DKIM Working Group D. Otis
Internet-Draft Trend Micro
Intended status: Standards Track D. Black
Expires: December 29, 2010 June 27, 2010
DKIM Third-Party Authorization Label
draft-otis-dkim-tpa-label-05
Abstract
A third party authorization label (TPA-Label) is a DNS-based
extension for DKIM ADSP records that allow domains in the From header
to authorize acceptable third-party signatures. This approach allows
autonomous and unilateral authorizations for a range of third-party
domains using scalable, individual DNS transactions. The extended
scope of DKIM signing practice assertions supplants more difficult to
administer transparent authorization schemes. Alternatives for
facilitating third-party authorizations currently necessitate
coordination between two or more domains to synchronously set up
selector/key DNS records, DNS zone delegations, and/or a regular
exchange of public/private keys.
Checking TPA-Label Resource Records for signing practices may
infrequently occur when a message is not compliant with restrictive
ADSP policies, where an Author Domain Signature is either missing or
invalid. When a third-party signature is found, TPA-Label Resource
Record transactions offer an efficient means for Author Domains to
authorize specific third-party signing domains. Recipients are
afforded a method to determine whether authorization exists in
situations where other modes of authorization are impractical. TPA-
Label Resource Records permit Author Domains a means to selectively
influence message handling, for messages otherwise lacking valid
Author Domain signatures.
Requirements Language
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 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Otis & Black Expires December 29, 2010 [Page 1]
Internet-Draft TPA-Label June 2010
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 29, 2010.
Copyright Notice
Copyright (c) 2010 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.
Otis & Black Expires December 29, 2010 [Page 2]
Internet-Draft TPA-Label June 2010
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 6
2.1. Terms Imported from other DKIM Specifications: . . . . . . 6
2.2. Terms Defined by this Specification: . . . . . . . . . . . 7
2.2.1. Third Party Signature . . . . . . . . . . . . . . . . 7
2.2.2. Third Party Signer . . . . . . . . . . . . . . . . . . 7
2.2.3. TPA-Label Listed Domain, TPA-LLD . . . . . . . . . . . 7
2.2.4. Author's Domain Acceptable Third-Party Signature . . . 7
2.2.5. Author's Domain Acceptable Third-Party Service . . . . 8
3. Combinatorial ADSP "dkim=" Values. . . . . . . . . . . . . . . 8
3.1. tpa-sig . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. tpa-path . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. TPA-Label Resource Record Authorization Considerations . . . . 9
5. Evaluating the Third-party Signing Domain or Service . . . . . 10
5.1. Third Party Authentication . . . . . . . . . . . . . . . . 10
5.1.1. Third Party Authentication - Web Email Provider
with Subscriber Pingbacks . . . . . . . . . . . . . . 10
5.1.2. Third Party Authentication - Closed Mailing List
Example . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.3. Third Party Authentication - Open Mailing List
Example . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.4. Third Party Authentication Example - Sender Header
Field . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.5. Services Lacking DKIM Signatures . . . . . . . . . . . 12
6. DNS Representation . . . . . . . . . . . . . . . . . . . . . . 13
7. TPA-Label and Tag Syntax Definitions . . . . . . . . . . . . . 14
8. TPA-Label Generation . . . . . . . . . . . . . . . . . . . . . 15
9. TPA-Label TXT Resource Record Structure . . . . . . . . . . . 15
9.1. TPA-Label Resource Record Scope Syntax . . . . . . . . . . 16
9.1.1. TPA-Label Listed Domain Authorization . . . . . . . . 16
9.1.2. Header Dependent Authorizations . . . . . . . . . . . 16
9.1.3. MailFrom Parameter . . . . . . . . . . . . . . . . . . 16
9.1.4. SMTP Host domains . . . . . . . . . . . . . . . . . . 17
10. Authorized Signing Domain . . . . . . . . . . . . . . . . . . 17
11. TPA-Label Resource Record Query Transactions . . . . . . . . . 17
12. TPA-Label Resource Record Compliance Assessment . . . . . . . 17
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
13.1. Author Domain Signing Practices (ADSP) Parameters . . . . 19
13.2. Email Authentication Method Registry . . . . . . . . . . . 19
13.3. Email Authentication Result Names Registry . . . . . . . . 21
13.4. Third Party Authorizations Labels Registry . . . . . . . . 21
13.5. Third Party Authorizations Scope Registry . . . . . . . . 22
14. Security Considerations . . . . . . . . . . . . . . . . . . . 23
14.1. Benefits to Recipients . . . . . . . . . . . . . . . . . . 23
14.2. Risks to Recipients . . . . . . . . . . . . . . . . . . . 23
14.3. Benefits to Author Domains . . . . . . . . . . . . . . . . 24
Otis & Black Expires December 29, 2010 [Page 3]
Internet-Draft TPA-Label June 2010
14.4. Risks to Author Domains . . . . . . . . . . . . . . . . . 24
14.5. Benefits to Third Party Signers . . . . . . . . . . . . . 25
14.6. Risks caused by Third Party Signers . . . . . . . . . . . 25
14.7. SHA-1 Collisions . . . . . . . . . . . . . . . . . . . . . 25
14.8. DNS Limits . . . . . . . . . . . . . . . . . . . . . . . . 26
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
16.1. Normative References . . . . . . . . . . . . . . . . . . . 27
16.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. DNS Example of TPA-Label Resource Record placement . 28
Appendix B. C code for label generation . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35
Otis & Black Expires December 29, 2010 [Page 4]
Internet-Draft TPA-Label June 2010
1. Introduction
A transparent method for DKIM authorization represents sharing a
number of details between the domain owner, and one or more providers
of email and DNS. Since there are many ways in which such
authorizations could be accomplished, it is unlikely standardized
formats will be developed to exchange necessary, and at times,
sensitive information. In addition, when there is a security breach
and authorization is transparent, the wrong party might be held
accountable for content they may have never seen nor logged. The
TPA-Label Resource Record supports a simple authorization method that
keeps visible which administrative entity signed a message, and
whether an Author Domain authorized the signature. The authorization
record may also impose additional header requirements.
