< draft-ietf-dane-openpgpkey-06.txt   draft-ietf-dane-openpgpkey-07.txt >
Network Working Group P. Wouters Network Working Group P. Wouters
Internet-Draft Red Hat Internet-Draft Red Hat
Intended status: Experimental October 20, 2015 Intended status: Experimental January 27, 2016
Expires: April 22, 2016 Expires: July 30, 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-06 draft-ietf-dane-openpgpkey-07
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 and public keys. DNS-Based Authentication of Named Entities ("DANE") is
locating OpenPGP public keys in DNS for a specific email address a method for publishing public keys in DNS. This document specifies
using a new OPENPGPKEY DNS Resource Record. Security is provided via a DANE method for publishing and locating OpenPGP public keys in DNS
Secure DNS, however the OPENPGPKEY record is not a replacement for for a specific email address using a new OPENPGPKEY DNS Resource
verification of authenticity via the "Web of Trust" or manual Record. Security is provided via Secure DNS, however the OPENPGPKEY
verification. The OPENPGPKEY record can be used to encrypt an email record is not a replacement for verification of authenticity via the
that would otherwise have to be send unencrypted. "Web of Trust" or manual verification. The OPENPGPKEY record can be
used to encrypt an email that would otherwise have to be send
unencrypted.
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.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 22, 2016. This Internet-Draft will expire on July 30, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Experiment goal . . . . . . . . . . . . . . . . . . . . . 3 1.1. Experiment goal . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4 2. The OPENPGPKEY Resource Record . . . . . . . . . . . . . . . 4
2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4 2.1. The OPENPGPKEY RDATA component . . . . . . . . . . . . . 4
2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 5 2.1.1. The OPENPGPKEY RDATA content . . . . . . . . . . . . 4
2.1.2. Reducing the Transferable Public Key size . . . . . . 5 2.1.2. Reducing the Transferable Public Key size . . . . . . 5
2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 6 2.2. The OPENPGPKEY RDATA wire format . . . . . . . . . . . . 6
2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 6 2.3. The OPENPGPKEY RDATA presentation format . . . . . . . . 6
3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 6 3. Location of the OPENPGPKEY record . . . . . . . . . . . . . . 6
4. Email address variants . . . . . . . . . . . . . . . . . . . 7 4. Email address variants . . . . . . . . . . . . . . . . . . . 7
5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 7 5. Application use of OPENPGPKEY . . . . . . . . . . . . . . . . 7
5.1. Obtaining an OpenPGP key for a specific email address . . 8 5.1. Obtaining an OpenPGP key for a specific email address . . 7
5.2. Confirming the validity of an OpenPGP key . . . . . . . . 8 5.2. Confirming that an OpenPGP key is current . . . . . . . . 8
5.3. Public Key UIDs and query names . . . . . . . . . . . . . 8 5.3. Public Key UIDs and query names . . . . . . . . . . . . . 8
6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 9 6. OpenPGP Key size and DNS . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7.1. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 10 7.1. MTA behaviour . . . . . . . . . . . . . . . . . . . . . . 10
7.2. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 11 7.2. MUA behaviour . . . . . . . . . . . . . . . . . . . . . . 10
7.3. Email client behaviour . . . . . . . . . . . . . . . . . 11 7.3. Email client behaviour . . . . . . . . . . . . . . . . . 11
7.4. Response size . . . . . . . . . . . . . . . . . . . . . . 11 7.4. Response size . . . . . . . . . . . . . . . . . . . . . . 11
7.5. Email address information leak . . . . . . . . . . . . . 11 7.5. Email address information leak . . . . . . . . . . . . . 11
7.6. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 12 7.6. Storage of OPENPGPKEY data . . . . . . . . . . . . . . . 12
7.7. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 12 7.7. Security of OpenPGP versus DNSSEC . . . . . . . . . . . . 12
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 12 8. Implementation Status . . . . . . . . . . . . . . . . . . . . 12
8.1. The GNU Privacy Guard (GNUpg) . . . . . . . . . . . . . . 13 8.1. The GNU Privacy Guard (GNUpg) . . . . . . . . . . . . . . 13
8.2. hash-slinger . . . . . . . . . . . . . . . . . . . . . . 14 8.2. hash-slinger . . . . . . . . . . . . . . . . . . . . . . 13
8.3. openpgpkey-milter . . . . . . . . . . . . . . . . . . . . 14 8.3. openpgpkey-milter . . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 15 9.1. OPENPGPKEY RRtype . . . . . . . . . . . . . . . . . . . . 14
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . 15 11.1. Normative References . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . 16 11.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 17 Appendix A. Generating OPENPGPKEY records . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 18
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, or verify a messages and files. To encrypt an email message, or verify a
sender's OpenPGP signature, the email client or MTA needs to locate sender's OpenPGP signature, the email client or MTA needs to locate
the recipient's OpenPGP public key. the recipient's OpenPGP public key.
