| Rfc | 7929 |
|---|
| Title | DNS-Based Authentication of Named Entities (DANE) Bindings for
OpenPGP |
|---|
| Author | P. Wouters |
|---|
| Date | August 2016 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | EXPERIMENTAL |
|---|
|
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.
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.
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
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
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
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)
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].
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>
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.
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,
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
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.
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.
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.
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>.
[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>.
[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/>.
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[...]
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
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