Tens of thousands of domains of various financial institutions are
frequently being phished. Phishing creates a nuisance for those who
aren't expecting these messages, and a threat for those who then
interact with them. Whenever institutions employ DKIM and utilize
various third-party services, the integrity of their Author Domain
Signature might be affected. Some assert less stringent Author
Domain Signing Policies on subdomains to accommodate the affect of
third-party services, as suggested by [I-D.ietf-dkim-mailinglists]
section 4.1, that recommends use of subdomains to assert less
restrictive ADSP policies.
As currently structured, ADSP does not offer an alternative to using
more domains, where only some are protected by a restrictive policy.
It lacks a simple method to retain an authentication and
authorization acceptance condition while using third party services.
This limitation forces use of a strategy that increases those being
deceived by phishing attempts. This is because people often do not
understand the significance of URI hierarchy, and become confused or
insensitive to domain changes. APWG phishing trends,
[apwg-globalphishingsurvey-2H2009] page 18, indicates phishing
commonly uses subdomains in a URL to fool potential victims.
Deterrents that utilize some authorized originating header are
ineffective. These headers often remain invisible to recipients, and
contain domains being exploited for periods measured in hours, in an
avoidance of a Wack-A-Mole like response. Even long term reputations
remain problematic due to the intermix of messages from compromised
accounts. Few recipients inspect the stack of message headers, or
are able to draw useful conclusions from a profusion of unfriendly
information. However, many recipients deal with abuse by sorting
messages into groups based upon an assumed source found in a few
headers.
Otis & Black Expires December 29, 2010 [Page 5]
Internet-Draft TPA-Label June 2010
Retaining authentication and authorization for the From, Sender, and
List-ID headers, and ensuring third-party inclusion of a Sender or
List-ID header, enhances protections afforded from source sorting to
reduce susceptibility to being deceived by look-alike phishing
attempts. However, when subdomains assert less stringent policies
that omit authentication, these messages might be combined with those
having more stringent policies when sorting is based upon parent
domains. Consistently using the same domain avoids confusion that
might otherwise be exploited to deceive recipients.
ADSP represents an open registry which offers domain specific
guidance for DKIM acceptance criteria, when determining whether
messages should be delivered, refused or discarded. However,
appropriate actions become unclear whenever third-party services are
involved. For example, it is not clear whether ADSP "dkim=all"
assertions include third-party services that could potentially damage
Author-Domain signatures. Although ADSP warns of a potential for
disruption, specific handling recommendations are limited to
"dkim=discardable". Administrative domains that assert all of their
outbound messages are signed offer significant forensic value.
However, the handling for their messages lacking an Author Domain
Signature with an ADSP "dkim=all" assertion remains unclear.
This document describes how any Author Domain publishing ADSP records
defined in [RFC5617], can autonomously authorize DKIM signatures
[RFC4871] (updated by [RFC5672]) by specific domains. TPA-Label
listed domains offer secondary signing practices for additional ADSP
compliance options, whenever no Author Domain Signature is present
within the message. The intended purpose of TPA-Label Resource
Records is to improve acceptance rates of genuine messages, to
minimize domain use, to minimize success rates for phishing, and to
minimize a recipient's administrative costs.
TPA-Label Resource Records authorize third-party signing domains and
services to extend DKIM compliance options for signing practices
defined by [RFC5617]. TPA-Label listed domains, TPA-LLD, are to be
considered equivalent to the authorizing Author Domain when assessing
compliance with ADSP. The TXT resource records, associated with TPA-
Label, start with the 'dkim' tag as defined by [RFC5617] for signing
practices, and may contain tags specifically defined for TPA-Label
Resource Records.
2. Language and Terminology
2.1. Terms Imported from other DKIM Specifications:
A "Valid Signature" is any signature on a message that correctly
Otis & Black Expires December 29, 2010 [Page 6]
Internet-Draft TPA-Label June 2010
verifies using the procedure described in Section 6.1 of
[RFC4871].
"Author Address" is defined in Section 2.3 of [RFC5617].
"Author Domain" is defined in section 2.4 of [RFC5617].
"Alleged Author" is defined in Section 2.5 of [RFC5617].
"Author Domain Signature" is defined in Section 2.7 of [RFC5617]
2.2. Terms Defined by this Specification:
2.2.1. Third Party Signature
A "Third Party Signature" is a Valid Signature that does not qualify
as an Author Domain Signature.
Editor's Note: While this term is defined in Section 6.3 of
[RFC5863] and in Section 2 of [RFC5016], this definition is in
terms of the Author Domain Signature and avoids statements about
any header field dependencies.
2.2.2. Third Party Signer
A "Third Party Signer" is a signer that adds a valid DKIM signature
that references a domain with the 'd=' tag in the DKIM-Signature
header field that is not the Author Domain.
2.2.3. TPA-Label Listed Domain, TPA-LLD
TPA-Label Listed Domain, TPA-LLD, is a TXT resource record that can
be referenced with a TPA-Label within an Author Domain. When a "tpa"
tag exists within the TXT resource record located at the TPA-Label,
the referenced domain must be within a listed domain. When the "tpa"
tag does not exist, the referenced domain is presumed listed. The
"scope" tag may stipulate the existence of additional headers or
which email elements are to confirm an administrative domain of a
service before being authorized to act on behalf of the Author
Domain. Following [RFC5321], domain name comparisons, as well as
TPA-Labels, are case insensitive.
2.2.4. Author's Domain Acceptable Third-Party Signature
An "Author's Domain Acceptable Third-Party Signature" is a Valid
Signature in which the domain name of the DKIM signing entity, i.e.,
the 'd=' tag in the DKIM-Signature header field, is the domain name
Otis & Black Expires December 29, 2010 [Page 7]
Internet-Draft TPA-Label June 2010
referenced in the TPA-Label Resource Record published by the Author
Domain with a scope of 'F', 'S', or 'L'. For 'S' and 'L' scopes, the
respective Sender header or a List-ID identifier of the List-ID
header must exist for either scope and contain a domain within the
TPA-LLD for authorization to be valid.
2.2.5. Author's Domain Acceptable Third-Party Service
An "Author's Domain Acceptable Third-Party Service" is a service that
is able to confirm the administrative domain with either forward or
reverse DNS references from either the client host name (EHLO/HELO)
or the return path (Mail From) as determined by the presence of the
'H' and 'M' scopes respectively. For additional 'S' or 'L' scopes,
the respective Sender header or a List-ID identifier of the List-ID
header must exist for either scope and contain a domain within the
TPA-LLD for authorization to be valid.
3. Combinatorial ADSP "dkim=" Values.
This document defines two new values listed with the ADSP "dkim" tag
in addition to those defined in [RFC5617] section 4.2.1. These
values can append to those currently defined, or used separately.
When used separately, the value "all" is to be assumed to prefix the
new values when recognized, otherwise the value "unknown" will be
assumed. It is not recommended to use any new value in conjunction
with "discardable", because when not understood, a message that
depends upon different handling might become lost.
3.1. tpa-sig
The ADSP dkim= value "tpa-sig" indicates that TPA-Labels will offer a
comprehensive list of Author's Domain Acceptable Third-Party
Signatures that may include header requirements. When there is no
valid Author Domain Signature or Author's Domain Acceptable Third-
Party Signature, the Author Domain recommends these messages be
refused.
3.2. tpa-path
The ADSP dkim= value "tpa-path" indicates that TPA-Labels will offer
a comprehensive list of Author's Domain Acceptable Third-Party
Signatures and Authorized Third-Party Services that may include
header requirements.
The "tpa-path" is used to accommodate third party services lacking
DKIM signatures, the confirmed path of the message determined by
either the client host name (EHLO/HELO) or the return path (Mail
Otis & Black Expires December 29, 2010 [Page 8]
Internet-Draft TPA-Label June 2010
From). The permitted path element's domain is authorized by a TPA-
Label Resource Record with the scope of 'H' or 'M' respectively.
Such messages are then in compliance with the Author Domain's
asserted Signing Policies. The leaf of the host name (left most
label) may need to be omitted when checking for TPA-Label Resource
Record authorization.
When there is no valid Author Domain Signature, or Author's Domain
Acceptable Third-Party Signature, or Author's Domain Acceptable
Third-Party Service, the Author Domain recommends these messages be
refused.
ADSP defends domains against spoofing. Any subdomain of a domain
publishing an ADSP with the "dkim" tag value containing "tpa-sig" or
"tpa-path" not also publishing an MX resource record, should be
assumed to have published the same ADSP records there as well.
4. TPA-Label Resource Record Authorization Considerations
When an Author Domain is not within the DKIM signing domain, the TPA-
LLD scheme can extend ADSP signing practice compliance. The TPA-LLD
scheme with an 'F', 'S', or 'L' scope permits a contained Third Party
Signature to be treated as an Author Domain Signature. The 'H' and
'M' scopes permit acceptance based upon confirmation of either the
client host name (EHLO/HELO) or the return path (Mail From)
respectively. For 'S' and 'L' scopes, the respective Sender header
or a List-ID identifier of the List-ID header must exist for either
scope and contain a domain within the TPA-LLD for authorization to be
valid. This allows Author Domains a means to extend restrictive
policy compliance. The TPA-LLD scheme for offering valid
authorization only requires DNS publications be made by the Author
Domain, even when signing domains and the Author Domain differ. This
approach avoids a need to exchange DKIM key related information.
Extended authorization will not ensure all possible spoofing is
prevented. However, by permitting broader use of restrictive
policies, this should generally reduce the level of spoofing over
what might be otherwise allowed. Authorized third party messages
should not receive annotations that indicate the message contains
authenticated identities. The TPA-LLD scope should include the 'S'
or 'L' scope where appropriate to allow recipients a means to isolate
different message sources.
The TPA-LLD scheme plays the role of only providing acceptable
signatures or services which might be suitable for non-critical
messages, with the goal of improving delivery acceptance, such as
those from specific mailing-lists. Before TPA-LLD authorization is
Otis & Black Expires December 29, 2010 [Page 9]
Internet-Draft TPA-Label June 2010
deployed, the Author Domain should be assured by domains being
authorized that appropriate measures are in place to authenticate
those who are submitting messages.
The "dkim=" tag within the TPA-Label Resource Record is expected to
normally contain a copy of the value asserted by the ADSP Resource
Record "dkim" tag. When the TPA-Label Resource Record "dkim" tag
value differs, and the message is compliant with the "scope" and "ad"
tag values, the TPA-Label Resource Record "dkim" tag value overrides
the ADSP Resource Record "dkim" tag value. Use of "tpa-path" should
selectively override the ADSP "tpa-sig" only where needed.
5. Evaluating the Third-party Signing Domain or Service
An Author Domain deploying a TPA-Label Resource Record does so on a
trust basis. Reasons for deploying TPA-Label Resource Records might
be to allow deployment of more stringent ADSP records while also
utilizing third-party signatures or services.
When an authorized Third Party Signer does not employ DKIM
authentication with ADSP or does not include Authentication-Results
headers, this could allow authorizations to be exploited.
5.1. Third Party Authentication
The Author Domain SHOULD ensure the Authorization Scope of the TPA-
Label Resource Record is authenticated. There are a number of ways
email can be authenticated, and different authentication mechanisms
validate different parts of the email. The following are examples of
how authorization might work.
5.1.1. Third Party Authentication - Web Email Provider with Subscriber
Pingbacks
The Author Domain "example.com" wants to deploy a TPA-Label Resource
Record to permit their traveling agents the use of
"webmail.example.net" services. This email provider has a closed
user policy and adds DKIM signatures to messages on behalf of the
"webmail.example.net" domain.
