Network Working Group P. Wouters
Internet-Draft Red Hat
Intended status: Standards Track March 9, 2015
Expires: September 10, 2015

Using DANE to Associate OpenPGP public keys with email addresses


OpenPGP is a message format for email (and file) encryption, that lacks a standardized lookup mechanism to obtain OpenPGP public keys. This document specifies a method for securely publishing and locating OpenPGP public keys in DNS using a new OPENPGPKEY DNS Resource Record.

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Table of Contents

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, the sender's email client or MTA needs to know where to find the recipient's public key. Once obtained, it needs to find some proof that the public key found actually belongs to the intended recipient.

Obtaining a public key is not a straightforward process as there are no trusted standardized locations for publishing OpenPGP public keys indexed by email address. Instead, OpenPGP clients rely on "well-known key servers" that are accessed using the HTTP Keyserver Protocol ("HKP") 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 lose their private key.

The lack of a secure means to look up a public key for an email address also prevents email clients and MUAs from encrypting a received email to the target recipient, forcing the software to send the message unencrypted. Currently deployed MTAs 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 Trust Signature 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 61. 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 consists of a single OpenPGP public key as defined in Section of [RFC4880]. Note that this format is without ASCII armor or base64 encoding.

2.3. The OPENPGPKEY RDATA presentation format

The RDATA Presentation Format, as visible in textual zone files, consists of a single OpenPGP public key as defined in Section of [RFC4880] encoded in Base64 [RFC4648]

3. Location of the OPENPGPKEY record

The DNS does not allow the use of all characters that are supported in the "local-part" of email addresses as defined in [RFC2822] and [RFC6530]. Therefore, email addresses are mapped into DNS using the following method:

8d[..]  IN OPENPGPKEY <base64 public key>

For example, to request an OPENPGPKEY resource record for a user whose email address is "", an OPENPGPKEY query would be placed for the following QNAME: "". The corresponding RR in the zone might look like (key shortened for formatting):

3.1. Email address variants

Some email service providers and email software perform automatic mappings of email addresses based on special characters. This can complicate finding the OPENPGPKEY record associated with the dynamically created email address. Some well known examples are listed below

Software implementing DNS lookup for the OPENPGPKEY RRtype MAY perform similar translations rules while trying to find the OPENPGPKEY record.

4. OpenPGP Key size and DNS

Due to the expected size of the OPENPGPKEY record, it is recommended to perform DNS queries for the OPENPGPKEY record using TCP, not UDP.

Although the reliability of the transport of large DNS Resource 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 accommodate 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. See Appendix A.

5. Security Considerations

OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE].

5.1. Response size

To prevent amplification attacks, an Authoritative DNS server MAY wish to prevent returning OPENPGPKEY records over UDP unless the source IP address has been verified with [DNS-COOKIES]. Such servers MUST NOT return REFUSED, but answer the query with an empty Answer Section and the truncation flag set ("TC=1).

5.2. Email address information leak

Email addresses are not secret. Using them causes their 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 across 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.

5.3. 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.

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.

Therefore, an OpenPGP key obtained via an OPENPGPKEY record can only be trusted as much as the DNS domain can be trusted, and is 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


This document uses a new DNS RR type, OPENPGPKEY, whose value 61 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. Edwin Taylor contributed language improvements for various iterations of this document.

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.
[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

[DNS-COOKIES] Eastlake, Donald., "Domain Name System (DNS) Cookies", Internet-Draft draft-ietf-dnsop-cookies, February 2015.
[OPENPGPKEY-USAGE] Wouters, P., "Usage considerations with the DNS OPENPGPKEY record", Internet-Draft draft-dane-openpgpkey-usage, October 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

gpg --export --export-options export-minimal \ | base64

The commonly available GnuPG software can be used to generate the RRdata portion of an OPENPGPKEY record:

gpg --export --export-options export-minimal \ | wc -c

gpg --export --export-options export-minimal \ | hexdump -e \
       '"\t" /1 "%.2x"' -e '/32 "\n"'

<SHA2-224(hugh)> IN TYPE61 \# \
    <numOctets> <keydata in hex>

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:

~> openpgpkey --output rfc

~> openpgpkey --output generic
8d[...] IN TYPE61 \# 2313 99008d03[...]

The openpgpkey command in the hash-slinger software can be used to generate complete OPENPGPKEY records

Author's Address

Paul Wouters Red Hat EMail: