GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc7929

Internet Engineering Task Force (IETF) P. Wouters Request for Comments: 7929 Red Hat Category: Experimental August 2016 ISSN: 2070-1721

DNS-Based Authentication of Named Entities (DANE) Bindings for OpenPGP

Abstract

 OpenPGP is a message format for email (and file) encryption that
 lacks a standardized lookup mechanism to securely obtain OpenPGP
 public keys.  DNS-Based Authentication of Named Entities (DANE) is a
 method for publishing public keys in DNS.  This document specifies a
 DANE 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 Secure DNS, however the OPENPGPKEY
 record is not a replacement for verification of authenticity via the
 "web of trust" or manual verification.  The OPENPGPKEY record can be
 used to encrypt an email that would otherwise have to be sent
 unencrypted.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7929.

Wouters Experimental [Page 1] RFC 7929 DANE for OpenPGP Keys August 2016

Copyright Notice

 Copyright (c) 2016 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.

Wouters Experimental [Page 2] RFC 7929 DANE for OpenPGP Keys August 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Experiment Goal . . . . . . . . . . . . . . . . . . . . .   4
   1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
 2.  The OPENPGPKEY Resource Record  . . . . . . . . . . . . . . .   5
   2.1.  The OPENPGPKEY RDATA Component  . . . . . . . . . . . . .   6
     2.1.1.  The OPENPGPKEY RDATA Content  . . . . . . . . . . . .   6
     2.1.2.  Reducing the Transferable Public Key Size . . . . . .   7
   2.2.  The OPENPGPKEY RDATA Wire Format  . . . . . . . . . . . .   7
   2.3.  The OPENPGPKEY RDATA Presentation Format  . . . . . . . .   7
 3.  Location of the OPENPGPKEY Record . . . . . . . . . . . . . .   8
 4.  Email Address Variants and Internationalization
     Considerations  . . . . . . . . . . . . . . . . . . . . . . .   9
 5.  Application Use of OPENPGPKEY . . . . . . . . . . . . . . . .  10
   5.1.  Obtaining an OpenPGP Key for a Specific Email Address . .  10
   5.2.  Confirming that an OpenPGP Key is Current . . . . . . . .  10
   5.3.  Public Key UIDs and Query Names . . . . . . . . . . . . .  10
 6.  OpenPGP Key Size and DNS  . . . . . . . . . . . . . . . . . .  11
 7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.1.  MTA Behavior  . . . . . . . . . . . . . . . . . . . . . .  12
   7.2.  MUA Behavior  . . . . . . . . . . . . . . . . . . . . . .  13
   7.3.  Response Size . . . . . . . . . . . . . . . . . . . . . .  14
   7.4.  Email Address Information Leak  . . . . . . . . . . . . .  14
   7.5.  Storage of OPENPGPKEY Data  . . . . . . . . . . . . . . .  14
   7.6.  Security of OpenPGP versus DNSSEC . . . . . . . . . . . .  15
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.1.  OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . .  15
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
   9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
 Appendix A.  Generating OPENPGPKEY Records  . . . . . . . . . . .  18
 Appendix B.  OPENPGPKEY IANA Template . . . . . . . . . . . . . .  19
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  20
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

Wouters Experimental [Page 3] RFC 7929 DANE for OpenPGP Keys August 2016

1. Introduction

 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 Mail User Agent (MUA) or
 the email server Mail Transfer Agent (MTA) needs to locate the
 recipient's OpenPGP public key.
 OpenPGP clients have relied on centralized "well-known" key servers
 that are accessed using the HTTP Keyserver Protocol [HKP].
 Alternatively, users need to manually browse a variety of different
 front-end websites.  These key servers do not require a confirmation
 of the email address used in the User ID (UID) 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.
 Therefore, these key servers are not well suited to support MUAs and
 MTAs 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 OPENPGPKEY data is secured using Secure DNS [RFC4035].
 The main goal of the OPENPGPKEY resource record is to stop passive
 attacks against plaintext emails.  While it can also thwart some
 active attacks (such as people uploading rogue keys to key servers in
 the hopes that others will encrypt to these rogue keys), this
 resource record is not a replacement for verifying OpenPGP public
 keys via the "web of trust" signatures, or manually via a fingerprint
 verification.

