pages. or manually by users using a variety of differently formatted front-
end web pages.
Currently deployed key servers have no method of validating any Currently deployed key servers have no method of validating any
uploaded OpenPGP public key. The key servers simply store and uploaded OpenPGP public key. The key servers simply store and
publish. Anyone can add public keys with any name or email address publish. Anyone can add public keys with any identities and anyone
and anyone can add signatures to any other public key using forged can add signatures to any other public key using forged malicious
malicious identities. For example, bogus keys of prominent identities. Furthermore, once uploaded, public keys cannot be
dissidents have been uploaded to these well-known key servers in deleted. People who did not pre-sign a key revocation can never
attempts to capture encrypted email. Furthermore, once uploaded,
public keys cannot be deleted. People who did not pre-sign a key
revocation and who have lost access to their private key can never
remove their public key from these key servers. remove their public key from these key servers.
The lack of association of email address and public key lookup is The lack of association of email address and public key lookup is
also preventing email clients, MTAs and MUAs from encrypting a also preventing email clients, MTAs and MUAs from encrypting a
received message to the target receipient forcing the software to received message to the target receipient forcing the software to
send the message unencryped. Currently deployed MTA's only support send the message unencryped. Currently deployed MTA's only support
encrypting the transport of the email, not the email contents itself. encrypting the transport of the email, not the email contents itself.
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 a new DNS RRtype. This is public key with their email address, using a new DNS RRtype. This is
skipping to change at page 4, line 10 skipping to change at page 3, line 36
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
This document also makes use of standard DNSSEC and DANE terminology. This document also makes use of standard DNSSEC and DANE terminology.
See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for
these terms. these terms.
2. The OPENPGPKEY Resource Record 2. The OPENPGPKEY record presence
A user who publishes an OPENPGPKEY record in DNS explicitly favours
receiving encrypted email instead of unencrypted email.
A user who publishes an OPENPGPKEY record in DNS still expects
senders to perform their due diligence by additional verification of
their public key via other out-of-band methods before sending any
confidential or sensitive information
In other words, the OPENPGPKEY record in DNS, without any additional
verification, should be used only as an alternative to sending
plaintext email. It SHOULD NOT be used to change one's opinion on
whether it is safe or appropriate to sent the content via email in
the first place.
3. The OPENPGPKEY Resource Record
The OPENPGPKEY DNS resource record (RR) is used to associate an end The OPENPGPKEY DNS resource record (RR) is used to associate an end
entity OpenPGP public key with an email address, thus forming a entity OpenPGP public key with an email address, thus forming a
"OpenPGP public key association". "OpenPGP public key association".
The type value allocated for the OPENPGPKEY RR type is [TBD]. The The type value allocated for the OPENPGPKEY RR type is [TBD]. The
OPENPGPKEY RR is class independent. The OPENPGPKEY RR has no special OPENPGPKEY RR is class independent. The OPENPGPKEY RR has no special
TTL requirements. TTL requirements. If an an OPENPGPKEY RR contains an expired OpenPGP
public key, it MUST NOT be used for encryption.
2.1. Location of the OpenPGPKEY record 3.1. Location of the OpenPGPKEY record
Domain names are prepared for requests in the following manner. Email addresses are mapped into DNS using the following method:
1. The user name (the "left-hand side" of the email address, called 1. The user name (the "left-hand side" of the email address, called
the "local-part" in the mail message format definition [RFC2822] the "local-part" in the mail message format definition [RFC2822]
and the "local part" in the specification for internationalized and the "local part" in the specification for internationalized
email [RFC6530]), is encoded with Base32 [RFC4648], to become the email [RFC6530]), is encoded with Base32 [RFC4648], to become the
left-most label in the prepared domain name. This does not left-most label in the prepared domain name. This does not
include the "@" character that separates the left and right sides include the at symbol ("@") that separates the left and right
of the email address. sides of the email address but does include any trailing equal
signs ("=") of base64 padding.
2. The string "_openpgpkey" becomes the second left-most label in 2. The string "_openpgpkey" becomes the second left-most label in
the prepared domain name. the prepared domain name.
3. The domain name (the "right-hand side" of the email address, 3. The domain name (the "right-hand side" of the email address,
called the "domain" in RFC 2822) is appended to the result of called the "domain" in RFC 2822) is appended to the result of
step 2 to complete the prepared domain name. step 2 to complete the prepared domain name.
