| Network Working Group | P. Wouters |
| Internet-Draft | Red Hat |
| Intended status: Experimental | August 27, 2015 |
| Expires: February 28, 2016 |
Using DANE to Associate OpenPGP public keys with email addresses
draft-ietf-dane-openpgpkey-04
OpenPGP is a message format for email (and file) encryption that lacks a standardized lookup mechanism to securely obtain OpenPGP public keys. This document specifies a method for publishing and locating OpenPGP public keys in DNS for a specific email address using a new OPENPGPKEY DNS Resource Record. Security is provided via DNSSEC.
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This Internet-Draft will expire on February 28, 2016.
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OpenPGP [RFC4880] public keys are used to encrypt or sign email messages and files. To encrypt an email message, or verify a sender's OpenPGP signature, the email client or MTA needs to locate the recipient's OpenPGP public key.
OpenPGP clients have relied on centralized "well-known" key servers that are accessed using either the HTTP Keyserver Protocol [HKP] Alternatively, users need to manually browse a variety of different front-end websites. These key servers do not validate the email address in the User ID of the uploaded OpenPGP public key. Attackers can - and have - uploaded rogue public keys with other people's email addresses to these key servers.
Once uploaded, public keys cannot be deleted. People who did not pre-sign a key revocation can never remove their OpenPGP public key from these key servers once they have lost access to their private key. This results in receiving encrypted email that cannot be decrypted.
Therefor, these keyservers are not well suited to support email clients and MTA's to automatically encrypt email - especially in the absence of an interactive user.
This document describes a mechanism to associate a user's OpenPGP public key with their email address, using the OPENPGPKEY DNS RRtype. These records are published in the DNS zone of the user's email address. If the user loses their private key, the OPENPGPKEY DNS record can simply be updated or removed from the zone.
The proposed new DNS Resource Record type is secured using DNSSEC. This trust model is not meant to replace the Web Of Trust model.
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.
The OPENPGPKEY DNS resource record (RR) is used to associate an end entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880] with an email address, thus forming a "OpenPGP public key association". A user that wishes to specify more than one OpenPGP key, for example because they are transitioning to a newer stronger key, can do so by adding multiple OPENPGPKEY records. A single OPENPGPKEY DNS record MUST only contain one OpenPGP key.
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.
The RDATA portion of an OPENPGPKEY Resource Record contains a single value consisting of a [RFC4880] formatted Transferable Public Key.
o The primary key X
o One User ID Y, which SHOULD match 'hugh@example.com'
o self-signature from X, binding X to Y
o The primary key X
o One User ID Y, which SHOULD match 'hugh@example.com'
o self-signature from X, binding X to Y
o encryption-capable subkey Z
o self-signature from X, binding Z to X
o [ other subkeys if relevant ... ]
o The primary key X
o One User ID Y, which SHOULD match 'hugh@example.com'
o self-signature from X, binding X to Y
o third-party certification from V, binding Y to X
o [ other third-party certifications if relevant ... ]
o encryption-capable subkey Z
o self-signature from X, binding Z to X
o [ other subkeys if relevant ... ]
An OpenPGP Transferable Public Key can be arbitrarily large. DNS records are limited in size. When creating OPENPGPKEY DNS records, the OpenPGP Transferable Public Key should be filtered to only contain appropriate and useful data. At a minimum, an OPENPGPKEY Transferable Public Key for the user hugh@example.com should contain:
When preparing a Transferable Public Key for a specific OPENPGPKEY RDATA format with the goal of minimizing certificate size, a user would typically want to:
The RDATA Wire Format consists of a single OpenPGP Transferable Public Key as defined in Section 11.1 of [RFC4880]. Note that this format is without ASCII armor or base64 encoding.
The RDATA Presentation Format, as visible in textual zone files, consists of a single OpenPGP Transferable Public Key as defined in Section 11,1 of [RFC4880] encoded in base64 as defined in Section 4 of [RFC4648].
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:
c9[..]d6._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: "c93f1e400f26708f98cb19d936620da35eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding RR in the example.com zone might look like (key shortened for formatting):
Mail systems usually handle variant forms of local-parts. The most common variants are upper and lower case, often automatically corrected when a name is recognized as such. Other variants include systems that ignore "noise" characters such as dots, so that local parts johnsmith and John.Smith would be equivalent. Many systems allow "extensions" such as john-ext or mary+ext where john or mary is treated as the effective local-part, and the ext is passed to the recipient for further handling. This can complicate finding the OPENPGPKEY record associated with the dynamically created email address.
[RFC5321] and its predecessors have always made it clear that only the recipient MTA is allowed to interpret the local-part of an address. A client supporting OPENPGPKEY therefor MUST NOT perform any kind of mapping rules based on the email address.
The OPENPGPKEY record allows an application or service to obtain or verify an OpenPGP public key. The lookup result MUST pass DNSSEC validation; if validation reaches any state other than "Secure", the verification MUST be treated as a failure.
If no OpenPGP public keys are known for an email address, an OPENPGPKEY lookup MAY be performed to discover the OpenPGP public key that belongs to a specific email address. This public key can then be used to verify a received signed message or can be used to send out an encrypted email message. An application that confirms the lack of an OPENPGPKEY record SHOULD remember this for some time to avoid sending out a DNS request for each email message that is sent out as this constitutes a privacy leak.
Locally stored OpenPGP public keys are not automatically refreshed. If the owner of that key creates a new OpenPGP public key, that owner is unable to securely notify all users and applications that have its old OpenPGP public key. Applications and users can perform an OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is still the correct key to use. If verifying a locally stored OpenPGP public key and the OpenPGP public key found through DNS is different from the locally stored OpenPGP public key, the verification MUST be treated as a failure. An application that can interact with the user MAY ask the user for guidance. For privacy reasons, an application MUST NOT attempt to validate a locally stored OpenPGP key using an OPENPGPKEY lookup at every use of that key.
Storage media can be signed using an OpenPGP public key. Even if the OpenPGP public key is included on the storage media, it needs to be independently validated. OpenPGP public keys contain one or more IDs than can have the syntax of an email address. An application can perform a lookup for an OPENPGPKEY at the expected location for the specific email address to confirm the validity of the OpenPGP public key. Once the key has been validated, all files on the storage media that have been signed by this key can now be verified.
Due to the expected size of the OPENPGPKEY record, applications SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY Resource Record.
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.
OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE].
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").
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 SHA2-256 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.
Users may have a local key store with OpenPGP public keys. An application supporting the use of OPENPGPKEY DNS records MUST NOT modify the local key store without explicit confirmation of the user, as the application is unaware of the user's personal policy for adding, removing or updating their local key store. An application MAY warn the user if an OPENPGPKEY record does not match the OpenPGP public key in the local key store.
Applications that do not have users associated with, such as daemon processes, SHOULD store OpenPGP public keys obtained via OPENPGPKEY up to their DNS TTL value. This avoids repeated DNS lookups that third parties could monitor to determine when an email is being sent to a particular user. If TLS is in use between MTA's, only the DNS lookup could happen unencrypted.
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.
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.
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. Daniel Kahn Gillmor provided the text describing the OpenPGP packet formats and filtering options. Edwin Taylor contributed language improvements for various iterations of this document. Text regarding email mappings was taken from draft-levine-dns-mailbox whose author is John Levine.
| [DNS-COOKIES] | Eastlake, Donald., "Domain Name System (DNS) Cookies", Internet-Draft draft-ietf-dnsop-cookies, August 2015. |
| [HKP] | Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)", Internet-Draft draft-shaw-openpgp-hkp, March 2013. |
| [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, DOI 10.17487/RFC2181, July 1997. |
| [RFC2822] | Resnick, P., "Internet Message Format", RFC 2822, DOI 10.17487/RFC2822, April 2001. |
| [RFC3597] | Gustafsson, A., "Handling of Unknown DNS Resource Record (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 2003. |
| [RFC5233] | Murchison, K., "Sieve Email Filtering: Subaddress Extension", RFC 5233, DOI 10.17487/RFC5233, January 2008. |
| [RFC5321] | Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008. |
| [RFC6530] | Klensin, J. and Y. Ko, "Overview and Framework for Internationalized Email", RFC 6530, DOI 10.17487/RFC6530, February 2012. |
| [RFC6672] | Rose, S. and W. Wijngaards, "DNAME Redirection in the DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012. |
| [RFC6698] | Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August 2012. |
| [RFC7129] | Gieben, R. and W. Mekking, "Authenticated Denial of Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, February 2014. |
gpg --export --export-options export-minimal,no-export-attributes \
hugh@example.com | base64
The commonly available GnuPG software can be used to generate a minimum Transferable Public Key for the RRdata portion of an OPENPGPKEY record:
gpg --export --export-options export-minimal,no-export-attributes \
hugh@example.com | wc -c
gpg --export --export-options export-minimal,no-export-attributes \
hugh@example.com | hexdump -e \
'"\t" /1 "%.2x"' -e '/32 "\n"'
<SHA2-256-trunc(hugh)>._openpgpkey.example.com. 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 hugh@example.com c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...] ~> openpgpkey --output generic hugh@example.com c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...]
The openpgpkey command in the hash-slinger software can be used to generate complete OPENPGPKEY records