1.1. Experiment Goal

 This specification is one experiment in improving access to public
 keys for end-to-end email security.  There are a range of ways in
 which this can reasonably be done for OpenPGP or S/MIME, for example,
 using the DNS, or SMTP, or HTTP.  Proposals for each of these have

Wouters Experimental [Page 4] RFC 7929 DANE for OpenPGP Keys August 2016

 been made with various levels of support in terms of implementation
 and deployment.  For each such experiment, specifications such as
 this will enable experiments to be carried out that may succeed or
 that may uncover technical or other impediments to large- or small-
 scale deployments.  The IETF encourages those implementing and
 deploying such experiments to publicly document their experiences so
 that future specifications in this space can benefit.
 This document defines an RRtype whose use is Experimental.  The goal
 of the experiment is to see whether encrypted email usage will
 increase if an automated discovery method is available to MTAs and
 MUAs to help the end user with email encryption key management.
 It is unclear if this RRtype will scale to some of the larger email
 service deployments.  Concerns have been raised about the size of the
 OPENPGPKEY record and the size of the resulting DNS zone files.  This
 experiment hopefully will give the working group some insight into
 whether or not this is a problem.
 If the experiment is successful, it is expected that the findings of
 the experiment will result in an updated document for standards track
 approval.
 The OPENPGPKEY RRtype somewhat resembles the generic CERT record
 defined in [RFC4398].  However, the CERT record uses sub-typing with
 many different types of keys and certificates.  It is suspected that
 its general application of very different protocols (PKIX versus
 OpenPGP) has been the cause for lack of implementation and
 deployment.  Furthermore, the CERT record uses sub-typing, which is
 now considered to be a bad idea for DNS.

1.2. 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 Transferable Public Key (see Section 11.1 of
 [RFC4880]) with an email address, thus forming an "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

Wouters Experimental [Page 5] RFC 7929 DANE for OpenPGP Keys August 2016

 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.

2.1. The OPENPGPKEY RDATA Component

 The RDATA portion of an OPENPGPKEY resource record contains a single
 value consisting of a Transferable Public Key formatted as specified
 in [RFC4880].

2.1.1. The OPENPGPKEY RDATA Content

 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:
           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
 If the primary key is not encryption-capable, at least one relevant
 subkey should be included, resulting in an OPENPGPKEY Transferable
 Public Key containing:
         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)
 The user can also elect to add a few third-party certifications,
 which they believe would be helpful for validation in the traditional
 "web of trust".  The resulting OPENPGPKEY Transferable Public Key
 would then look like:
         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)

Wouters Experimental [Page 6] RFC 7929 DANE for OpenPGP Keys August 2016

2.1.2. Reducing the Transferable Public Key Size

 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:
 o  Where one User ID from the certifications matches the looked-up
    address, strip away non-matching User IDs and any associated
    certifications (self-signatures or third-party certifications).
 o  Strip away all User Attribute packets and associated
    certifications.
 o  Strip away all expired subkeys.  The user may want to keep revoked
    subkeys if these were revoked prior to their preferred expiration
    time to ensure that correspondents know about these earlier than
    expected revocations.
 o  Strip away all but the most recent self-signature for the
    remaining User IDs and subkeys.
 o  Optionally strip away any uninteresting or unimportant third-party
    User ID certifications.  This is a value judgment by the user that
    is difficult to automate.  At the very least, expired and
    superseded third-party certifications should be stripped out.  The
    user should attempt to keep the most recent and most well-
    connected certifications in the "web of trust" in their
    Transferable Public Key.

2.2. The OPENPGPKEY RDATA Wire Format

 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.

2.3. The OPENPGPKEY RDATA Presentation Format

 The RDATA Presentation Format, as visible in master files [RFC1035],
 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].

Wouters Experimental [Page 7] RFC 7929 DANE for OpenPGP Keys August 2016

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 [RFC5322] and
 [RFC6530].  Therefore, email addresses are mapped into DNS using the
 following method:
 1.  The "left-hand side" of the email address, called the "local-
     part" in both the mail message format definition [RFC5322] and in
     the specification for internationalized email [RFC6530]) is
     encoded in UTF-8 (or its subset ASCII).  If the local-part is
     written in another charset, it MUST be converted to UTF-8.
 2.  The local-part is first canonicalized using the following rules.
     If the local-part is unquoted, any comments and/or folding
     whitespace (CFWS) around dots (".") is removed.  Any enclosing
     double quotes are removed.  Any literal quoting is removed.
 3.  If the local-part contains any non-ASCII characters, it SHOULD be
     normalized using the Unicode Normalization Form C from
     [Unicode90].  Recommended normalization rules can be found in
     Section 10.1 of [RFC6530].
 4.  The local-part is 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.
 5.  The string "_openpgpkey" becomes the second left-most label in
     the prepared domain name.
 6.  The domain name (the "right-hand side" of the email address,
     called the "domain" in [RFC5322]) 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>