For example, to request an OPENPGPKEY resource record for a user For example, to request an OPENPGPKEY resource record for a user
whose address is "hugh@example.com", you would use whose address is "hugh@example.com", you would use
"d1qmeq0._openpgpkey.example.com" in the request. The corresponding "nb2wo2a=._openpgpkey.example.com" in the request. The corresponding
RR in the example.com zone might look like: RR in the example.com zone might look like:
d1qmeq0._openpgpkey.example.com. IN OPENPGPKEY <encoded public key> nb2wo2a=._openpgpkey.example.com. IN OPENPGPKEY <encoded public key>
Design note: Encoding the user name with Base32 allows local parts Design note: Encoding the user name with Base32 allows local parts
that have characters that would prevent their use in domain names. that have characters that would prevent their use in domain names.
For example, a period (".") is a valid character in a local part, but For example, a period (".") is a valid character in a local part, but
would wreak havoc in a domain name. Similarly, RFC 6530 allows non- would wreak havoc in a domain name. Similarly, RFC 6530 allows non-
ASCII characters in local parts, and encoding a local part with non- ASCII characters in local parts, and encoding a local part with non-
ASCII characters with Base32 renders the name usable in the DNS. ASCII characters with Base32 renders the name usable in the DNS. The
equal sign ("=") is a valid character for a DNS label, even though it
is not a valid character for a DNS hostname.
2.2. The OPENPGPKEY RDATA Format 3.2. The OPENPGPKEY RDATA Format
The RDATA (or RHS) of an OPENPGPKEY Resource Record contains a single The RDATA (or RHS) of an OPENPGPKEY Resource Record contains a single
value consisting of a [RFC4880] formatted OpenPGP public keyring value consisting of a [RFC4880] formatted OpenPGP public keyring
encoded in base32 as specified in [RFC4648]. encoded in base64 as specified in [RFC4648]. This is not equivalent
to an "ascii armor export" which adds a header, a footer, and
sometimes additional items, to the exported data.
3. OpenPGP public key considerations 4. OpenPGP public key considerations
Once an OPENPGPKEY resource record has been found and the OpenPGP Once an OPENPGPKEY resource record has been found and the OpenPGP
public keyring has been base32 decoded, the right public key must be public keyring has been decoded, the right public key must be located
located inside the keyring. For a public key in the keyring to be inside the keyring. For a public key in the keyring to be usable,
usable, the public key has to have a key uid as specified in the public key has to have a key uid as specified in [RFC4648] that
[RFC4648] that matches the email address for which the OPENPGPKEY RR matches the email address for which the OPENPGPKEY RR lookup was
lookup was performed. performed.
3.1. Public Key UIDs and email addresses 4.1. Public Key UIDs and email addresses
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 target recipient's from a OPENPGPKEY RR can be used as long as the target recipient's
email address appears as one of the OpenPGP public key uids. The email address appears as one of the OpenPGP public key uids. The
name part (left of the @) should appear in the native format, not its name part (left of the @) should appear in the native format, not its
base32 encoding that was used to lookup the OPENPGPKEY RR. base32 encoding that was used to lookup the OPENPGPKEY RR.
3.2. Public Key UIDs and IDNA 4.2. Public Key UIDs and IDNA
Internationalized domains that use non-ascii characters (U-label) are Internationalized domains that use non-ascii characters (U-label) are
encoded in DNS using IDNA [RFC5891] - also referred to as punycode or encoded in DNS using IDNA [RFC5891] - also referred to as punycode or
A-label. When matching OpenPGP public key uids, both the email A-label. When matching OpenPGP public key uids, both the email
address specified using U-label and A-label should be considered as address specified using U-label and A-label should be considered as
valid public key uids. valid public key uids.
3.3. Public Key UIDs and synthesized DNS records 4.3. Public Key UIDs and synthesized DNS records
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
address appears as one of the OpenPGP public key uids. For example, address appears as one of the OpenPGP public key uids. For example,
if the OPENPGPKEY RR query for hugh@example.com if the OPENPGPKEY RR query for hugh@example.com
(d1qmeq0._openpgpkey.example.com) yields a CNAME to (b2wo2a=._openpgpkey.example.com) yields a CNAME to
d1qmeq0._openpgpkey.example.net, and an OPENPGPKEY RR for b2wo2a=._openpgpkey.example.net, and an OPENPGPKEY RR for
d1qmeq0._openpgpkey.example.net exists, then this OpenPGP public key b2wo2a=._openpgpkey.example.net exists, then this OpenPGP public key
can be used, provided one of the key uids contains can be used, provided one of the key uids contains
"hugh@example.com". This public key cannot be used if it would only "hugh@example.com". This public key cannot be used if it would only
contain the key uid "hugh@example.net". contain the key uid "hugh@example.net".