The closed user policy of "webmail.example.net" permits subscribers
to post messages with Author Domains that are not
"webmail.example.net" in the From header fields, only when control of
the Author Address has been validated by a response to an encoded
"pingback" email. The "webmail.example.net" service also establishes
accounts to authenticate all users sending messages through their
Otis & Black Expires December 29, 2010 [Page 10]
Internet-Draft TPA-Label June 2010
service. Therefore, the referenced TPA-Label Resource Record can
include an 'F' scope value to authorize Author Domain messages signed
by this Third-Party Signer.
5.1.2. Third Party Authentication - Closed Mailing List Example
The Author Domain wants to deploy a TPA-Label Resource Record for a
mailing list with a closed posting policy that redistributes email in
a way which breaks Author Domain Signatures, but adds a DKIM
signature on behalf of the mailing list domain and includes an
Authentication-Results header field for posted messages. The closed
posting policy is enforced by requiring subscribers to validate their
control of their Author Address by responding to encoded "pingback"
email sent to this address.
Since the mailing list management always verifies control of the
Author Address, and is configured to include Authentication-Results
headers, and includes a List-ID header, the referenced TPA-Label
Resource Record can include an 'L' scope value to permit Author
Domain messages containing an authorized List-ID domain to be signed
by this Third-Party Signer.
5.1.3. Third Party Authentication - Open Mailing List Example
The Author Domain wants to deploy a TPA-Label Resource Record for a
mailing list with an open posting policy that redistributes email in
a way that breaks Author Domain Signatures, but that adds a DKIM
signature on behalf of the mailing list domain and includes an
Authentication-Results header field for posted messages. The open
posting policy will refuse messages lacking Author Domain Signatures
for domains that have deployed an ADSP signing practice of "dkim=all"
or "dkim=discardable".
Since the list management always refuses to post an Author Address
lacking a Author Domain Signature when the domain has deployed an
ADSP record with an "dkim=all" or "dkim=discardable", and is
configured to include Authentication-Results headers, and includes a
List-ID header, the referenced TPA-Label Resource Record can include
an 'L' scope value to permit Author Domain messages containing an
authorized List-ID domain to be signed by this Third-Party Signer.
5.1.4. Third Party Authentication Example - Sender Header Field
Author Domain "example.com" wishes to temporarily employ the service
agency "temp.example.org" to handle overflow secretarial support.
The agency "temp.example.org" sends email on behalf of the executive
staff of "example.com" and adds the Sender header field of
"secretary@temp.example.org" in the email. Since "temp.example.org"
Otis & Black Expires December 29, 2010 [Page 11]
Internet-Draft TPA-Label June 2010
only allows its own staff to email through its server which adds
"temp.example.org" DKIM signatures, a TPA-LLD can include the
"temp.example.org" domain with an 'S' scope to specifically authorize
messages containing the Sender header field to help ensure these
messages are not handled as phishing attempts.
5.1.5. Services Lacking DKIM Signatures
5.1.5.1. Abuse and DSN Reporting
There is likely little interest for an otherwise uninvolved domain to
receive a massive number of bogus messages being returned as
feedback. Often the purpose of feedback is to discover compromised
systems or accounts actively being exploited in some manner. Unless
there is evidence that the Author Domain either handled or authorized
the handling of the message, only statistics and samples should be
reported to the associated Autonomous System, and perhaps to the
Author Domain when an interest is expressed.
The 'H' and 'M' scopes available within the TPA-LLD records allow the
Author Domain to be associated with SMTP Clients publicly
transmitting messages, and/or the Mail return path when these domains
differ, and when DKIM is not employed by the third-party service. In
this case, appropriate DSN or abuse reporting to the Author Domain is
better assured as a result. The correspondence between SMTP Client
hosts and Mail return path can be affirmed by the TPA-LLD scheme with
a scope of 'H' or 'M' that might be used to categorize feedback data
or confirm DSN destinations.
Services that depend only upon path authorizations, will permit other
domains to spoof the Author Domain, and yet obtain acceptance.
During such events, the Author Domain might need to retract their
authorization from the service. For this reason, "tpa-path"
authorization should only be used as a carefully monitored interim
solution.
5.1.5.2. Third Party Authentication Example - SMTP Host
Author Domain "example.com" makes use of invite services. This
service does not utilize DKIM, where the host name given by the EHLO
command is "invite.example.net". The Author Domain can authorize the
domain "invite.example.net" or "example.net" with the scope of 'H' to
improve acceptance of messages that are sent on behalf of
"example.com" from this outbound server.
Otis & Black Expires December 29, 2010 [Page 12]
Internet-Draft TPA-Label June 2010
5.1.5.3. Third Party Authentication Example - Return Path
Author Domain "example.com" makes use of tell-a-friend services.
This service does not utilize DKIM with their own return path as
"customer@taf.example.net" in the SMTP exchange. The Author Domain
can authorize the domain "taf.example.net" with the scope of 'M' to
improve acceptance of messages that are on behalf of "example.com"
from this outbound server.
5.1.5.4. Use of Path Authorization
Those using the "tpa-path" value should not authorize domains
requiring more than a few DNS transactions to confirm a domain.
Those implementing this ADSP extension should also limit the number
of DNS transactions that might be attempted, or this could negatively
impact unrelated domains when evaluating path related protocols.
Editor's Note: This option was added for better coverage during
initial deployment of DKIM and ADSP. Earlier efforts to employ
SRV records to resolve the SMTP client failed adoption.
Current experimental path protocols combine into a single set of
all IPv4 and IPv6 addresses for all outbound servers handling a
domain's messages. To aggregate this potentially large set of
resources, path protocols provide up to one hundred and eleven
separate DNS transactions. One to obtain the initial record, one
for each of the ten permitted mechanisms, which may in turn
require up to ten transactions to resolve the mechanism's target
list.
Path protocol libraries expand macros containing email address
local-parts as locations for subsequent resource records. This
allows a cached path related resource record to produce a new set
of DNS transactions whenever the local-part of a spam campaign
changes.