OpenPGP clients have relied on centralized "well-known" key servers OpenPGP clients have relied on centralized "well-known" key servers
that are accessed using either the HTTP Keyserver Protocol [HKP] that are accessed using either the HTTP Keyserver Protocol [HKP]
Alternatively, users need to manually browse a variety of different Alternatively, users need to manually browse a variety of different
front-end websites. These key servers do not validate the email front-end websites. These key servers do not require a confirmation
address in the User ID of the uploaded OpenPGP public key. Attackers of the email address used in the User ID of the uploaded OpenPGP
can - and have - uploaded rogue public keys with other people's email public key. Attackers can - and have - uploaded rogue public keys
addresses to these key servers. with other people's email addresses to these key servers.
Once uploaded, public keys cannot be deleted. People who did not Once uploaded, public keys cannot be deleted. People who did not
pre-sign a key revocation can never remove their OpenPGP public key pre-sign a key revocation can never remove their OpenPGP public key
from these key servers once they have lost access to their private from these key servers once they have lost access to their private
key. This results in receiving encrypted email that cannot be key. This results in receiving encrypted email that cannot be
decrypted. decrypted.
Therefor, these keyservers are not well suited to support email Therefor, these keyservers are not well suited to support email
clients and MTA's to automatically encrypt email - especially in the clients and MTA's to automatically encrypt email - especially in the
absence of an interactive user. absence of an interactive user.
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 the OPENPGPKEY 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 These records are published in the DNS zone of the user's email
address. If the user loses their private key, the OPENPGPKEY DNS address. If the user loses their private key, the OPENPGPKEY DNS
record can simply be updated or removed from the zone. record can simply be updated or removed from the zone.
The OPENPGPKEY data is secured using Secure DNS. The OPENPGPKEY data is secured using Secure DNS [RFC4035]
The main goal of the OPENPGPKEY resource record is to stop passive The main goal of the OPENPGPKEY resource record is to stop passive
attacks against plaintext emails. While it can also twart some attacks against plaintext emails. While it can also thwart some
active attacks (such as people uploading rogue keys to keyservers in active attacks (such as people uploading rogue keys to keyservers in
the hopes that others will encrypt to these rogue keys), this the hopes that others will encrypt to these rogue keys), this
resource record is not a replacement for verifying OpenPGP public resource record is not a replacement for verifying OpenPGP public
keys via the web of trust signatures, or manually via a fingerprint keys via the web of trust signatures, or manually via a fingerprint
verification. verification.
1.1. Experiment goal 1.1. Experiment goal
This document defines an Experimental RRtype. The goal of the This document defines an RRtype whose use is Experimental. The goal
experiment is to see whether encrypted email usage will increase if of the experiment is to see whether encrypted email usage will
an automated discovery method is available to MTA's and MUA's to help increase if an automated discovery method is available to MTA's and
the enduser with email encryption key management. MUA's to help the enduser with email encryption key management.
It is unclear if this RRtype will scale to some of the larger email 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 service deployments. Concerns have been raised about the size of the
OPENPGPKEY record and the size of the resulting DNS zone files. This OPENPGPKEY record and the size of the resulting DNS zone files. This
experiment hopefully will give the working group some insight into experiment hopefully will give the working group some insight into
whether this is a problem or not. whether this is a problem or not.
If the experiment is successful, it is expected that the findings of If the experiment is successful, it is expected that the findings of
the experiment will result in an updated document for standards track the experiment will result in an updated document for standards track
approval. approval.
