< draft-ietf-dane-openpgpkey-03.txt   draft-ietf-dane-openpgpkey-04.txt >
Network Working Group P. Wouters Network Working Group P. Wouters
Internet-Draft Red Hat Internet-Draft Red Hat
Intended status: Standards Track April 01, 2015 Intended status: Experimental August 27, 2015
Expires: October 03, 2015 Expires: February 28, 2016
Using DANE to Associate OpenPGP public keys with email addresses Using DANE to Associate OpenPGP public keys with email addresses
draft-ietf-dane-openpgpkey-03 draft-ietf-dane-openpgpkey-04
Abstract Abstract
OpenPGP is a message format for email (and file) encryption that OpenPGP is a message format for email (and file) encryption that
lacks a standardized lookup mechanism to securely obtain OpenPGP lacks a standardized lookup mechanism to securely obtain OpenPGP
public keys. This document specifies a method for publishing, public keys. This document specifies a method for publishing and
locating and verifying OpenPGP public keys in DNS for a specific locating OpenPGP public keys in DNS for a specific email address
email address using a new OPENPGPKEY DNS Resource Record. Security using a new OPENPGPKEY DNS Resource Record. Security is provided via
is provided via DNSSEC. DNSSEC.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 03, 2015. This Internet-Draft will expire on February 28, 2016.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 3
2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4 2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4
2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 4 2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 4
2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 4 2.1.2. Reducing the Transferable Public Key size . . . . . . 5
3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 4 2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 5
3.1. Email address variants . . . . . . . . . . . . . . . . . 5 2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 5
4. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 6 3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 5
4.1. Obtaining an OpenPGP key for a specific email address . . 6 4. Email address variants . . . . . . . . . . . . . . . . . . . 6
4.2. Confirming the validity of an OpenPGP key . . . . . . . . 6 5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 7
4.3. Verifying an unknown OpenPGP signature . . . . . . . . . 6 5.1. Obtaining an OpenPGP key for a specific email address . . 7
5. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 6 5.2. Confirming the validity of an OpenPGP key . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 5.3. Verifying an unknown OpenPGP signature . . . . . . . . . 7
6.1. Response size . . . . . . . . . . . . . . . . . . . . . . 7 6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 7
6.2. Email address information leak . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6.3. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 8 7.1. Response size . . . . . . . . . . . . . . . . . . . . . . 8
6.4. Forward security of OpenPGP versus DNSSEC . . . . . . . . 8 7.2. Email address information leak . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 7.3. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 9
7.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 8 7.4. Forward security of OpenPGP versus DNSSEC . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 9 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 10 10.1. Normative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11 10.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
OpenPGP [RFC4880] public keys are used to encrypt or sign email OpenPGP [RFC4880] public keys are used to encrypt or sign email
messages and files. To encrypt an email message, the sender's email messages and files. To encrypt an email message, or verify a
client or MTA needs to know where to find the recipient's OpenPGP sender's OpenPGP signature, the email client or MTA needs to locate
public key. Once obtained, it needs to find some proof that the the recipient's OpenPGP public key.
OpenPGP public key found actually belongs to the intended recipient.
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 OpenPGP clients have relied on centralized "well-known" key servers
straightforward process as there are no trusted standardized that are accessed using either the HTTP Keyserver Protocol [HKP]
locations for publishing OpenPGP public keys indexed by email Alternatively, users need to manually browse a variety of different
address. Instead, OpenPGP clients rely on "well-known key servers" front-end websites. These key servers do not validate the email
that are accessed using the HTTP Keyserver Protocol ("HKP") or address in the User ID of the uploaded OpenPGP public key. Attackers
manually by users using a variety of differently formatted front-end can - and have - uploaded rogue public keys with other people's email
web pages. Worse, some OpenPGP users announce their OpenPGP public addresses to these key servers.
key fingerprint on social media with no method of validation
whatsoever.
Currently deployed key servers have no method of validating any Once uploaded, public keys cannot be deleted. People who did not
uploaded OpenPGP public key. The key servers simply store and pre-sign a key revocation can never remove their OpenPGP public key
publish. Anyone can add public keys with any identities and anyone from these key servers once they have lost access to their private
can add signatures to any other public key using forged malicious key. This results in receiving encrypted email that cannot be
identities. Furthermore, once uploaded, public keys cannot be decrypted.