6. DNS Representation
The receiver obtains domain authorizations with a DNS query for an IN
class TXT TPA-Label resource record located below the location
specified in [RFC4871] section 7.4 and the label "_tpa.". The TPA-
Label itself is generated by processing the domain in question, which
normally matches the DKIM signature's "d=" parameter. A TPA-Label
Resource Record is published adjacent to the [RFC5617] conventional
ADSP record, for example below "_tpa._domainkey.<Author-Domain>".
The Author Domain provides authorization for other domains with the
existence of a TPA-Label TXT resource record. When a "tpa" tag value
Otis & Black Expires December 29, 2010 [Page 13]
Internet-Draft TPA-Label June 2010
exists, it must include the referenced domain for authorization to be
valid. Authorization to act on behalf of the Author Domain can also
be limited by the "scope" tag value for specific message elements.
An Author Domain may wish to delegate the listing of third-party
services to a different administrative domain. Ideally, this would
be accomplished by delegating the _tpa._domainkey.<Author-Domain>
zone to the administrative entity handling publication of TPA-Label
Resource Records. This delegation could also be done unilaterally
with a DNAME resource record published at _tpa._domainkey.<Author-
Domain>.
Character-strings contained within the TXT resource record are
concatenated into forming a single string. A character-string is a
single length octet followed by that number of characters treated as
binary information.
The TPA-Label Resource Records should be located at these domains:
<tpa-label>._tpa._domainkey.<Author-Domain>.
7. TPA-Label and Tag Syntax Definitions
"base32" function is defined in [RFC4648].
"sha1" function is defined in [FIPS.180-2.2002].
"lcase" converts upper-case ALPHA characters to lower-case.
"signing-domain" is the "d=" tag value defined in Section 3.5 of
[RFC4871].
Augmented BNF for Syntax Specifications:
asterisk = %x2A ; "*"
dash = %x2D ; "-"
dot = %x2E ; "."
underscore = %x5F ; "_"
ANY = asterisk dot ; "*."
dns-char = ALPHA / DIGIT / dash
id-prefix = ALPHA / DIGIT
label = id-prefix [*61dns-char id-prefix]
sldn = label dot label
base-char = (dns-char / underscore)
domain = *(label dot) sldn
tpa-label = underscore base32( sha1( lcase(signing-domain)))
Otis & Black Expires December 29, 2010 [Page 14]
Internet-Draft TPA-Label June 2010
8. TPA-Label Generation
The TPA-Label is created from the hash value returned by the "sha1"
function of the signing-domain expressed in lower case ASCII. The
hash is then converted to a base32 character set, with the resulting
label prefixed with an underscore. Any terminating period is not
included with the signing-domain, as indicated by the ABNF
definition.
Note: No newline character, 0x0A, is to be appended to the end of
the domain name, as might occur with the command line generation
of sha1 values. For example, these command line appended newlines
are avoided by using the 'echo -n" option.
9. TPA-Label TXT Resource Record Structure
Every TPA-Label TXT resource record MUST start with an outbound
signing-practices tag, so the first four characters of the record are
lowercase "dkim", followed by optional whitespace and "=". In
addition to the tags defined by [RFC5617], TPA-Label syntax
descriptions for additional tags follow the tag-value syntax
described in section 4.2.1 of [RFC5617] and section 3.2 of [RFC4871].
Unrecognized tags and tags with illegal values MUST be ignored. In
the ABNF below, the WSP token is inherited from [RFC5322]. The ALPHA
and DIGIT tokens are imported from [RFC5234].
The tags used in TPA-Label resource records are as follows:
+--------+------------------------------------+
| Tag | Function |
+--------+------------------------------------+
| scope= | Authorization Scope List (as-list) |
| tpa= | Authorized Domains List (ad-list) |
+--------+------------------------------------+
TPA-Label Extended Tags
+--------------+----------------------+
| Scope Values | Field or Parameter |
+--------------+----------------------+
| F | From (Author) Header |
| L | List-ID |
| S | Sender Header |
| M | MailFrom |
| H | SMTP Host |
+--------------+----------------------+
Otis & Black Expires December 29, 2010 [Page 15]
Internet-Draft TPA-Label June 2010
TPA-Label Scope Values
9.1. TPA-Label Resource Record Scope Syntax
scope= Authorization Scope List (Optional). This tag defines a list
of scoping assertions for various email-address locations within the
message. Only recognized scope values offer any form of ADSP
authorization.
scope = "F" / "L" / "S" / "M" / "H"
as-list = "scope" [WSP] "=" [WSP] scope 0*([WSP] ":" [WSP] scope)
9.1.1. TPA-Label Listed Domain Authorization
9.1.1.1. From (Author) Header Field
The "F" scope asserts that messages carrying the Author Domain within
the From header field are authorized to be signed by the TPA-LLD.
9.1.2. Header Dependent Authorizations
9.1.2.1. List-ID Header Field
The "L" scope asserts that authorization is valid only when a List-ID
identifier of the List-ID header field [RFC2919] is within the TPA-
LLD.
9.1.2.2. Sender Header Field
The "S" scope asserts that authorization is valid only when the
domain in the Sender header is within the TPA-LLD.
9.1.2.3. Combined 'L' or 'S' Scopes
When combined, the scopes 'L' and 'S' require that either a List-ID
identifier of the List-ID header field or the Sender header must
contain a domain within the TPA-LLD for the authorization to be
valid.
9.1.3. MailFrom Parameter
The "M" scope asserts that an email-address domain, that is within a
TPA-LLD used in the [RFC5321] MAIL command is authorized.
Otis & Black Expires December 29, 2010 [Page 16]
Internet-Draft TPA-Label June 2010
9.1.4. SMTP Host domains
The "H" scope asserts that host names given in [RFC5321] EHLO or HELO
commands within TPA-LLD is authorized.