The OPENPGPKEY RRtype somewhat resembles the generic CERT record The OPENPGPKEY RRtype somewhat resembles the generic CERT record
defined in [RFC4398]. However, the CERT record uses sub-typing with defined in [RFC4398]. However, the CERT record uses sub-typing with
many different types of keys and certificates. It is suspected that many different types of keys and certificates. It is suspected that
its generality of very different protocols (PKIX versus OpenPGP) has its general application of very different protocols (PKIX versus
been the cause for lack of implementation and deployment. OpenPGP) has been the cause for lack of implementation and
Furthermore, the CERT record uses sub-typing, which is now considered deployment. Furthermore, the CERT record uses sub-typing, which is
to be a bad idea for DNS. now considered to be a bad idea for DNS.
1.2. Terminology 1.2. 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.
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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 Transferable Public Key (see Section 11.1 of [RFC4880] entity OpenPGP Transferable Public Key (see Section 11.1 of [RFC4880]
with an email address, thus forming a "OpenPGP public key with an email address, thus forming a "OpenPGP public key
association". A user that wishes to specify more than one OpenPGP association". A user that wishes to specify more than one OpenPGP
key, for example because they are transitioning to a newer stronger key, for example because they are transitioning to a newer stronger
key, can do so by adding multiple OPENPGPKEY records. A single key, can do so by adding multiple OPENPGPKEY records. A single
OPENPGPKEY DNS record MUST only contain one OpenPGP key. 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.
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 Transferable Public Key. value consisting of a [RFC4880] formatted Transferable Public Key.
2.1.1. The OPENPGPKEY RDATA content 2.1.1. The OPENPGPKEY RDATA content
An OpenPGP Transferable Public Key can be arbitrarily large. DNS An OpenPGP Transferable Public Key can be arbitrarily large. DNS
records are limited in size. When creating OPENPGPKEY DNS records, records are limited in size. When creating OPENPGPKEY DNS records,
skipping to change at page 6, line 36 skipping to change at page 6, line 29
Public Key. 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 Transferable The RDATA Wire Format consists of a single OpenPGP Transferable
Public Key as defined in Section 11.1 of [RFC4880]. Note that this Public Key as defined in Section 11.1 of [RFC4880]. Note that this
format is 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 master files [RFC1035],
consists of a single OpenPGP Transferable Public Key as defined in consists of a single OpenPGP Transferable Public Key as defined in
Section 11.1 of [RFC4880] encoded in base64 as defined in Section 4 Section 11.1 of [RFC4880] encoded in base64 as defined in 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 [RFC5322] and in the "local-part" of email addresses as defined in [RFC5322] 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 [RFC5322] the "local-part" in the mail message format definition [RFC5322]
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]) is encoded in UTF-8 (or its subset ASCII). If
ASCII). If it is written in another encoding it should be the local-part is written in another encoding it MUST be converted
converted to UTF-8 and then hashed using the SHA2-256 [RFC5754] to UTF-8.
algorithm, with the hash truncated to 28 octets and represented in
its hexadecimal representation, to become the left-most label in o The local-part is hashed using the SHA2-256 [RFC5754] algorithm,
the prepared domain name. Truncation comes from the right-most with the hash truncated to 28 octets and represented in its
octets. This does not include the at symbol ("@") that separates hexadecimal representation, to become the left-most label in the
the left and right sides of the email address. prepared domain name.
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 5322) is appended to the result of step called the "domain" in [RFC5322]) is appended to the result of
2 to complete the prepared domain name. step 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>
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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. any kind of mapping rules based on the email address.
5. 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 an
verify an OpenPGP public key. The lookup result MUST pass DNSSEC OpenPGP public key and use it for verifying a digital signature or
validation; if validation reaches any state other than "Secure", the encrypting a message to the public key. The DNS answer MUST pass
verification MUST be treated as a failure. 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 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 MAY be performed to discover the OpenPGP public key OPENPGPKEY DNS lookup MAY be performed to seek the OpenPGP public key
that belongs to a specific email address. This public key can then that corresponds to that email address. This public key can then be
be used to verify a received signed message or can be used to send used to verify a received signed message or can be used to send out
out an encrypted email message. An application that confirms the an encrypted email message. An application whose attempt fails to
lack of an OPENPGPKEY record SHOULD remember this for some time to retrieve a DNSSEC verified OPENPGPKEY RR from the DNS should remember
avoid sending out a DNS request for each email message that is sent that failure for some time to avoid sending out a DNS request for
out as this constitutes a privacy leak. each email message the application is sending out; such DNS requests
constitute a privacy leak
5.2. Confirming the validity of an OpenPGP key 5.2. Confirming that an OpenPGP key is current
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 the locally stored OpenPGP public
public key and the OpenPGP public key found through DNS is different key is different from the DNSSEC validated OpenPGP public key
from the locally stored OpenPGP public key, the verification MUST be currently published in DNS, the verification MUST be treated as a
treated as a failure. An application that can interact with the user failure unless the locally stored OpenPGP key signed the newly
MAY ask the user for guidance. For privacy reasons, an application published OpenPGP public key found in DNS. An application that can
MUST NOT attempt to validate a locally stored OpenPGP key using an interact with the user MAY ask the user for guidance. For privacy
OPENPGPKEY lookup at every use of that key. reasons, an application MUST NOT attempt to lookup an OpenPGP key
from DNSSEC at every use of that key.