Wouters Experimental [Page 8] RFC 7929 DANE for OpenPGP Keys August 2016

4. Email Address Variants and Internationalization Considerations

 Mail systems usually handle variant forms of local-parts.  The most
 common variants are upper- and lowercase, 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 '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.  Therefore, sending MUAs and MTAs supporting OPENPGPKEY MUST
 NOT perform any kind of mapping rules based on the email address.  In
 order to improve chances of finding OPENPGP RRs for a particular
 local-part, domains that allow variant forms (such as treating local-
 parts as case-insensitive) might publish OPENPGP RRs for all variants
 of local-parts, might publish variants on first use (for example, a
 webmail provider that also controls DNS for a domain can publish
 variants as used by owner of a particular local-part) or just publish
 OPENPGP RRs for the most common variants.
 Section 3 above defines how the local-part is used to determine the
 location where one looks for an OPENPGPKEY record.  Given the variety
 of local-parts seen in email, designing a good experiment for this is
 difficult, as: a) some current implementations are known to lowercase
 at least US-ASCII local-parts, b) we know from (many) other
 situations that any strategy based on guessing and making multiple
 DNS queries is not going to achieve consensus for good reasons, and
 c) the underlying issues are just hard -- see Section 10.1 of
 [RFC6530] for discussion of just some of the issues that would need
 to be tackled to fully address this problem.
 However, while this specification is not the place to try to address
 these issues with local-parts, doing so is also not required to
 determine the outcome of this experiment.  If this experiment
 succeeds, then further work on email addresses with non-ASCII local-
 parts will be needed and, based on the findings from this experiment,
 that would be better than doing nothing or starting this experiment
 based on a speculative approach to what is a very complex topic.

Wouters Experimental [Page 9] RFC 7929 DANE for OpenPGP Keys August 2016

5. Application Use of OPENPGPKEY

 The OPENPGPKEY record allows an application or service to obtain an
 OpenPGP public key and use it for verifying a digital signature or
 encrypting a message to the public key.  The DNS answer MUST pass
 DNSSEC validation; if DNSSEC validation reaches any state other than
 "Secure" (as specified in [RFC4035]), the DNSSEC validation MUST be
 treated as a failure.

5.1. Obtaining an OpenPGP Key for a Specific Email Address

 If no OpenPGP public keys are known for an email address, an
 OPENPGPKEY DNS lookup MAY be performed to seek the OpenPGP public key
 that corresponds to that 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 whose attempt fails to
 retrieve a DNSSEC-verified OPENPGPKEY RR from the DNS should remember
 that failure for some time to avoid sending out a DNS request for
 each email message the application is sending out; such DNS requests
 constitute a privacy leak.

5.2. Confirming that an OpenPGP Key is Current

 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 that the locally stored OpenPGP public
 key is still the correct key to use.  If the locally stored OpenPGP
 public key is different from the DNSSEC-validated OpenPGP public key
 currently published in DNS, the confirmation MUST be treated as a
 failure unless the locally stored OpenPGP key signed the newly
 published OpenPGP public key found in DNS.  An application that can
 interact with the user MAY ask the user for guidance; otherwise, the
 application will have to apply local policy.  For privacy reasons, an
 application MUST NOT attempt to look up an OpenPGP key from DNSSEC at
 every use of that key.

5.3. Public Key UIDs and Query Names

 An OpenPGP public key can be associated with multiple email addresses
 by specifying multiple key UIDs.  The OpenPGP public key obtained
 from an OPENPGPKEY RR can be used as long as the query and resulting
 data form a proper email to the UID identity association.
 CNAMEs (see [RFC2181]) and DNAMEs (see [RFC6672]) can be followed to
 obtain an OPENPGPKEY RR, as long as the original recipient's email
 address appears as one of the OpenPGP public key UIDs.  For example,

Wouters Experimental [Page 10] RFC 7929 DANE for OpenPGP Keys August 2016

 if the OPENPGPKEY RR query for hugh@example.com
 (8d57[...]b7._openpgpkey.example.com) yields a CNAME to
 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for
 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public
 key can be used, provided one of the key UIDs contains
 "hugh@example.com".  This public key cannot be used if it would only
 contain the key UID "hugh@example.net".
 If one of the OpenPGP key UIDs contains only a single wildcard as the
 left-hand side of the email address, such as "*@example.com", the
 OpenPGP public key may be used for any email address within that
 domain.  Wildcards at other locations (e.g., "hugh@*.com") or regular
 expressions in key UIDs are not allowed, and any OPENPGPKEY RR
 containing these MUST be ignored.