10. Authorized Signing Domain
tpa= Authorized Signing Domain list. (optional) This tag, when
present, MUST repeat all or portions of the domain encoded within the
TPA-Label Resource Record. This option ensures the proper handling
of possible hash collisions. When a domain is prefixed with the "*."
ANY label, then all subdomains of this domain are to be considered
included within the list. When the 'tpa' tag is not present or has
no value, it should be assumed to compare with the domain used to
generate the TPA-Label.
ad = [ANY] domain
ad-list = "tpa" [WSP] "=" [WSP] ad 0*([WSP] ":" [WSP] ad)
11. TPA-Label Resource Record Query Transactions
The discovery of TPA-Label resource records need not be subsequent to
the discovery of the ADSP record specified by [RFC5617]. However,
when no ADSP record is discovered or when it does not contain a
"dkim" tag value of either "tpa-sig" or "tpa-path", the verifier MAY
assume that no TPA-Label Resource Records have been published.
Otherwise, when there is a Third Party Signature without any Author
Domain Signature, the discovery of TPA-Label Resource Records should
be attempted.
12. TPA-Label Resource Record Compliance Assessment
The signing practice compliance assessment of Third Party Signatures
is a discretionary operation performed by the verifier. Messages
that have valid Author Domain Signatures are already considered to
result in a pass. When a verifier decides to assess compliance for
Third Party Signatures with an Author Domain ADSP "dkim" tag value
"tpa-sig" or "tpa-path", then, checked in succession, one of the
following sets of conditions MUST be met for the result to be
considered a pass.
For Third Party Signatures, the following represents the set of "tpa-
sig" assessment conditions to be checked:
Otis & Black Expires December 29, 2010 [Page 17]
Internet-Draft TPA-Label June 2010
o The Third Party Signature MUST validate according to [RFC4871].
o A TXT Resource Record, referenced by a TPA-Label created by the
DKIM signature "d=" tag, MUST exist in DNS.
o The discovered TPA-Label TXT Resource Record Structure MUST be
valid.
o The domain that created the TPA-Label MUST be within the TPA-LLD.
o Where a scope of 'F', 'S', or 'L' is specified, the Author Domain
MUST have an Author's Domain Acceptable Third-Party Signature.
o Where a scope of 'L' or 'S' is specified, a List-ID identifier in
the List-ID header field or a Sender header MUST contain a domain
within the TPA-LLD.
Meeting all the conditions in this set results in a "tpa-sig" pass,
where subsequent checks are then skipped.
For Third Party Services where the Author Domain ADSP "dkim" tag
value contains "tpa-path", and where the preceding assessment
conditions were not met, then the following represents "tpa-path"
assessment conditions to be checked:
One of three possible TXT Resource Records are checked in succession.
These are referenced by an 'H' related TPA-Label created either from
the domain given by [RFC5321] EHLO or HELO command, this domain with
left-most label omitted, or by an 'M' related email-address domain
within the [RFC5321] MAIL command.
The TXT record discovery process continues until a TPA-Label TXT
Resource Record Structure is found where:
o The discovered TPA-Label TXT Resource Record Structure is valid.
o The domain that created the TPA-Label is within the TPA-LLD.
o The domain that created the TPA-Label corresponds to the scope of
'H', 'H' or 'M' respectively.
o Where a scope of 'L' or 'S' is specified, either the domain in
List-ID given by [RFC2919] in the List-ID header is within the
TPA-LLD, or a Sender header contains a domain within the TPA-LLD
respectively.
o Once these four conditions have been met, the domain MUST be
confirmed using either forward or reverse DNS references. (A path
related protocol dataset might also provide confirmation, but
conservative transaction limits should be imposed.)
Meeting all four conditions in this set, and confirming the domain,
results in a "tpa-path" pass.
Otis & Black Expires December 29, 2010 [Page 18]
Internet-Draft TPA-Label June 2010
When the TPA-Label TXT Resource Record can not be retrieved due to
some error that is likely transient in nature, as specified in
[RFC5617] Section 4.3. such as "SERVFAIL" for example, the result of
the TPA-Label Resource Record compliance assessment is "temperror".
When the TPA-Label TXT Resource Record retrieval returns a DNS
"NOERROR", but not with a single record, the result of the TPA-Label
Resource Record compliance assessment is "permerror".
When the TPA-Label TXT Resource Record can not be retrieved with a
DNS "NXDOMAIN",the result of the TPA-Label Resource Record compliance
assessment is "nxdomain".
The following pass conditions are combined to provide a single pass
value.
o A "tpa-sig" pass confirms an Author Domain Acceptable Third-Party
Signature.
o A "tpa-path" pass confirms an Author Domain Acceptable Service.