5.3. Public Key UIDs and query names 5.3. Public Key UIDs and query names
An OpenPGP public key can be associated with multiple email addresses An OpenPGP public key can be associated with multiple email addresses
by specifying multiple key uids. The OpenPGP public key obtained by specifying multiple key uids. The OpenPGP public key obtained
from a OPENPGPKEY RR can be used as long as the query and resulting from a OPENPGPKEY RR can be used as long as the query and resulting
data form a proper email to uid identity association. data form a proper email to uid identity association.
CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed CNAME's (see [RFC2181]) and DNAME's (see [RFC6672]) can be followed
to obtain an OPENPGPKEY RR, as long as the original recipient's email to obtain an OPENPGPKEY RR, as long as the original recipient's email
skipping to change at page 9, line 4 skipping to change at page 8, line 50
8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for 8d57[...]b7._openpgpkey.example.net, and an OPENPGPKEY RR for
8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public 8d57[...]b7._openpgpkey.example.net exists, then this OpenPGP public
key can be used, provided one of the key uids contains key can be used, provided one of the key uids contains
"hugh@example.com". This public key cannot be used if it would only "hugh@example.com". This public key cannot be used if it would only
contain the key uid "hugh@example.net". contain the key uid "hugh@example.net".
If one of the OpenPGP key uids contains only a single wildcard as the If one of the OpenPGP key uids contains only a single wildcard as the
LHS of the email address, such as "*@example.com", the OpenPGP public LHS of the email address, such as "*@example.com", the OpenPGP public
key may be used for any email address within that domain. Wildcards key may be used for any email address within that domain. Wildcards
at other locations (eg hugh@*.com) or regular expressions in key uids at other locations (eg hugh@*.com) or regular expressions in key uids
are not allowed, and any OPENPGPKEY RR containing these should be are not allowed, and any OPENPGPKEY RR containing these MUST be
ignored. ignored.
6. 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, applications
SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY SHOULD use TCP - not UDP - to perform queries for the OPENPGPKEY
Resource Record. 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 not be exported to OpenPGP public keyrings that
that are used to create OPENPGPKEY Resource Records. Some OpenPGP are used to create OPENPGPKEY Resource Records. Some OpenPGP
software, for example GnuPG, have support for a "minimal key export" software, for example GnuPG, support a "minimal key export" that is
that is well suited to use as OPENPGPKEY RDATA. See Appendix A. well suited to use as OPENPGPKEY RDATA. See Appendix A.
7. Security Considerations 7. Security Considerations
DNSSEC is not an alternative for the "web of trust" or for manual DNSSEC is not an alternative for the "web of trust" or for manual
fingerprint verification by humans. It is a solution aimed to ease fingerprint verification by users. DANE for OpenPGP as specified in
obtaining someone's public key, and without manual verification this document is a solution aimed to ease obtaining someone's public
should be treated as "better then plaintext" only. While this twarts key. Without manual verification of the OpenPGP key obtained via
all passive attacks that simply capture and log all plaintext email DANE, this retrieved key should only be used for encryption if the
content, it is not a security measure against active attacks. A user only other alternative is sending the message in plaintext. While
who publishes an OPENPGPKEY record in DNS still expects senders to this thwarts all passive attacks that simply capture and log all
perform their due diligence by additional verification of their plaintext email content, it is not a security measure against active
public key via other out-of-band methods before sending any attacks. A user who publishes an OPENPGPKEY record in DNS still
confidential or sensitive information. 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 In other words, the OPENPGPKEY record MUST NOT be used to send
sensitive information without additional verification or confirmation sensitive information without additional verification or confirmation
that the OpenPGP key actually belongs to the target recipient. that the OpenPGP key actually belongs to the target recipient.