6. OpenPGP Key Size and DNS

 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 not be exported to OpenPGP public keyrings that
 are used to create OPENPGPKEY resource records.  Some OpenPGP
 software (for example, GnuPG) supports a "minimal key export" that is
 well suited to use as OPENPGPKEY RDATA.  See Appendix A.

7. Security Considerations

 DNSSEC is not an alternative for the "web of trust" or for manual
 fingerprint verification by users.  DANE for OpenPGP, as specified in
 this document, is a solution aimed to ease obtaining someone's public
 key.  Without manual verification of the OpenPGP key obtained via
 DANE, this retrieved key should only be used for encryption if the
 only other alternative is sending the message in plaintext.  While
 this thwarts all passive attacks that simply capture and log all
 plaintext email content, it is not a security measure against active
 attacks.  A user who publishes an OPENPGPKEY record in DNS still

Wouters Experimental [Page 11] RFC 7929 DANE for OpenPGP Keys August 2016

 expects senders to perform their due diligence by additional (non-
 DNSSEC) verification of their public key via other out-of-band
 methods before sending any confidential or sensitive information.
 In other words, the OPENPGPKEY record MUST NOT be used to send
 sensitive information without additional verification or confirmation
 that the OpenPGP key actually belongs to the target recipient.
 DNSSEC does not protect the queries from Pervasive Monitoring as
 defined in [RFC7258].  Since DNS queries are currently mostly
 unencrypted, a query to look up a target OPENPGPKEY record could
 reveal that a user using the (monitored) recursive DNS server is
 attempting to send encrypted email to a target.  This information is
 normally protected by the MUAs and MTAs by using Transport Layer
 Security (TLS) encryption using STARTTLS.  The DNS itself can
 mitigate some privacy concerns, but the user needs to select a
 trusted DNS server that supports these privacy-enhancing features.
 Recursive DNS servers can support DNS Query Name Minimalisation
 [RFC7816], which limits leaking the QNAME to only the recursive DNS
 server and the nameservers of the actual zone being queried for.
 Recursive DNS servers can also support TLS [RFC7858] to ensure that
 the path between the end user and the recursive DNS server is
 encrypted.
 Various components could be responsible for encrypting an email
 message to a target recipient.  It could be done by the sender's MUA
 or a MUA plug-in or the sender's MTA.  Each of these have their own
 characteristics.  A MUA can ask the user to make a decision before
 continuing.  The MUA can either accept or refuse a message.  The MTA
 must deliver the message as-is, or encrypt the message before
 delivering.  Each of these components should attempt to encrypt an
 unencrypted outgoing message whenever possible.
 In theory, two different local-parts could hash to the same value.
 This document assumes that such a hash collision has a negligible
 chance of happening.
 Organizations that are required to be able to read everyone's
 encrypted email should publish the escrow key as the OPENPGPKEY
 record.  Mail servers of such organizations MAY optionally re-encrypt
 the message to the individual's OpenPGP key.

7.1. MTA Behavior

 An MTA could be operating in a stand-alone mode, without access to
 the sender's OpenPGP public keyring, or in a way where it can access
 the user's OpenPGP public keyring.  Regardless, the MTA MUST NOT
 modify the user's OpenPGP keyring.

Wouters Experimental [Page 12] RFC 7929 DANE for OpenPGP Keys August 2016

 An MTA sending an email MUST NOT add the public key obtained from an
 OPENPGPKEY resource record to a permanent public keyring for future
 use beyond the TTL.
 If the obtained public key is revoked, the MTA MUST NOT use the key
 for encryption, even if that would result in sending the message in
 plaintext.
 If a message is already encrypted, the MTA SHOULD NOT re-encrypt the
 message, even if different encryption schemes or different encryption
 keys would be used.
 If the DNS request for an OPENPGPKEY record returned an Indeterminate
 or Bogus answer as specified in [RFC4035], the MTA MUST NOT send the
 message and queue the plaintext message for encrypted delivery at a
 later time.  If the problem persists, the email should be returned
 via the regular bounce methods.
 If multiple non-revoked OPENPGPKEY resource records are found, the
 MTA SHOULD pick the most secure RR based on its local policy.

