Internet DRAFT - draft-ietf-dane-openpgpkey-usage
draft-ietf-dane-openpgpkey-usage
Network Working Group P. Wouters
Internet-Draft Red Hat
Intended status: Standards Track October 27, 2014
Expires: April 30, 2015
Best Common Practise for using OPENPGPKEY records
draft-ietf-dane-openpgpkey-usage-01
Abstract
The OPENPGPKEY DNS Resource Record can be used to match an email
address to an OpenPGP key. This document specifies a Best Common
Practise ("BCP") for email clients, MUA's and MTA's for using the
OPENPGPKEY DNS Resource Record.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 30, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. The OPENPGPKEY record presence . . . . . . . . . . . . . . . 2
3. OpenPGP public key considerations . . . . . . . . . . . . . . 3
3.1. Public Key UIDs and email addresses . . . . . . . . . . . 3
3.2. Public Key UIDs and IDNA . . . . . . . . . . . . . . . . 3
3.3. Public Key UIDs and synthesized DNS records . . . . . . . 3
3.4. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4.1. Email address information leak . . . . . . . . . . . . . 4
4.2. OpenPGP security and DNSSEC . . . . . . . . . . . . . . . 5
4.3. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 5
4.4. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Email client behaviour . . . . . . . . . . . . . . . . . 6
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document describes a Best Current Practise ("BCP") for using
OPENPGPKEY DNS Resource Records xref target="OPENPGPKEY"/ in email
clients, MUA's and MTA.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document also makes use of standard DNSSEC and DANE terminology.
See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for
these terms.
2. The OPENPGPKEY record presence
A user who publishes an OPENPGPKEY record in DNS explicitly prefers
receiving encrypted email over receiving 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
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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. OpenPGP public key considerations
Once an OPENPGPKEY resource record has been found and the OpenPGP
public keyring has been decoded, the right public key must be located
inside the keyring. For a public key in the keyring to be usable,
the public key has to have a key uid as specified in [RFC4648] that
matches the email address for which the OPENPGPKEY RR lookup was
performed.
3.1. Public Key UIDs and email addresses
An OpenPGP public key can be associated with multiple email addresses
by specifying multiple key uids. The OpenPGP public key obtained
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
name part (left of the @) should appear in the native format, not its
SHA2-224 hash that was used to lookup the OPENPGPKEY RR.
3.2. Public Key UIDs and IDNA
Internationalized domains that use non-ascii characters (U-label) are
encoded in DNS using IDNA [RFC5891] - also referred to as punycode or
A-label. When matching OpenPGP public key uids, both the email
address specified using U-label and A-label should be considered as
valid public key uids.
3.3. Public Key UIDs and synthesized DNS records
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
address appears as one of the OpenPGP public key uids. For example,
if the OPENPGPKEY RR query for hugh@example.com
(8d57[...]b7._openpgpkey.example.com) yields a CNAME to
8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for
8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public
key can be used, provided one of the key uids contains
"hugh@example.com". This public key cannot be used if it would only
contain the key uid "hugh@example.net".
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
key may be used for any email address within that domain. Wildcards
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at other locations (eg hugh@*.com) or regular expressions in key uids
are not allowed, and any OPENPGPKEY RR containing these should be
ignored.
3.4. OpenPGP Key size and DNS
Although the reliability of the transport of large DNS Resoruce
Records has improved in the last years, it is still recommended to
keep the DNS records as small as possible without sacrificing the
security properties of the public key. The algorithm type and key
size of OpenPGP keys should not be modified to accomodate this
section.
OpenPGP supports various attributes that do not contribute to the
security of a key, such as an embedded image file. It is recommended
that these properties are not exported to OpenPGP public keyrings
that are used to create OPENPGPKEY Resource Records. Some OpenPGP
software, for example GnuPG, have support for a "minimal key export"
that is well suited to use as OPENPGPKEY RDATA.