Various components could be responsible for encrypting an email Various components could be responsible for encrypting an email
message to a target recipient. It could be done by the sender's message to a target recipient. It could be done by the sender's
email client or software plugin, the sender's Mail User Agent (MUA) email client or software plugin, the sender's Mail User Agent (MUA)
or the sender's Mail Transfer Agent (MTA). Each of these have their or the sender's Mail Transfer Agent (MTA). Each of these have their
own characteristics. An email client can direct the human to make a own characteristics. An email client can ask the user to make a
decision before continuing. The MUA can either accept or refuse 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. The MTA must deliver the message as-is, or encrypt the
message before delivering. Each of these programs should attempt to message before delivering. Each of these programs should attempt to
encrypt an unencrypted received message whenever possible. encrypt an unencrypted received 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 negliable
chance of happening.
Organisations that are required to be able to read everyone's Organisations that are required to be able to read everyone's
encrypted email should publish the escrow key as the OPENPGPKEY encrypted email should publish the escrow key as the OPENPGPKEY
record. Upon receipt, such mail servers MAY optionally re-encrypt record. Mail servers of such organizations MAY optionally re-encrypt
the message to the individual's OpenPGP key. the message to the individual's OpenPGP key.
7.1. MTA behaviour 7.1. MTA behaviour
An MTA could be operating in a stand-alone mode, without access to 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 sender's OpenPGP public keyring, or in a way where it can access
the user's OpenPGP public keyring. Regardless, the MTA MUST NOT the user's OpenPGP public keyring. Regardless, the MTA MUST NOT
modify the user's OpenPGP keyring. modify the user's OpenPGP keyring.
An MTA sending an email MUST NOT add the public key obtained from an An MTA sending an email MUST NOT add the public key obtained from an
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use beyond the TTL. use beyond the TTL.
If the obtained public key is revoked, the MTA MUST NOT use the key 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 for encryption, even if that would result in sending the message in
plaintext. plaintext.
If a message is already encrypted, the MTA SHOULD NOT re-encrypt the If a message is already encrypted, the MTA SHOULD NOT re-encrypt the
message, even if different encryption schemes or different encryption message, even if different encryption schemes or different encryption
keys would be used. keys would be used.
If the DNS request for an OPENPGPKEY record returned an If the DNS request for an OPENPGPKEY record returned an Indeterminate
"indeterminate" or "bogus" answer, the MTA MUST NOT sent the message or Bogus answer as specified in [RFC4035], the MTA MUST NOT send the
and queue the plaintext message for encrypted delivery at a later message and queue the plaintext message for encrypted delivery at a
time. If the problem persists, the email should be returned via the later time. If the problem persists, the email should be returned
regular bounce methods. via the regular bounce methods.
If multiple non-revoked OPENPGPKEY resource records are found, the If multiple non-revoked OPENPGPKEY resource records are found, the
MTA SHOULD pick the most secure RR based on its local policy. MTA SHOULD pick the most secure RR based on its local policy.
7.2. MUA behaviour 7.2. MUA behaviour
If the public key for a recipient obtained from the locally stored If the public key for a recipient obtained from the locally stored
sender's public keyring differs from the recipient's OPENPGPKEY RR, sender's public keyring differs from the recipient's OPENPGPKEY RR,
the MUA MUST NOT accept the message for delivery. the MUA MUST NOT accept the message for delivery.
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policy. policy.
7.3. Email client behaviour 7.3. Email client behaviour
Email clients should adhere to the above listed MUA behaviour. Email clients should adhere to the above listed MUA behaviour.
Additionally, an email client MAY interact with the user to resolve Additionally, an email client MAY interact with the user to resolve
any conflicts between locally stored keyrings and OPENPGPKEY RRdata. any conflicts between locally stored keyrings and OPENPGPKEY RRdata.
An email client that is encrypting a message SHOULD clearly indicate An email client that is encrypting a message SHOULD clearly indicate
to the user the difference between encrypting to a locally stored and to the user the difference between encrypting to a locally stored and
humanly verified public key and encrypting to an unverified (by the user verified public key and encrypting to an unverified public key
human sender) public key obtained via an OPENPGPKEY resource record. obtained via an OPENPGPKEY resource record.
7.4. Response size 7.4. 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 [EDNS-COOKIE]. Such servers source IP address has been confirmed with [EDNS-COOKIE]. Such
MUST NOT return REFUSED, but answer the query with an empty Answer servers MUST NOT return REFUSED, but answer the query with an empty
Section and the truncation flag set ("TC=1"). Answer Section and the truncation flag set ("TC=1").
7.5. Email address information leak 7.5. Email address information leak
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.
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to a particular user. to a particular user.
7.7. Security of OpenPGP versus DNSSEC 7.7. Security of OpenPGP versus DNSSEC
Anyone who can obtain a DNSSEC private key of a domain name via Anyone who can obtain a DNSSEC private key of a domain name via
coercion, theft or brute force calculations, can replace any coercion, theft or brute force calculations, can replace any
OPENPGPKEY record in that zone and all of the delegated child zones. OPENPGPKEY record in that zone and all of the delegated child zones.
Any future messages encrypted with the malicious OpenPGP key could Any future messages encrypted with the malicious OpenPGP key could
then be read. then be read.
Therefore, an OpenPGP key obtained via an OPENPGPKEY record can only Therefore, an OpenPGP key obtained via a DNSSEC validated OPENPGPKEY
be trusted as much as the DNS domain can be trusted, and is no record can only be trusted as much as the DNS domain can be trusted,
substitute for in-person key verification or verification via the and is no substitute for in-person OpenPGP key verification or
"Web of Trust". additional Openpgp verification via "Web of Trust" signatures present
on the OpenPGP in question.
8. Implementation Status 8. Implementation Status
[RFC Editor Note: Please remove this entire seciton prior to [RFC Editor Note: Please remove this entire seciton prior to
publication as an RFC.] publication as an RFC.]
This section records the status of known implementations of the This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982]. Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their be construed to be, a catalog of available implementations or their
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Implementation Experience: Currrent experience limited to small test Implementation Experience: Currrent experience limited to small test
networks only networks only
Contact Information: https://gnupg.org/ Contact Information: https://gnupg.org/
Interoperability: No report. Interoperability: No report.
8.2. hash-slinger 8.2. hash-slinger
Implementation Name and Details: The hash-slinger software is a Implementation Name and Details: The hash-slinger software is a
collection of tools to generate and verify application DNS records collection of tools to generate, download and verify application
written by the author of this document. It is available at http:/ public keys and application fingerprints. It uses DNSSEC
/people.redhat.com/pwouters/ validation. The tool is written by the author of this document.
It is available at http://people.redhat.com/pwouters/
Brief Description: Support has been added in the form of an Brief Description: Support has been added in the form of an
"openpgpkey" command that can generate, fetch and verify "openpgpkey" command that can generate, fetch, validate the DNSSEC
OPENPGPKEY records. authentication and verify OPENPGPKEY records.
Level of Maturity: The implementation has been around for a few Level of Maturity: The implementation has been around for a few
months but has not seen widespread deployment. months but has not seen widespread deployment.
Coverage: The implementation follows the latest draft with the Coverage: The implementation follows the latest draft with the
exception that it first performs a lowercase of the local-part exception that it first performs a lowercase of the local-part
before hashing. before hashing.
Licensing: All code is covered under the GNU Public License version Licensing: All code is covered under the GNU Public License version
3 or later. 3 or later.
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Implementation Experience: Currrent experience limited to small test Implementation Experience: Currrent experience limited to small test
networks only networks only
Contact Information: pwouters@redhat.com Contact Information: pwouters@redhat.com
Interoperability: No report. Interoperability: No report.
9. IANA Considerations 9. IANA Considerations
9.1. OPENPGPKEY RRtype 9.1. 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.
10. Acknowledgments 10. 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. Willem Toorop contributed the gpg and hexdump command options.
Daniel Kahn Gillmor provided the text describing the OpenPGP packet Daniel Kahn Gillmor provided the text describing the OpenPGP packet
formats and filtering options. Edwin Taylor contributed language formats and filtering options. Edwin Taylor contributed language
improvements for various iterations of this document. Text regarding improvements for various iterations of this document. Text regarding
email mappings was taken from draft-levine-dns-mailbox whose author email mappings was taken from draft-levine-dns-mailbox whose author
is John Levine. is John Levine.
11. References 11. References
11.1. Normative References 11.1. 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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997, RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<http://www.rfc-editor.org/info/rfc2181>. <http://www.rfc-editor.org/info/rfc2181>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
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