7.2. MUA Behavior

 If the public key for a recipient obtained from the locally stored
 sender's public keyring differs from the recipient's OPENPGPKEY RR,
 the MUA SHOULD halt processing the message and interact with the user
 to resolve the conflict before continuing to process the message.
 If the public key for a recipient obtained from the locally stored
 sender's public keyring contains contradicting properties for the
 same key obtained from an OPENPGPKEY RR, the MUA SHOULD NOT accept
 the message for delivery.
 If multiple non-revoked OPENPGPKEY resource records are found, the
 MUA SHOULD pick the most secure OpenPGP public key based on its local
 policy.
 The MUA MAY interact with the user to resolve any conflicts between
 locally stored keyrings and OPENPGPKEY RRdata.
 A MUA that is encrypting a message SHOULD clearly indicate to the
 user the difference between encrypting to a locally stored and
 previously user-verified public key and encrypting to a public key
 obtained via an OPENPGPKEY resource record that was not manually
 verified by the user in the past.

Wouters Experimental [Page 13] RFC 7929 DANE for OpenPGP Keys August 2016

7.3. 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 confirmed with [RFC7873].  Such servers
 MUST NOT return REFUSED, but answer the query with an empty answer
 section and the truncation flag set ("TC=1").

7.4. Email Address Information Leak

 The hashing of the local-part in this document is not a security
 feature.  Publishing OPENPGPKEY records 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 local-parts, as
 single rainbow tables can be re-used across domains.  This can be
 somewhat countered by using NextSECure version 3 (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 local-
 parts (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.

7.5. 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.
 Applications that cannot interact with users, 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.

Wouters Experimental [Page 14] RFC 7929 DANE for OpenPGP Keys August 2016

7.6. Security of OpenPGP versus DNSSEC

 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.
 Any future messages encrypted with the malicious OpenPGP key could
 then be read.
 Therefore, an OpenPGP key obtained via a DNSSEC-validated OPENPGPKEY
 record can only be trusted as much as the DNS domain can be trusted,
 and is no substitute for in-person OpenPGP key verification or
 additional OpenPGP verification via "web of trust" signatures present
 on the OpenPGP in question.

8. IANA Considerations

8.1. 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.
 The IANA template for OPENPGPKEY is listed in Appendix B.  It was
 submitted to IANA for review on July 23, 2014 and approved on August
 12, 2014.

9. References

9.1. Normative References

 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <http://www.rfc-editor.org/info/rfc1035>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
            Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
            <http://www.rfc-editor.org/info/rfc2181>.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, DOI 10.17487/RFC4033, March 2005,
            <http://www.rfc-editor.org/info/rfc4033>.

Wouters Experimental [Page 15] RFC 7929 DANE for OpenPGP Keys August 2016

 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource Records for the DNS Security Extensions",
            RFC 4034, DOI 10.17487/RFC4034, March 2005,
            <http://www.rfc-editor.org/info/rfc4034>.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol Modifications for the DNS Security
            Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
            <http://www.rfc-editor.org/info/rfc4035>.
 [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
            Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
            <http://www.rfc-editor.org/info/rfc4648>.
 [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
            Thayer, "OpenPGP Message Format", RFC 4880,
            DOI 10.17487/RFC4880, November 2007,
            <http://www.rfc-editor.org/info/rfc4880>.
 [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
            Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
            2010, <http://www.rfc-editor.org/info/rfc5754>.

9.2. Informative References

 [HKP]      Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
            Work in Progress, draft-shaw-openpgp-hkp-00, March 2003.
 [MAILBOX]  Levine, J., "Encoding mailbox local-parts in the DNS",
            Work in Progress, draft-levine-dns-mailbox-01, September
            2015.
 [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
            (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
            2003, <http://www.rfc-editor.org/info/rfc3597>.
 [RFC4255]  Schlyter, J. and W. Griffin, "Using DNS to Securely
            Publish Secure Shell (SSH) Key Fingerprints", RFC 4255,
            DOI 10.17487/RFC4255, January 2006,
            <http://www.rfc-editor.org/info/rfc4255>.
 [RFC4398]  Josefsson, S., "Storing Certificates in the Domain Name
            System (DNS)", RFC 4398, DOI 10.17487/RFC4398, March 2006,
            <http://www.rfc-editor.org/info/rfc4398>.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <http://www.rfc-editor.org/info/rfc5321>.

