Internet DRAFT - draft-wouters-dane-openpgp
draft-wouters-dane-openpgp
Network Working Group P. Wouters
Internet-Draft Red Hat
Intended status: Standards Track February 13, 2014
Expires: August 17, 2014
Using DANE to Associate OpenPGP public keys with email addresses
draft-wouters-dane-openpgp-02
Abstract
OpenPGP is a message format for email (and file) encryption, that
lacks a standarized lookup mechanism to obtain OpenPGP public keys.
This document specifies a standarized method for securely publishing
and locating OpenPGP public keys in DNS using a new 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 August 17, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 3
2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 3
2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 3
2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 3
3. Location of the OpenPGPKEY record . . . . . . . . . . . . . . 4
4. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5.1. Email address information leak . . . . . . . . . . . . . 5
5.2. Forward security of OpenPGP versus DNSSEC . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
To encrypt a message to a target recipient using OpenPGP [RFC4880],
possession of the recipient's OpenPGP public key is required. To
obtain that public key, two problems need to be solved by 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
actually belongs to the intended recipient.
Obtaining a public key is not a straightforward process as there are
no trusted standarized locations for publishing OpenPGP public keys
indexed by email address. Instead, OpenPGP clients rely on "well-
known key servers" that are accessed using the web based HKP protocol
or manually by users using a variety of differently formatted front-
end web pages.
Currently deployed key servers have no method of validating any
uploaded OpenPGP public key. The key servers simply store and
publish. Anyone can add public keys with any identities and anyone
can add signatures to any other public key using forged malicious
identities. Furthermore, once uploaded, public keys cannot be
deleted. People who did not pre-sign a key revocation can never
remove their public key from these key servers once they lost their
private key.
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The lack of association of email address and public key lookup is
also preventing email clients, MTAs and MUAs from encrypting a
received message to the target receipient forcing the software to
send the message unencryped. Currently deployed MTA's only support
encrypting the transport of the email, not the email contents itself.
This document describes a mechanism to associate a user's OpenPGP
public key with their email address, using a new DNS RRtype.
The proposed new DNS Resource Record type is secured using DNSSEC.
This trust model is not meant to replace the "web of trust" model.
However, it can be used to encrypt a message that would otherwise
have to be sent out unencrypted, where it could be monitored by a
third party in transit or located in plaintext on a storage or email
server.
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 Resource Record
The OPENPGPKEY DNS resource record (RR) is used to associate an end
entity OpenPGP public key with an email address, thus forming a
"OpenPGP public key association".
The type value allocated for the OPENPGPKEY RR type is [TBD]. The
OPENPGPKEY RR is class independent. The OPENPGPKEY RR has no special
TTL requirements.
2.1. The OPENPGPKEY RDATA component
The RDATA (or RHS) of an OPENPGPKEY Resource Record contains a single
value consisting of a [RFC4880] formatted OpenPGP public keyring.
2.2. The OPENPGPKEY RDATA wire format
The RDATA Wire Format is the binary OpenPGP public keyring as
specified in [RFC4880] without any ascii armor or base64 encoding.
2.3. The OPENPGPKEY RDATA presentation format
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The RDATA Presentation Format, as visible in textual zone files,
consists of the [RFC4880] formatted OpenPGP public keyring encoded in
Base64 [RFC4648]
3. Location of the OpenPGPKEY record
Email addresses are mapped into DNS using the following method:
1. The user name (the "left-hand side" of the email address, called
the "local-part" in the mail message format definition [RFC2822]
and the "local part" in the specification for internationalized
email [RFC6530]), is hashed using the SHA2-224 [RFC5754]
algorithm to become the left-most label in the prepared domain
name. This does not include the at symbol ("@") that separates
the left and right sides of the email address.
2. The DNS does not allow the use of all characters that are
supported in "local-part" of email addresses as defined in
[RFC2822] and [RFC6530] . The SHA2-224 hashing of the user name
ensures that none of these characters would need to be placed
directly in the DNS.
3. The string "_openpgpkey" becomes the second left-most label in
the prepared domain name.
4. The domain name (the "right-hand side" of the email address,
called the "domain" in RFC 2822) is appended to the result of
step 2 to complete the prepared domain name.
For example, to request an OPENPGPKEY resource record for a user
whose email address is "hugh@example.com", an OPENPGPKEY query would
be placed for the following QNAME: "8d5730bd8d76d417bf974c03f59eedb7a
f98cb5c3dc73ea8ebbd54b7._openpgpkey.example.com" The corresponding RR
in the example.com zone might look like:
8d5730bd8d76d417bf974c03f59eedb7af98cb5c3dc73ea8ebbd54b7._openpgpkey.example.com. IN OPENPGPKEY <encoded public key>
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.
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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. See Appendix A
5. Security Considerations
OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE]
5.1. Email address information leak
Email addresses are not secret. Using them causes its publication.
The hashing of the user name in this document is not a security
feature. Publishing OPENPGPKEY records however, will create a list
of hashes of valid email addresses, which could simplify obtaining a
list of valid email addresses for a particular domain. It is
desirable to not ease the harvesting of email addresses where
possible.
The domain name part of the email address is not used as part of the
hash so that hashes can be used in multiple zones deployed using
DNAME [RFC6672]. This does makes it slightly easier and cheaper to
brute-force the SHA2-224 hashes into common and short user names, as
single rainbow tables can be re-used accross domains. This can be
somewhat countered by using NSEC3.
DNS zones that are signed with DNSSEC using NSEC for denial of
existence are susceptible to zone-walking, a mechanism that allows
someone to enumerate all the OPENPGPKEY hashes in a zone. This can
be used in combination with previously hashed common or short user
names (in rainbow tables) to deduce valid email addresses. DNSSEC-
signed zones using NSEC3 for denial of existence instead of NSEC are
significantly harder to brute-force after performing a zone-walk.
5.2. Forward security of OpenPGP versus 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. On the other hand, OpenPGP key sizes
are chosen based on how many years (or decades) their encryption
should remain unbreakable by adversaries that own large scale
computational resources.
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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 size of the OpenPGP
keypair. Any future messages encrypted with the malicious OpenPGP
key could then be read.
Therefor, an OpenPGP key obtained via an OPENPGPKEY record can only
be trusted as much as the DNS domain can be trusted, and are no
substitute for in-person key verification of the "Web of Trust". See
[OPENPGPKEY-USAGE] for more in-depth information on safe usage of
OPENPGPKEY based OpenPGP keys.
6. IANA Considerations
6.1. OPENPGPKEY RRtype
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.
7. Acknowledgements
This document is based on RFC-4255 and draft-ietf-dane-smime whose
authors are Paul Hoffman, Jacob Schlyter and W. Griffin. Olafur
Gudmundsson provided feedback and suggested various improvements.
Willem Toorop contributed the gpg and hexdump command options.
8. References
8.1. Normative References
[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.
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[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.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
8.2. Informative References
[OPENPGPKEY-USAGE]
Wouters, P., "Usage considerations with the DNS OPENPGPKEY
record", draft-wouters-openpgpkey-usage (work in
progress), January 2014.
[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.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
[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.
Appendix A. Generating OPENPGPKEY records
The commonly available GnuPG software can be used to generate the
RRdata portion of an OPENPGPKEY record:
gpg --export --export-options export-minimal \
hugh@example.com | base64
The --armor or -a option of the gpg command should NOT be used, as it
adds additional markers around the armored key.
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When DNS software reading or signing the zone file does not yet
support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597]
can be used to generate the RDATA. One needs to calculate the number
of octets and the actual data in hexadecimal:
gpg --export --export-options export-minimal \
hugh@example.com | wc -c
gpg --export --export-options export-minimal \
hugh@example.com | hexdump -e \
'"\t" /1 "%.2x"' -e '/32 "\n"'
These values can then be used to generate a generic record:
<SHA2-224(hugh)>._openpgpkey.example.com. IN TYPE65280 \# <numOctets> <keydata in hex>
The openpgpkey command in the hash-slinger software can be used to
generate complete OPENPGPKEY records
~> openpgpkey --output rfc hugh@example.com
8d5730bd8d[...]bbd54b7._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...]
~> openpgpkey --output generic hugh@example.com
8d5730bd8d[...]bbd54b7._openpgpkey.example.com. IN TYPE65280 \# 2313 99008d03[...]
Author's Address
Paul Wouters
Red Hat
Email: pwouters@redhat.com
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