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 Therefor, these keyservers are not well suited to support email
address prevents email clients and MUAs from encrypting an outgoing clients and MTA's to automatically encrypt email - especially in the
email to the target recipient, forcing the software to send the absence of an interactive user.
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 This document describes a mechanism to associate a user's OpenPGP
public key with their email address, using a new DNS RRtype. 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. The proposed new DNS Resource Record type is secured using DNSSEC.
This trust model is not meant to replace the Trust Signature model. 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. This method can also be used to obtain the OpenPGP public
key which can then be used for manual verification.
1.1. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
This document also makes use of standard DNSSEC and DANE terminology. This document also makes use of standard DNSSEC and DANE terminology.
See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for See DNSSEC [RFC4033], [RFC4034], [RFC4035], and DANE [RFC6698] for
these terms. these terms.
2. The OPENPGPKEY Resource Record 2. The OPENPGPKEY Resource Record
The OPENPGPKEY DNS resource record (RR) is used to associate an end The OPENPGPKEY DNS resource record (RR) is used to associate an end
entity OpenPGP public key with an email address, thus forming a entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880]
"OpenPGP public key association". 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 The type value allocated for the OPENPGPKEY RR type is 61. The
OPENPGPKEY RR is class independent. The OPENPGPKEY RR has no special OPENPGPKEY RR is class independent. The OPENPGPKEY RR has no special
TTL requirements. TTL requirements.
2.1. The OPENPGPKEY RDATA component 2.1. The OPENPGPKEY RDATA component
The RDATA portion of an OPENPGPKEY Resource Record contains a single The RDATA portion of an OPENPGPKEY Resource Record contains a single
value consisting of a [RFC4880] formatted OpenPGP public keyring. value consisting of a [RFC4880] formatted Transferable Public Key.
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, a 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 ... ]
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 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 then expected revocations.
o strip away all but the most recent self-sig 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 certifcations 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 2.2. The OPENPGPKEY RDATA wire format
The RDATA Wire Format consists of a single OpenPGP public key as The RDATA Wire Format consists of a single OpenPGP Transferable
defined in Section 5.5.1.1 of [RFC4880]. Note that this format is Public Key as defined in Section 11.1 of [RFC4880]. Note that this
without ASCII armor or base64 encoding. format is without ASCII armor or base64 encoding.
2.3. The OPENPGPKEY RDATA presentation format 2.3. The OPENPGPKEY RDATA presentation format
The RDATA Presentation Format, as visible in textual zone files, The RDATA Presentation Format, as visible in textual zone files,
consists of a single OpenPGP public key as defined in consists of a single OpenPGP Transferable Public Key as defined in
Section 5.5.1.1. of [RFC4880] encoded in base64 as defined in Section 11,1 of [RFC4880] encoded in base64 as defined in Section 4
Section 4 of [RFC4648]. of [RFC4648].
3. Location of the OPENPGPKEY record 3. Location of the OPENPGPKEY record
The DNS does not allow the use of all characters that are supported 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 in the "local-part" of email addresses as defined in [RFC2822] and
[RFC6530]. Therefore, email addresses are mapped into DNS using the [RFC6530]. Therefore, email addresses are mapped into DNS using the
following method: following method:
o The user name (the "left-hand side" of the email address, called o The user name (the "left-hand side" of the email address, called
the "local-part" in the mail message format definition [RFC2822] the "local-part" in the mail message format definition [RFC2822]
and the "local part" in the specification for internationalized and the local-part in the specification for internationalized
email [RFC6530]) should already be encoded in UTF-8 (or its subset email [RFC6530]) should already be encoded in UTF-8 (or its subset
ASCII). If it is written in another encoding it should be ASCII). If it is written in another encoding it should be
converted to UTF-8. Next, it is turned into lowercase and hashed converted to UTF-8 and then hashed using the SHA2-256 [RFC5754]
using the SHA2-256 [RFC5754] algorithm, with the hash truncated to algorithm, with the hash truncated to 28 octets and represented in
28 octets and represented in its hexadecimal representation, to its hexadecimal representation, to become the left-most label in
become the left-most label in the prepared domain name. the prepared domain name. Truncation comes from the right-most
Truncation comes from the right-most octets. This does not octets. This does not include the at symbol ("@") that separates
include the at symbol ("@") that separates the left and right the left and right sides of the email address.
sides of the email address.
o The string "_openpgpkey" becomes the second left-most label in the o The string "_openpgpkey" becomes the second left-most label in the
prepared domain name. prepared domain name.
o The domain name (the "right-hand side" of the email address, 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 called the "domain" in RFC 2822) is appended to the result of step
2 to complete the prepared domain name. 2 to complete the prepared domain name.