13. IANA Considerations
13.1. Author Domain Signing Practices (ADSP) Parameters
To accommodate the extensions to ADSP Signing Practices, the IANA
Registry "ADSP Outbound Signing Practices" defined by Section 4.2.1
of [RFC5617] needs the following elements to be added:
Note to RFC EDITOR: This is currently located at:
http://www.iana.org/assignments/adsp-parameters/adsp-parameters.xhtml
+----------+-----------------+
| Type | Reference |
+----------+-----------------+
| tpa-sig | [THIS DOCUMENT] |
| tpa-path | [THIS DOCUMENT] |
+----------+-----------------+
TPA-Label Resource Record validation Method
13.2. Email Authentication Method Registry
To accommodate the method derived from TPA-Label Resource Record
processing, the IANA Registry "Email Authentication Method" defined
Otis & Black Expires December 29, 2010 [Page 19]
Internet-Draft TPA-Label June 2010
by Section 6.2 of [RFC5451] needs the following elements to be added:
Note to RFC EDITOR: This is currently located at: http://
www.iana.org/assignments/email-auth/
email-auth.xhtml#email-auth-methods
+---------+-----------+--------+----------+-------------------------+
| Method | Defined | ptype | property | value |
+---------+-----------+--------+----------+-------------------------+
| tpa-lld | [THIS | header | d | value of signature "d" |
| | DOCUMENT] | | | tag. The dkim method |
| | | | | results from [RFC5451] |
| | | | | should also be included |
| | | | | in a Authenticated |
| | | | | Results header field |
| | | | scope | value of scope |
| | | | | (Section 13.5) tag. |
| | | | | (When 'scope' contains |
| | | | | 'H', the iprev |
| | | | | [RFC5451] (Section 3) |
| | | | | method results should |
| | | | | also be included in the |
| | | | | Authenticated-Results |
| | | | | header field) |
| | | | ca-scope | The scopes |
| | | | | (Section 13.5) with a |
| | | | | compliance assessment |
| | | | | as pass |
| | | | tpa | Value of tpa |
| | | | | (Section 10) tag at |
| | | | | time of compliance |
| | | | | assessment |
+---------+-----------+--------+----------+-------------------------+
TPA-Label Resource Record validation Method
Otis & Black Expires December 29, 2010 [Page 20]
Internet-Draft TPA-Label June 2010
13.3. Email Authentication Result Names Registry
To accommodate the results derived from TPA-Label Resource Record
processing, the IANA Registry "Email Authentication Method" defined
by Section 6.3 of [RFC5451] needs the following elements added:
Note to RFC EDITOR: This is currently located at: http://
www.iana.org/assignments/email-auth/
email-auth.xhtml#email-auth-result-names
+--------------+---------+------------------------------------------+
| code | method | meaning |
+--------------+---------+------------------------------------------+
| none | tpa-lld | No TPA-Label was published |
| pass | tpa-lld | section Section 12 |
| tempfail | tpa-lld | section Section 12 |
| permfail | tpa-lld | section Section 12 |
| unknown | tpa-lld | The TPA-Label Resource Record had a |
| | | tag/value of "dkim=unknown" and the |
| | | Third Party Signature failed its |
| | | compliance assessment. |
| discard | tpa-lld | The TPA-Label Resource Record had a |
| | | tag/value of dkim=discard and the Third |
| | | Party Signature failed its compliance |
| | | assessment. |
| fail | tpa-lld | The TPA-Label Resource Record had a |
| | | tag/value of dkim=all and the Third |
| | | Party Signature failed to its compliance |
| | | assessment. |
| nxdomain | tpa-lld | When obtaining the TPA-Label Resource |
| | | Record, DNS indicated this domain does |
| | | not exist. |
| Other value | tpa-lld | The TPA-Label Resource Record had a |
| defined in | | tag/value of dkim={other value} and the |
| the IANA | | Third Party Signature failed its |
| ADSP | | compliance assessment. |
| Outbound | | |
| Signing | | |
| Practices | | |
| Registry | | |
+--------------+---------+------------------------------------------+
TPA-Label Resource Record complaince assessment Results
13.4. Third Party Authorizations Labels Registry
Names of tags that are valid in TPA-Label Resource Records with the
exception of experimental tags Section 9 MUST be registered in this
Otis & Black Expires December 29, 2010 [Page 21]
Internet-Draft TPA-Label June 2010
created IANA registry.
New entries are assigned only for values that have been documented in
a published RFC that has had IETF Review, per IANA CONSIDERATIONS
[RFC5226].
Each tag registered must correspond to a definition.
The initial set of values for this registry is:
+----------+-------------+------------------------------------------+
| tag | defined | definition |
+----------+-------------+------------------------------------------+
| dkim | Section 9 | As per IANA Registry ADSP Outbound |
| | | Signing Practices |
| scope | Section 9.1 | Section 13.5 |
| tpa-sig | Section 10 | List of authorized domains |
| tpa-path | Section 10 | List of authorized domains |
+----------+-------------+------------------------------------------+
TPA-Label Resource Record compliance assessment Results
13.5. Third Party Authorizations Scope Registry
Values that correspond to Section 9.1 MUST be registered in this
created registry:
New entries are assigned only for values that have been documented in
a published RFC that has had IETF Review, per IANA CONSIDERATIONS
[RFC5226].
Each value registered must correspond to a definition.
The initial set of values for this registry is:
+-------+-----------------+
| value | defined |
+-------+-----------------+
| F | Section 9.1.1 |
| L | Section 9.1.2.1 |
| S | Section 9.1.2.2 |
| M | Section 9.1.3 |
| H | Section 9.1.4 |
+-------+-----------------+
TPA-Label Resource Record compliance assessment Results
Otis & Black Expires December 29, 2010 [Page 22]
Internet-Draft TPA-Label June 2010
14. Security Considerations
This draft extends signing practices for [RFC4871] where most generic
DKIM Signature related security matters are discussed. Additional
considerations are also included in [I-D.ietf-dkim-mailinglists].
Security considerations for the TPA-LLD scheme are mostly related to
attempts on the part of malicious senders to falsely represent
themselves as other senders, often in an attempt to defraud either
the recipient or the alleged originator.
Additional security considerations regarding DKIM signing practices
may be found in the DKIM threat analysis [RFC4686].
14.1. Benefits to Recipients
The verifier, after finding either an Author's Domain Acceptable
Third-Party Signature or Author's Domain Acceptable Third-Party
Service in a message, will have significantly greater confidence in
the Third-Party, than when no TPA-Label Resource Record is obtained.
This enhanced confidence may, at the recipients' discretion, cause a
message to be delivered to the recipient without further source
related assessment.
14.2. Risks to Recipients
The decisions a recipient makes in regard to message filtering based
on TPA-Label Resource Records are likely to depend on the system
integrity of the Third Party with respect to the Authentication (see
Section 5.1) and the provided scope labels. When the 'H' or 'M'
scoped element is not authenticated by the Third Party or a domain is
not confirmed, there is a risk of accepting potentially spoofed
messages. When there is no out-of-band authentication confirming the
sender, Authentication-Results headers then play an important role.
Without proper Authentication-Results handling by the third-party,
there is also risk of accepting potentially spoofed messages.