Wouters Experimental [Page 16] RFC 7929 DANE for OpenPGP Keys August 2016

 [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
            DOI 10.17487/RFC5322, October 2008,
            <http://www.rfc-editor.org/info/rfc5322>.
 [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
            Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
            February 2012, <http://www.rfc-editor.org/info/rfc6530>.
 [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
            DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
            <http://www.rfc-editor.org/info/rfc6672>.
 [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, <http://www.rfc-editor.org/info/rfc6698>.
 [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
            Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
            2014, <http://www.rfc-editor.org/info/rfc7258>.
 [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
            Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
            <http://www.rfc-editor.org/info/rfc7816>.
 [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
            and P. Hoffman, "Specification for DNS over Transport
            Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
            2016, <http://www.rfc-editor.org/info/rfc7858>.
 [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
            Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
            <http://www.rfc-editor.org/info/rfc7873>.
 [SMIME]    Hoffman, P. and J. Schlyter, "Using Secure DNS to
            Associate Certificates with Domain Names For S/MIME", Work
            in Progress, draft-ietf-dane-smime-12, July 2016.
 [Unicode90]
            The Unicode Consortium, "The Unicode Standard, Version
            9.0.0", (Mountain View, CA: The Unicode Consortium,
            2016. ISBN 978-1-936213-13-9),
            <http://www.unicode.org/versions/Unicode9.0.0/>.

Wouters Experimental [Page 17] RFC 7929 DANE for OpenPGP Keys August 2016

Appendix A. Generating OPENPGPKEY Records

 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 | 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 of 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,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"'
 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[...]

Wouters Experimental [Page 18] RFC 7929 DANE for OpenPGP Keys August 2016

Appendix B. OPENPGPKEY IANA Template

 This is a copy of the original registration template submitted to
 IANA; the text (including the references) has not been updated.
A. Submission Date: 23-07-2014
B.1 Submission Type: [x] New RRTYPE [ ] Modification to RRTYPE
B.2 Kind of RR: [x] Data RR [ ] Meta-RR
C. Contact Information for submitter (will be publicly posted):
   Name: Paul Wouters         Email Address: pwouters@redhat.com
   International telephone number: +1-647-896-3464
   Other contact handles: paul@nohats.ca
D. Motivation for the new RRTYPE application.
   Publishing RFC-4880 OpenPGP formatted keys in DNS with DNSSEC
   protection to faciliate automatic encryption of emails in
   defense against pervasive monitoring.
E. Description of the proposed RR type.
http://tools.ietf.org/html/draft-ietf-dane-openpgpkey-00#section-2
F. What existing RRTYPE or RRTYPEs come closest to filling that need
   and why are they unsatisfactory?
   The CERT RRtype is the closest match. It unfortunately depends on
   subtyping, and its use in general is no longer recommended. It
   also has no human usable presentation format. Some usage types of
   CERT require external URI's which complicates the security model.
   This was discussed in the dane working group.
G. What mnemonic is requested for the new RRTYPE (optional)?
   OPENPGPKEY
H. Does the requested RRTYPE make use of any existing IANA registry
   or require the creation of a new IANA subregistry in DNS
   Parameters? If so, please indicate which registry is to be used
   or created. If a new subregistry is needed, specify the
   allocation policy for it and its initial contents. Also include
   what the modification procedures will be.
   The RDATA part uses the key format specified in RFC-4880, which
   itself use
   https://www.iana.org/assignments/pgp-parameters/pgp-parameters.xhtm

Wouters Experimental [Page 19] RFC 7929 DANE for OpenPGP Keys August 2016

   This RRcode just uses the formats specified in those registries for
   its RRdata part.
I. Does the proposal require/expect any changes in DNS
   servers/resolvers that prevent the new type from being processed
   as an unknown RRTYPE (see [RFC3597])?
   No.
J. Comments:
   Currently, three software implementations of
   draft-ietf-dane-openpgpkey are using a private number.

Acknowledgments

 This document is based on [RFC4255] and [SMIME] whose authors are
 Paul Hoffman, Jakob 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 [MAILBOX] whose author is John Levine.

Author's Address

 Paul Wouters
 Red Hat
 Email: pwouters@redhat.com

Wouters Experimental [Page 20]

/data/webs/external/dokuwiki/data/pages/rfc/rfc7929.txt · Last modified: 2016/08/05 17:45 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki