Using DANE to Associate OpenPGP public keys with email addresses
draft-ietf-dane-openpgpkey-01

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 April 30, 2015.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. The OPENPGPKEY Resource Record
• 2.1. The OPENPGPKEY RDATA component
• 2.2. The OPENPGPKEY RDATA wire format
• 2.3. The OPENPGPKEY RDATA presentation format
• 3. Location of the OpenPGPKEY record
• 4. OpenPGP Key size and DNS
• 5. Security Considerations
• 5.1. Email address information leak
• 5.2. Forward security of OpenPGP versus DNSSEC
• 6. IANA Considerations
• 6.1. OPENPGPKEY RRtype
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Generating OPENPGPKEY records
• Author's Address

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.

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 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 is the binary OpenPGP public keyring as specified in [RFC4880] without any ascii armor or base64 encoding.

2.3. The OPENPGPKEY RDATA presentation format

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.

8d[..]b7._openpgpkey.example.com.  IN OPENPGPKEY <base64 public key>

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: "8d5730bd8d76d417bf974c03f59eedb7af98cb5c3dc73ea8ebbd54b7._openpgpkey.example.com" The corresponding RR in the example.com zone might look like (key shortened for formatting):

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. 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.

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 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.

8. References

8.1. Normative References

8.2. Informative References

Appendix A. Generating OPENPGPKEY records

gpg --export --export-options export-minimal \
    hugh@example.com | base64

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 | wc -c

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

<SHA2-224(hugh)>._openpgpkey.example.com. IN TYPE65280 \# \
    <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 hugh@example.com
8d[...]b7._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...]

~> openpgpkey --output generic hugh@example.com
8d[...]b7._openpgpkey.example.com. IN TYPE65280 \# 2313 99008d03[...]

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

Author's Address