4. Security Considerations
The main goal of the OPENPGPKEY resource record is to stop passive
attacks against plaintext emails. While it can also twart some
active attacks (such as people uploading rogue keys to keyservers in
the hopes that others will encrypt to these rogue keys), this
resource record is not a replacement for verifying OpenPGP public
keys via the web of trust signatures, or manually via a fingerprint
verification.
Various components could be responsible for encrypting an email
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)
or the sender's Mail Transfer Agent (MTA). Each of these have their
own characteristics. An email client can direct the human to make a
decision before continuing. The MUA can either accept or refuse a
message. The MTA must deliver the message as-is, or encrypt the
message before delivering. Each of these programs should ensure that
the security of an email message is never downgraded, and that an
unencrypted received message will be encrypted whenever possible.
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.
4.1. Email address information leak
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DNS zones that are signed with DNSSEC using NSEC for denial of
existence are susceptible to zone-walking, a mechanism that allow
someone to enumerate all the names in the zone. Someone who wanted
to collect email addresses from a zone that uses OPENPGPKEY might use
such a mechanism. DNSSEC-signed zones using NSEC3 for denial of
existence are significantly less susceptible to zone-walking.
Someone could still attempt a dictionary attack on the zone to find
OPENPGPKEY records, just as they can use dictionary attacks on an
SMTP server or grab the entire contents of existing PGP key servers
to see which addresses are valid.
4.2. OpenPGP security and DNSSEC
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
one doubts the strength or security of the DNSSEC key for whatever
reason, one simply rolls to a new DNSSEC key with a stronger
algorithm or larger key size.
This effectively means that anyone who can obtain a DNSSEC private
key of a domain name via coercion, theft or brute force calculations,
can replace any OPENPGPKEY record in that zone and all of the
delegated child zones, irrespective of the key length strength of the
OpenPGP keypair.
Therefor, DNSSEC is not an alternative for the "web of trust" or for
manual fingerprint verification by humans. It is a solution aimed to
ease obtaining someone's public key, and without manual verification
should be treated as "better then plaintext" only. While this twarts
all passive attacks that simply capture and log all plaintext email
content, it is not a security measure against active attacks.
4.3. MTA behaviour
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 user's OpenPGP public keyring. Regardless, the MTA MUST NOT
modify the user's OpenPGP keyring.
An MTA sending an email MUST NOT add the public key obtained from an
OPENPGPKEY resource record to a permanent public keyring for future
use beyond the TTL.
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
plaintext.
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If a message is already encrypted, the MTA SHOULD NOT re-encrypt the
message, even if different encryption schemes or different encryption
keys were used.
If an OPENPGPKEY resource record is received without DNSSEC
protection, it MAY still be used for encryption.
If the DNS request for an OPENPGPKEY record 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
MTA SHOULD pick the most secure RR based on its local policy. [or
should it encrypt to both?]
4.4. MUA behaviour
If the public key for a recipient obtained from the locally stored
sender's public keyring differs from the recipient's OPENPGPKEY RR,
the MUA MUST NOT accept the message for delivery.
If the public key for a recipient obtained from the locally stored
sender's public keyring contains contradicting properties for the
same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept
the message for delivery.
If multiple non-revoked OPENPGPKEY resource records are found, the
MUA SHOULD pick the most secure OpenPGP public key based on its local
policy.
4.5. Email client behaviour
Email clients should adhere to the above listed MUA behaviour.
Additionally, an email client MAY interact with the user to resolve
any conflicts between locally stored keyrings and OPENPGPKEY RRdata.
An email client that is encrypting a message SHOULD clearly indicate
to the user the difference between encrypting to a locally stored and
humanly verified public key and encrypting to an unverified (by the
human sender) public key obtained via an OPENPGPKEY resource record.
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5. References
5.1. Normative References
[OPENPGPKEY]
Wouters, P., "DANE for OpenPGP public keys", draft-ietf-
dane-openpgp (work in progress), October 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
5.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely
Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
January 2006.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
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[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, June 2012.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
Author's Address
Paul Wouters
Red Hat
Email: pwouters@redhat.com
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