For example, to request an OPENPGPKEY resource record for a user For example, to request an OPENPGPKEY resource record for a user
whose email address is "hugh@example.com", an OPENPGPKEY query would whose email address is "hugh@example.com", an OPENPGPKEY query would
be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35 be placed for the following QNAME: "c93f1e400f26708f98cb19d936620da35
eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding eec8f72e57f9eec01c1afd6._openpgpkey.example.com". The corresponding
RR in the example.com zone might look like (key shortened for RR in the example.com zone might look like (key shortened for
formatting): formatting):
c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key> c9[..]d6._openpgpkey.example.com. IN OPENPGPKEY <base64 public key>
3.1. Email address variants 4. Email address variants
Mail systems usually handle variant forms of local-parts. The most Mail systems usually handle variant forms of local-parts. The most
common variants are upper and lower case, which are now invariably common variants are upper and lower case, often automatically
treated as equivalent. But many other variants are possible. Some corrected when a name is recognized as such. Other variants include
systems allow and ignore "noise" characters such as dots, so local systems that ignore "noise" characters such as dots, so that local
parts johnsmith and John.Smith would be equivalent. Many systems parts johnsmith and John.Smith would be equivalent. Many systems
allow "extensions" such as john-ext or mary+ext where john or mary is 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 treated as the effective local-part, and the ext is passed to the
recipient for further handling. This can complicate finding the recipient for further handling. This can complicate finding the
OPENPGPKEY record associated with the dynamically created email OPENPGPKEY record associated with the dynamically created email
address. address.
[RFC5321] and its predecessors have always made it clear that only [RFC5321] and its predecessors have always made it clear that only
the recipient MTA is allowed to interpret the local-part of an the recipient MTA is allowed to interpret the local-part of an
address. A client supporting OPENPGPKEY therefor MUST NOT perform address. A client supporting OPENPGPKEY therefor MUST NOT perform
any kind of mapping rules based on the email address. As the local- any kind of mapping rules based on the email address.
part is converted to lowercase before hashing, case sensitivity will
not cause problems for the OPENPGPKEY lookup.
4. Application use of OPENPGPKEY 5. Application use of OPENPGPKEY
The OPENPGPKEY record allows an application or service to obtain or The OPENPGPKEY record allows an application or service to obtain or
verify an OpenPGP public key. The lookup result MUST pass DNSSEC verify an OpenPGP public key. The lookup result MUST pass DNSSEC
validation; if validation reaches any state other than "Secure", the validation; if validation reaches any state other than "Secure", the
verification MUST be treated as a failure. verification MUST be treated as a failure.
4.1. Obtaining an OpenPGP key for a specific email address 5.1. Obtaining an OpenPGP key for a specific email address
If no OpenPGP public keys are known for an email address, an If no OpenPGP public keys are known for an email address, an
OPENPGPKEY lookup can be performed to discover the OpenPGP public key OPENPGPKEY lookup MAY be performed to discover the OpenPGP public key
that belongs to a specific email address. This public key can then 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 be used to verify a received signed message or can be used to send
out an encrypted email message. 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.
4.2. Confirming the validity of an OpenPGP key 5.2. Confirming the validity of an OpenPGP key
Locally stored OpenPGP public keys are not automatically refreshed. Locally stored OpenPGP public keys are not automatically refreshed.
If the owner of that key creates a new OpenPGP public key, that owner 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 is unable to securely notify all users and applications that have its
old OpenPGP public key. Applications and users can perform an old OpenPGP public key. Applications and users can perform an
OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is OPENPGPKEY lookup to confirm the locally stored OpenPGP public key is
still the correct key to use. If verifying a locally stored OpenPGP still the correct key to use. If verifying a locally stored OpenPGP
public key and the OpenPGP public key found through DNS is different public key and the OpenPGP public key found through DNS is different
from the locally stored OpenPGP public key, the verification MUST be from the locally stored OpenPGP public key, the verification MUST be
treated as a failure. An application that can interact with the user treated as a failure. An application that can interact with the user
MAY ask the user for guidance. 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.
4.3. Verifying an unknown OpenPGP signature 5.3. Verifying an unknown OpenPGP signature
Storage media can be signed using an OpenPGP public key. Even if the 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 OpenPGP public key is included on the storage media, it needs to be
independently validated. OpenPGP public keys contain one or more IDs independently validated. OpenPGP public keys contain one or more IDs
than can have the syntax of an email address. An application can than can have the syntax of an email address. An application can
perform a lookup for an OPENPGPKEY at the expected location for the perform a lookup for an OPENPGPKEY at the expected location for the
specific email address to confirm the validity of the OpenPGP public specific email address to confirm the validity of the OpenPGP public
key. Once the key has been validated, all files on the storage media key. Once the key has been validated, all files on the storage media
that have been signed by this key can now be verified. that have been signed by this key can now be verified.
5. OpenPGP Key size and DNS 6. OpenPGP Key size and DNS
Due to the expected size of the OPENPGPKEY record, applications
Due to the expected size of the OPENPGPKEY record, it is recommended SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY
to perform DNS queries for the OPENPGPKEY record using TCP, not UDP. Resource Record.
Although the reliability of the transport of large DNS Resource Although the reliability of the transport of large DNS Resource
Records has improved in the last years, it is still recommended to Records has improved in the last years, it is still recommended to
keep the DNS records as small as possible without sacrificing the keep the DNS records as small as possible without sacrificing the
security properties of the public key. The algorithm type and key security properties of the public key. The algorithm type and key
size of OpenPGP keys should not be modified to accommodate this size of OpenPGP keys should not be modified to accommodate this
section. section.
OpenPGP supports various attributes that do not contribute to the OpenPGP supports various attributes that do not contribute to the
security of a key, such as an embedded image file. It is recommended security of a key, such as an embedded image file. It is recommended
that these properties are not exported to OpenPGP public keyrings that these properties are not exported to OpenPGP public keyrings
that are used to create OPENPGPKEY Resource Records. Some OpenPGP that are used to create OPENPGPKEY Resource Records. Some OpenPGP
software, for example GnuPG, have support for a "minimal key export" software, for example GnuPG, have support for a "minimal key export"
that is well suited to use as OPENPGPKEY RDATA. See Appendix A. that is well suited to use as OPENPGPKEY RDATA. See Appendix A.
6. Security Considerations 7. Security Considerations
OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE]. OPENPGPKEY usage considerations are published in [OPENPGPKEY-USAGE].
6.1. Response size 7.1. Response size
To prevent amplification attacks, an Authoritative DNS server MAY To prevent amplification attacks, an Authoritative DNS server MAY
wish to prevent returning OPENPGPKEY records over UDP unless the wish to prevent returning OPENPGPKEY records over UDP unless the
source IP address has been verified with [DNS-COOKIES]. Such servers source IP address has been verified with [DNS-COOKIES]. Such servers
MUST NOT return REFUSED, but answer the query with an empty Answer MUST NOT return REFUSED, but answer the query with an empty Answer
Section and the truncation flag set ("TC=1"). Section and the truncation flag set ("TC=1").
6.2. Email address information leak 7.2. 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 The hashing of the user name in this document is not a security
feature. Publishing OPENPGPKEY records however, will create a list feature. Publishing OPENPGPKEY records however, will create a list
of hashes of valid email addresses, which could simplify obtaining a of hashes of valid email addresses, which could simplify obtaining a
list of valid email addresses for a particular domain. It is list of valid email addresses for a particular domain. It is
desirable to not ease the harvesting of email addresses where desirable to not ease the harvesting of email addresses where
possible. possible.
The domain name part of the email address is not used as part of the 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 hash so that hashes can be used in multiple zones deployed using
DNAME [RFC6672]. This does makes it slightly easier and cheaper to DNAME [RFC6672]. This does makes it slightly easier and cheaper to
brute-force the SHA2-224 hashes into common and short user names, as 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 single rainbow tables can be re-used across domains. This can be
somewhat countered by using NSEC3. somewhat countered by using NSEC3.
DNS zones that are signed with DNSSEC using NSEC for denial of DNS zones that are signed with DNSSEC using NSEC for denial of
existence are susceptible to zone-walking, a mechanism that allows existence are susceptible to zone-walking, a mechanism that allows
someone to enumerate all the OPENPGPKEY hashes in a zone. This can someone to enumerate all the OPENPGPKEY hashes in a zone. This can
be used in combination with previously hashed common or short user be used in combination with previously hashed common or short user
names (in rainbow tables) to deduce valid email addresses. DNSSEC- names (in rainbow tables) to deduce valid email addresses. DNSSEC-
signed zones using NSEC3 for denial of existence instead of NSEC are signed zones using NSEC3 for denial of existence instead of NSEC are
significantly harder to brute-force after performing a zone-walk. significantly harder to brute-force after performing a zone-walk.
6.3. Storage of OPENPGPKEY data 7.3. Storage of OPENPGPKEY data
Users may have a local key store with OpenPGP public keys. An Users may have a local key store with OpenPGP public keys. An
application supporting the use of OPENPGPKEY DNS records MUST NOT application supporting the use of OPENPGPKEY DNS records MUST NOT
modify the local key store without explicit confirmation of the user, modify the local key store without explicit confirmation of the user,
as the application is unaware of the user's personal policy for as the application is unaware of the user's personal policy for
adding, removing or updating their local key store. An application adding, removing or updating their local key store. An application
MAY warn the user if an OPENPGPKEY record does not match the OpenPGP MAY warn the user if an OPENPGPKEY record does not match the OpenPGP
public key in the local key store. public key in the local key store.
OpenPGP public keys obtained via OPENPGPKEY records should not be Applications that do not have users associated with, such as daemon
stored beyond their DNS TTL value. 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.
6.4. Forward security of OpenPGP versus DNSSEC 7.4. Forward security of OpenPGP versus DNSSEC
DNSSEC key sizes are chosen based on the fact that these keys can be 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 rolled with next to no requirement for security in the future. If
one doubts the strength or security of the DNSSEC key for whatever one doubts the strength or security of the DNSSEC key for whatever
reason, one simply rolls to a new DNSSEC key with a stronger reason, one simply rolls to a new DNSSEC key with a stronger
algorithm or larger key size. On the other hand, OpenPGP key sizes algorithm or larger key size. On the other hand, OpenPGP key sizes
are chosen based on how many years (or decades) their encryption are chosen based on how many years (or decades) their encryption
should remain unbreakable by adversaries that own large scale should remain unbreakable by adversaries that own large scale
computational resources. computational resources.
skipping to change at page 8, line 42 skipping to change at page 10, line 5
delegated child zones, irrespective of the key size of the OpenPGP delegated child zones, irrespective of the key size of the OpenPGP
keypair. Any future messages encrypted with the malicious OpenPGP keypair. Any future messages encrypted with the malicious OpenPGP
key could then be read. key could then be read.
Therefore, an OpenPGP key obtained via an OPENPGPKEY record can only 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 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 substitute for in-person key verification of the "Web of Trust". See
[OPENPGPKEY-USAGE] for more in-depth information on safe usage of [OPENPGPKEY-USAGE] for more in-depth information on safe usage of
OPENPGPKEY based OpenPGP keys. OPENPGPKEY based OpenPGP keys.
7. IANA Considerations 8. IANA Considerations
7.1. OPENPGPKEY RRtype 8.1. OPENPGPKEY RRtype
This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has This document uses a new DNS RR type, OPENPGPKEY, whose value 61 has
been allocated by IANA from the Resource Record (RR) TYPEs been allocated by IANA from the Resource Record (RR) TYPEs
subregistry of the Domain Name System (DNS) Parameters registry. subregistry of the Domain Name System (DNS) Parameters registry.
8. Acknowledgments 9. Acknowledgments
This document is based on RFC-4255 and draft-ietf-dane-smime whose This document is based on RFC-4255 and draft-ietf-dane-smime whose
authors are Paul Hoffman, Jacob Schlyter and W. Griffin. Olafur authors are Paul Hoffman, Jacob Schlyter and W. Griffin. Olafur
Gudmundsson provided feedback and suggested various improvements. Gudmundsson provided feedback and suggested various improvements.
Willem Toorop contributed the gpg and hexdump command options. Edwin Willem Toorop contributed the gpg and hexdump command options.
Taylor contributed language improvements for various iterations of Daniel Kahn Gillmor provided the text describing the OpenPGP packet
this document. Text regarding email mappings was taken from draft- formats and filtering options. Edwin Taylor contributed language
levine-dns-mailbox whose author is John Levine. improvements for various iterations of this document. Text regarding
email mappings was taken from draft-levine-dns-mailbox whose author
is John Levine.
9. References 10. References
9.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005. 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions", Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005. RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005. Extensions", RFC 4035, March 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006. 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. [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880, November 2007. 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 [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010. Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
2010, <http://www.rfc-editor.org/info/rfc5754>.
9.2. Informative References 10.2. Informative References
[DNS-COOKIES] [DNS-COOKIES]
Eastlake, Donald., "Domain Name System (DNS) Cookies", Eastlake, Donald., "Domain Name System (DNS) Cookies",
draft-ietf-dnsop-cookies (work in progress), February draft-ietf-dnsop-cookies (work in progress), August 2015.
2015.
[HKP] Shaw, D., "The OpenPGP HTTP Keyserver Protocol (HKP)",
draft-shaw-openpgp-hkp (work in progress), March 2013.
[OPENPGPKEY-USAGE] [OPENPGPKEY-USAGE]
Wouters, P., "Usage considerations with the DNS OPENPGPKEY Wouters, P., "Usage considerations with the DNS OPENPGPKEY
record", draft-dane-openpgpkey-usage (work in progress), record", draft-dane-openpgpkey-usage (work in progress),
October 2014. October 2014.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997. Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<http://www.rfc-editor.org/info/rfc2181>.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822, DOI
2001. 10.17487/RFC2822, April 2001,
<http://www.rfc-editor.org/info/rfc2822>.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003. (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
2003, <http://www.rfc-editor.org/info/rfc3597>.
[RFC5233] Murchison, K., "Sieve Email Filtering: Subaddress [RFC5233] Murchison, K., "Sieve Email Filtering: Subaddress
Extension", RFC 5233, January 2008. Extension", RFC 5233, DOI 10.17487/RFC5233, January 2008,
<http://www.rfc-editor.org/info/rfc5233>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008. DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for [RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012. 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 [RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, June 2012. 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 [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS) of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012. Protocol: TLSA", RFC 6698, August 2012.
[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
Existence in the DNS", RFC 7129, February 2014. Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
February 2014, <http://www.rfc-editor.org/info/rfc7129>.
Appendix A. Generating OPENPGPKEY records Appendix A. Generating OPENPGPKEY records
The commonly available GnuPG software can be used to generate the The commonly available GnuPG software can be used to generate a
RRdata portion of an OPENPGPKEY record: minimum Transferable Public Key for the RRdata portion of an
OPENPGPKEY record:
gpg --export --export-options export-minimal \ gpg --export --export-options export-minimal,no-export-attributes \
hugh@example.com | base64 hugh@example.com | base64
The --armor or -a option of the gpg command should NOT be used, as it The --armor or -a option of the gpg command should NOT be used, as it
adds additional markers around the armored key. adds additional markers around the armored key.
When DNS software reading or signing the zone file does not yet When DNS software reading or signing the zone file does not yet
support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597] support the OPENPGPKEY RRtype, the Generic Record Syntax of [RFC3597]
can be used to generate the RDATA. One needs to calculate the number can be used to generate the RDATA. One needs to calculate the number
of octets and the actual data in hexadecimal: of octets and the actual data in hexadecimal:
gpg --export --export-options export-minimal \ gpg --export --export-options export-minimal,no-export-attributes \
hugh@example.com | wc -c hugh@example.com | wc -c
gpg --export --export-options export-minimal \ gpg --export --export-options export-minimal,no-export-attributes \
hugh@example.com | hexdump -e \ hugh@example.com | hexdump -e \
'"\t" /1 "%.2x"' -e '/32 "\n"' '"\t" /1 "%.2x"' -e '/32 "\n"'
These values can then be used to generate a generic record (line These values can then be used to generate a generic record (line
break has been added for formatting): break has been added for formatting):
<SHA2-256-trunc(hugh)>._openpgpkey.example.com. IN TYPE61 \# \ <SHA2-256-trunc(hugh)>._openpgpkey.example.com. IN TYPE61 \# \
<numOctets> <keydata in hex> <numOctets> <keydata in hex>
The openpgpkey command in the hash-slinger software can be used to The openpgpkey command in the hash-slinger software can be used to
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