Network Working Group                                         P. WoutersInternet-Draft                                                   Red HatIntended status: Standards Track                          April 01, 2015Expires: October 03, 2015    Using DANE to Associate OpenPGP public keys with email addresses                     draft-ietf-dane-openpgpkey-03Abstract   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,   locating and verifying OpenPGP public keys in DNS for a specific   email address using a new OPENPGPKEY DNS Resource Record.  Security   is provided via DNSSEC.Status of This Memo   This Internet-Draft is submitted in full conformance with the   provisions of BCP 78 and BCP 79.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at http://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on October 03, 2015.Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.Wouters                 Expires October 03, 2015                [Page 1]Internet-Draft        DANE for OpenPGP public keys            April 2015   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  . . . . . . . . . . . . . . . . . . . . . . . .   2     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4   2.  The OPENPGPKEY Resource Record  . . . . . . . . . . . . . . .   4     2.1.  The OPENPGPKEY RDATA component  . . . . . . . . . . . . .   4     2.2.  The OPENPGPKEY RDATA wire format  . . . . . . . . . . . .   4     2.3.  The OPENPGPKEY RDATA presentation format  . . . . . . . .   4   3.  Location of the OPENPGPKEY record . . . . . . . . . . . . . .   4     3.1.  Email address variants  . . . . . . . . . . . . . . . . .   5   4.  Application use of OPENPGPKEY . . . . . . . . . . . . . . . .   6     4.1.  Obtaining an OpenPGP key for a specific email address . .   6     4.2.  Confirming the validity of an OpenPGP key . . . . . . . .   6     4.3.  Verifying an unknown OpenPGP signature  . . . . . . . . .   6   5.  OpenPGP Key size and DNS  . . . . . . . . . . . . . . . . . .   6   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7     6.1.  Response size . . . . . . . . . . . . . . . . . . . . . .   7     6.2.  Email address information leak  . . . . . . . . . . . . .   7     6.3.  Storage of OPENPGPKEY data  . . . . . . . . . . . . . . .   8     6.4.  Forward security of OpenPGP versus DNSSEC . . . . . . . .   8   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8     7.1.  OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . .   8   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9   Appendix A.  Generating OPENPGPKEY records  . . . . . . . . . . .  10   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  111.  Introduction   OpenPGP [RFC4880] public keys are used to encrypt or sign email   messages and files.  To encrypt an email message, the sender's email   client or MTA needs to know where to find the recipient's OpenPGP   public key.  Once obtained, it needs to find some proof that the   OpenPGP public key found actually belongs to the intended recipient.Wouters                 Expires October 03, 2015                [Page 2]Internet-Draft        DANE for OpenPGP public keys            April 2015   Similarly, when files on a storage media are signed with an OpenPGP   public key that is included on the storage media, this key needs to   be independently verified.   Obtaining and verifying an OpenPGP 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.  Worse, some OpenPGP users announce their OpenPGP public   key fingerprint on social media with no method of validation   whatsoever.   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 prevents email clients and MUAs from encrypting an outgoing   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, leaving   the content of the email exposed to anyone with access to any of the   mail or storage servers used to transport the email from the sender   to the receiver.   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.  This method can also be used to obtain the OpenPGP public   key which can then be used for manual verification.Wouters                 Expires October 03, 2015                [Page 3]Internet-Draft        DANE for OpenPGP public keys            April 20151.1.  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 portion 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 5.5.1.1 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 5.5.1.1. of [RFC4880] encoded in base64 as defined in   Section 4 of [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:   o  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 internationalizedWouters                 Expires October 03, 2015                [Page 4]Internet-Draft        DANE for OpenPGP public keys            April 2015      email [RFC6530]) should already be encoded in UTF-8 (or its subset      ASCII).  If it is written in another encoding it should be      converted to UTF-8.  Next, it is turned into lowercase and hashed      using the SHA2-256 [RFC5754] algorithm, with the hash truncated to      28 octets and represented in its hexadecimal representation, to      become the left-most label in the prepared domain name.      Truncation comes from the right-most octets.  This does not      include the at symbol ("@") that separates the left and right      sides of the email address.   o  The string "_openpgpkey" becomes the second left-most label in the      prepared domain name.   o  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: "c93f1e400f26708f98cb19d936620da35   eec8f72e57f9eec01c1afd6._openpgpkey.example.com".  The corresponding   RR in the example.com zone might look like (key shortened for   formatting):   c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key>3.1.  Email address variants   Mail systems usually handle variant forms of local-parts.  The most   common variants are upper and lower case, which are now invariably   treated as equivalent.  But many other variants are possible.  Some   systems allow and ignore "noise" characters such as dots, so 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.  As the local-   part is converted to lowercase before hashing, case sensitivity will   not cause problems for the OPENPGPKEY lookup.Wouters                 Expires October 03, 2015                [Page 5]Internet-Draft        DANE for OpenPGP public keys            April 20154.  Application use of OPENPGPKEY   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.4.1.  Obtaining an OpenPGP key for a specific email address   If no OpenPGP public keys are known for an email address, an   OPENPGPKEY lookup can 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.4.2.  Confirming the validity of an OpenPGP key   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.4.3.  Verifying an unknown OpenPGP signature   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.5.  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 keyWouters                 Expires October 03, 2015                [Page 6]Internet-Draft        DANE for OpenPGP public keys            April 2015   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.6.  Security Considerations   OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE].6.1.  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").6.2.  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.Wouters                 Expires October 03, 2015                [Page 7]Internet-Draft        DANE for OpenPGP public keys            April 20156.3.  Storage of OPENPGPKEY data   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.   OpenPGP public keys obtained via OPENPGPKEY records should not be   stored beyond their DNS TTL value.6.4.  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.7.  IANA Considerations7.1.  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.8.  AcknowledgmentsWouters                 Expires October 03, 2015                [Page 8]Internet-Draft        DANE for OpenPGP public keys            April 2015   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.  Text regarding email mappings was taken from draft-   levine-dns-mailbox whose author is John Levine.9.  References9.1.  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 2010.9.2.  Informative References   [DNS-COOKIES]              Eastlake, Donald., "Domain Name System (DNS) Cookies",              draft-ietf-dnsop-cookies (work in progress), February              2015.   [OPENPGPKEY-USAGE]              Wouters, P., "Usage considerations with the DNS OPENPGPKEY              record", draft-dane-openpgpkey-usage (work in progress),              October 2014.Wouters                 Expires October 03, 2015                [Page 9]Internet-Draft        DANE for OpenPGP public keys            April 2015   [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.   [RFC5233]  Murchison, K., "Sieve Email Filtering: Subaddress              Extension", RFC 5233, January 2008.   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,              October 2008.   [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.   [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of              Existence in the DNS", RFC 7129, February 2014.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.   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:Wouters                 Expires October 03, 2015               [Page 10]Internet-Draft        DANE for OpenPGP public keys            April 2015   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 (line   break has been added for formatting):   <SHA2-256-trunc(hugh)>._openpgpkey.example.com. IN TYPE61 \# \       <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   c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY mQCNAzIG[...]   ~> openpgpkey --output generic hugh@example.com   c9[..]d6._openpgpkey.example.com. IN TYPE61 \# 2313 99008d03[...]Author's Address   Paul Wouters   Red Hat   Email: pwouters@redhat.comWouters                 Expires October 03, 2015               [Page 11]