If one of the OpenPGP key uids contains only a single wildcard as the If one of the OpenPGP key uids contains only a single wildcard as the
LHS of the email address, such as "*@example.com", the OpenPGP public LHS of the email address, such as "*@example.com", the OpenPGP public
key may be used for any email address within that domain. Wildcards key may be used for any email address within that domain. Wildcards
at other locations (eg hugh@*.com) or regular expressions in key uids at other locations (eg hugh@*.com) or regular expressions in key uids
are not allowed, and any OPENPGPKEY RR containing these should be are not allowed, and any OPENPGPKEY RR containing these should be
ignored. ignored.
3.4. Public Key size and DNS record size 4.4. Public Key size and DNS record size
Although the reliability of the transport of large DNS Resoruce Although the reliability of the transport of large DNS Resoruce
Records has improved in the last few years, it is still recommended Records has improved in the last few years, it is still recommended
to keep the DNS records as small as possible without sacrificing the to keep the DNS records as small as possible without sacrificing the
security properties of the public key. The algorithm type and key security properties of the public key. The algorithm type and key
size of the OpenPGP keypair should not be modified to accomodate this size of the OpenPGP keypair should not be modified to accomodate this
section. section.
[Should a statement be made on the number of signatures left on the [Should a statement be made on the number of signatures left on the
key? Should there be _any_ signatures other than the self-signed key? Should there be _any_ signatures other than the self-signed
one?] one?]
OpenPGP supports various attributes that do not contribute to the OpenPGP supports various attributes that do not contribute to the
security of a key, such as an embedded image file. It is recommended security of a key, such as an embedded image file. It is recommended
that these properties are not exported to OpenPGP public keyrings that these properties are not exported to OpenPGP public keyrings
that are used to create OPENPGPKEY Resource Records. that are used to create OPENPGPKEY Resource Records.
4. Security Considerations 5. Security Considerations
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 twart some attacks against plaintext emails. While it can also twart 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.
Various components could be responsible for encrypting an email Various components could be responsible for encrypting an email
message to a target recipient. It could be done by the sender's message to a target recipient. It could be done by the sender's
email client or software plugin, the sender's Mail User Agent (MUA) email client or software plugin, the sender's Mail User Agent (MUA)
or the sender's Mail Transfer Agent (MTA). Each of these have their or the sender's Mail Transfer Agent (MTA). Each of these have their
own characteristics. An email client can interact with the user to own characteristics. An email client can direct the human to make a
make a decision before continuing. The MUA can only accept or refuse decision before continuing. The MUA can either accept or refuse a
a message. The MTA must deliver the message, either as-is, or message. The MTA must deliver the message as-is, or encrypt the
encrypted. Each of these programs should ensure that an unencrypted message before delivering. Each of these programs should ensure that
received email message will be encrypted whenever possible. the security of an email message is never downgraded, and that an
unencrypted received message will be encrypted whenever possible.
4.1. Email address information leak Organisations that require to be able to read everyone's encrypted
email should publish the escrow key as the OPENPGPKEY record. Upon
receipt, such mail servers can optionally re-encrypt the message to
the individual's OpenPGP key.
5.1. Email address information leak
DNS zones that are signed with DNSSEC using NSEC for denial of DNS zones that are signed with DNSSEC using NSEC for denial of
existence are susceptible to zone-walking, a mechanism that allow existence are susceptible to zone-walking, a mechanism that allow
someone to enumerate all the names in the zone. Someone who wanted someone to enumerate all the names in the zone. Someone who wanted
to collect email addresses from a zone that uses OPENPGPKEY might use to collect email addresses from a zone that uses OPENPGPKEY might use
such a mechanism. DNSSEC-signed zones using NSEC3 for denial of such a mechanism. DNSSEC-signed zones using NSEC3 for denial of
existence are significantly less susceptible to zone-walking. existence are significantly less susceptible to zone-walking.
Someone could still attempt a dictionary attack on the zone to find Someone could still attempt a dictionary attack on the zone to find
OPENPGPKEY records, just as they can use dictionary attacks on an OPENPGPKEY records, just as they can use dictionary attacks on an
SMTP server or grab the entire contents of existing PGP key servers SMTP server or grab the entire contents of existing PGP key servers
to see which addresses are valid. to see which addresses are valid.
4.2. OpenPGP security and DNSSEC 5.2. OpenPGP security and DNSSEC
DNSSEC key sizes are chosen based on the fact that these keys can be DNSSEC key sizes are chosen based on the fact that these keys can be
rolled with next to no requirement for security in the future. If rolled with next to no requirement for security in the future. If
one doubts the strength or security of the DNSSEC key for whatever one doubts the strength or security of the DNSSEC key for whatever
reason, one simply rolls to a new DNSSEC key with a stronger reason, one simply rolls to a new DNSSEC key with a stronger
algorithm or larger key size. algorithm or larger key size.
The same does not apply to OpenPGP encrypted messages. Users have an
expectation that their OpenPGP encrypted messages cannot be decrypted
for years or decades into the future. Changing to a new OpenPGP
keypair is also a costly and manual process that people tend to avoid
when possible.
This effectively means that anyone who can obtain a DNSSEC private This effectively means that anyone who can obtain a DNSSEC private
key of a domain name via coercion, theft or brute force calculations, key of a domain name via coercion, theft or brute force calculations,
can replace any OPENPGPKEY record in that zone and all of the can replace any OPENPGPKEY record in that zone and all of the
delegated child zones, irrespective of the key length strength of the delegated child zones, irrespective of the key length strength of the
OpenPGP keypair. OpenPGP keypair.
Therefore, DNSSEC is not an alternative for the "web of trust" or for Therefor, DNSSEC is not an alternative for the "web of trust" or for
manual fingerprint verification by humans. It is a solution aimed to manual fingerprint verification by humans. It is a solution aimed to
ease obtaining someone's public key, and without manual verification ease obtaining someone's public key, and without manual verification
should be treated as "better then plaintext" only. While this twarts should be treated as "better then plaintext" only. While this twarts
all passive attacks that simply capture and log all plaintext email all passive attacks that simply capture and log all plaintext email
content, it is not a security measure against active attacks. content, it is not a security measure against active attacks.
4.3. MTA behaviour 5.3. MTA behaviour
An MTA could be operating in a stand-alone mode, without access to An MTA could be operating in a stand-alone mode, without access to
the sender's OpenPGP public keyring, or in a way where it can access the sender's OpenPGP public keyring, or in a way where it can access
the user's OpenPGP public keyring. Regardless, the MTA SHOULD NOT the user's OpenPGP public keyring. Regardless, the MTA MUST NOT
modify the user's OpenPGP keyring. modify the user's OpenPGP keyring.
An MTA sending an email SHOULD NOT add the public key obtained from An MTA sending an email MUST NOT add the public key obtained from an
an OPENPGPKEY resource record to a permanent public keyring for OPENPGPKEY resource record to a permanent public keyring for future
future use beyond the TTL. use beyond the TTL.
If the obtained public key is revoked, the MTA MUST NOT use the key If the obtained public key is revoked, the MTA MUST NOT use the key
for encryption, even if that would result in sending the message in for encryption, even if that would result in sending the message in
plaintext. plaintext.
[What is the correct behaviour of an MTA when it receives an If a message is already encrypted, the MTA SHOULD NOT re-encrypt the
encrypted message from a MUA that is encrypted to a different key message, even if different encryption schemes or different encryption
then the one listed in the recipient's OPENPGPKEY record? Encrypt keys were used.
the encrypted message? Refuse to send out the message? Don't even
look up the OPENPGPKEY record and pass unmodified?]
If an OPENPGPKEY resource record is received without DNSSEC If an OPENPGPKEY resource record is received without DNSSEC
protection, it MUST NOT be used. If the DNS request returned an protection, it MAY still be used for encryption.
"indeterminate" or "bogus" answer, the MTA should queue the plaintext
message and try encryption and delivery again at a later time. If the DNS request for an OPENPGPKEY returned an "indeterminate" or
"bogus" answer, the MTA MUST NOT sent the message and queue the
plaintext message for delivery at a later time. If the problem
persists, the email should be returned via the regular bounce
methods.
If multiple non-revoked OPENPGPKEY resource records are found, the If multiple non-revoked OPENPGPKEY resource records are found, the
MTA should pick the most secure RR based on its local policy. MTA SHOULD pick the most secure RR based on its local policy. [or
should it encrypt to both?]
4.4. MUA behaviour 5.4. MUA behaviour
If the public key for a recipient obtained from the locally stored If the public key for a recipient obtained from the locally stored
public keyring differs from the recipient's OPENPGPKEY resource sender's public keyring differs from the recipient's OPENPGPKEY RR,
record, the MUA SHOULD NOT accept the message for delivery. the MUA MUST NOT accept the message for delivery.
If a MUA detects that a locally stored public key is present in an If the public key for a recipient obtained from the locally stored
OPENPGPKEY resource record, and the OPENPGPKEY RR version of the sender's public keyring contains contradicting properties for the
public key is revoked, the MUA SHOULD reject the message for same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept
delivery. the message for delivery.
If multiple non-revoked OPENPGPKEY resource records are found, the If multiple non-revoked OPENPGPKEY resource records are found, the
MUA should pick the most secure RR based on its local policy. MUA SHOULD pick the most secure OpenPGP public key based on its local
policy.
4.5. Email client behaviour
An email client MAY interact with a user to add the contents from an 5.5. Email client behaviour
OPENPGPKEY resource record into the user's permanent public keyring.
If the public key for a recipient obtained from the locally stored Email clients should adhere to the above listed MUA behaviour.
public keyring differs from the recipient's OPENPGPKEY resource Additionally, an email client MAY interact with the user to resolve
record, the email client SHOULD ask the user which key to use for any conflicts between locally stored keyrings and OPENPGPKEY RRdata.
encryption. The email cilent SHOULD allow encrypting to both public
keys.
An email client that is encrypting a message SHOULD clearly indicate An email client that is encrypting a message SHOULD clearly indicate
to the user the difference between encrypting to a locally stored and to the user the difference between encrypting to a locally stored and
manually verified public key and encrypting to an automatically humanly verified public key and encrypting to an unverified (by the
obtained public key via an OPENPGPKEY resource record that has not human sender) public key obtained via an OPENPGPKEY resource record.
been manually verified.
If a MUA detects that a locally stored and manually verified public 6. IANA Considerations
key is present in an OPENPGPKEY resource record, and the OPENPGPKEY
RR version of the public key is revoked, the MUA SHOULD warn the user
and give them the chance to not sent the message at all.
If multiple non-revoked OPENPGPKEY resource records are found, the 6.1. OPENPGPKEY RRtype
MUA should pick the most secure RR based on its local policy.
4.6. Subject: line encryption This document uses a new DNS RR type, OPENPGPKEY, whose value [TBD]
has been allocated by IANA from the Resource Record (RR) TYPEs
subregistry of the Domain Name System (DNS) Parameters registry.
Often, encrypting an email does not cause its Subject: line to be 7. Generating OPENPGPKEY RRdata
encrypted. If the email client, MUA or MTA automatically encrypt an
email based on the existence of an OPENPGPKEY record, it should clear
or replace the Subject: header with a notification that does not
expose the original subject line. It should prepend the original
Subject: line to the first line of the body of the email message
before encryption. This allows a receiving email client to decrypt
the message and replace the Subject: line to its original decrypted
form when presenting the user with the decrypted email message.
5. IANA Considerations The commonly available GnuPG software can be used to generate the
RRdata portion of an OPENPGPKEY record:
5.1. OPENPGPKEY RRtype gpg --export --export-options export-minimal \
your@email.com | base64
This document uses a new DNS RR type, OPENPGPKEY, whose value [TBD] The --armor or -a option should NOT be used. While it also provides
has been allocated by IANA from the Resource Record (RR) TYPEs a base64 encoded copy of the binary openpgk key data, it adds a
subregistry of the Domain Name System (DNS) Parameters registry. header and footer to the output.
6. Acknowledgements 8. Acknowledgements
This document is based on RFC-4255 and draft-ietf-dane-smime whose This document is based on RFC-4255 and draft-ietf-dane-smime whose
authors are Paul Hoffman, Jacob Schlyter and W. Griffin. authors are Paul Hoffman, Jacob Schlyter and W. Griffin.
7. References 9. References
7.1. Normative References 9.1. Normative References
[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, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
Rose, "DNS Security Introduction and Requirements", 4033, March 2005.
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions", Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005. RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005. Extensions", RFC 4035, March 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006. Encodings", RFC 4648, October 2006.
[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, November 2007. Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC5891] Klensin, J., "Internationalized Domain Names in [RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010. Applications (IDNA): Protocol", RFC 5891, August 2010.
7.2. Informative References 9.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, July 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, [RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
April 2001. 2001.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006. January 2006.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012. Internationalized Email", RFC 6530, February 2012.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, June 2012. DNS", RFC 6672, June 2012.
 End of changes. 57 change blocks. 
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