With this specification, third party signatures have verifiable
value. When implementing the compliance assessment of third party
signatures and TPA-Label Resource Records, implementers need to
consider the possibility that a Bad Actor will send the recipient a
message with a large number of valid DKIM Signatures. Verifying all
of these may consume a large amount of processing resources such that
it may be worth checking the existence of a TPA-Label Resource Record
first. Section 11 describes a quick check to see if TPA-Label
Resource Records may exist. Additionally validating DKIM signatures
and obtaining related resource records might be limited to known
trustworthy domains.
Otis & Black Expires December 29, 2010 [Page 23]
Internet-Draft TPA-Label June 2010
14.3. Benefits to Author Domains
TPA-Label resource records can replace domain delegations, selector/
key record mirroring, or key exchanges. A significant number of
details are associated with selector/key records. These details
include user limitations, suitable services, key resource record's
Time-To-Live, revocation and update procedures, and how the DKIM
Signature header field's 'i=' semantics are to be applied. In
addition, services that depend upon DKIM keys are better secured by
not delegating these DKIM keys, where instead the TPA-LLD scheme
allows Author Domains an ability to limit the scope of their
authorizations, while also not being mistaken for having
authenticated the entity submitting the message.
TPA-Label Resource Records convey which domains are authoritative
even when they are not the Author Domain. However, authorized
domains are unable to utilize the DKIM signature's 'i=' semantics to
directly assert which identifiers on whose behalf a signature was
added. As such, no domain should be authorized unless it is trusted
to ensure the Alleged Author of an email undergoes authentication
that offers acceptable protections for the Author Domain. For
example, such authentication might ensure submitting entities have
demonstrated receipt of "pingback" messages sent to the Author
Address contained within the messages being signed.
By deploying TPA-Label Resource Records, Author Domains benefit when
recipients assess signing practice compliance by using the TPA-LLD
scheme. These recipients will be less likely to drop the Author
Domain's genuine messages, whenever the Author Domain attempts to
restrict acceptance. Restricting acceptance of non-compliant
messages is the basic motivation for publishing ADSP records. In
addition, recipients are more likely to validate Authorized Third
Party Domain Signatures.
Broader use of restrictive ADSP policies provides a better likelihood
of being able to eliminate a greater range of non-compliant messages,
in addition to improving acceptance from authorized sources. With
authorization, scope labels allow the Author Domain to control
message attributes even from the authorized third parties.
Signing domains having good reputations referenced by a TPA-LLD might
therefore provide a means to safely extend limited compliance
assessment resources to otherwise unknown domains or SMTP Clients.
14.4. Risks to Author Domains
As indicated in Section 5, there is ultimately a trust of the third
party domain to do the right thing and not generate, or allow others
Otis & Black Expires December 29, 2010 [Page 24]
Internet-Draft TPA-Label June 2010
to generate, messages that falsely appear to be from the Author
Domain. The authentication methods in place for different email
elements need to be carefully reflected in the scope of the TPA
records.
By authorizing mailing lists with TPA-Label Resource Records, this
could cause a loss of confidentiality in mailing list participation
by the Author Domain. This might help Bad Actors deduce which
subscription related email the Author Domain may receive. Because of
the hashing function in generating the TPA-label, anyone wishing to
discover which domains are being authorized, has to probe each TPA-
label based on the exact signing domain. In addition, service
organizations or community groups are able to share comprehensive
lists which means, even though the domain has been authorized, that
in itself does not mean the Author Domain is exchanging messages with
the authorized domain.
14.5. Benefits to Third Party Signers
Third Party Signers benefit by allowing those using their service,
the autonomy to authorize their service without needing to exchange
DKIM key related details. This is particularly useful for mailing
lists.
14.6. Risks caused by Third Party Signers
As mentioned before, Third Party Signers need to authenticate
messages from Author Domains. This authentication provides a safety
mechanism for the Author Domain and their recipients. The Third
Party may not be aware of the authentication value or the message
elements involved and make changes without understanding the impact
this may have upon targeted Author Domains and their recipients. For
example, the Third Party might stop DKIM signing or stop applying
Authentication-Results headers. The unexpected exposure might enable
wide spread abuse and prove detrimental for both the Author Domain
and their recipients.
14.7. SHA-1 Collisions
The use of the SHA-1 hash algorithm does not represent a security
concern. The hash simply ensures a deterministic domain-name size is
achieved. Unexpected collisions can be detected and handled by using
the extended TPA-Label Resource Record "tpa=" option. The use of
TPA-Label Resource Records without the TPA-Label "tpa=" options does
present an opportunity for an adversary to attempt to find a hash
collision. Message spoofing outside the realm of DKIM protection is
likely easier to achieve than finding hash collisions. There is
minimal risk of TPA-Labels colliding. Listing 3 x 10^45 domains has
Otis & Black Expires December 29, 2010 [Page 25]
Internet-Draft TPA-Label June 2010
less than a 0.1 percent risk of any two domain labels colliding.
14.8. DNS Limits
Use of the TPA-Label Resource Records, rather than simply listing the
authorized domain, ensures the DNS record size is independent of the
Third Party Domain. The typical domain name size has been steadily
increasing. This increase has been caused by domain names that
encode international character sets. Perhaps soon there will be a
futher increase spurred by an expanse of TLDs having larger
international labels.
The maximum domain name size allowed, per [RFC1034] Section 3, is 255
bytes (or octets). Each label has a byte for its length. Every
domain name has a right most label representing the root with a zero
length, for another byte. A scheme that concatenates a listed domain
with the publishing domain, separated by some conventional label,
reduces the maximal domain name in half, where the conventional label
reduces this further.
If "_tpa." were used as the conventional label with a simple listing
method, the maximum domain name size this supports would be 122
bytes. The suffix for TPA-Labels is "_tpa.domainkey." which consumes
16 bytes. The TPA-Label itself consumes 34 bytes. A domain that
publishes the TPA labels in their domain, would then have 205 bytes
available for their Author Domain. Since an Author's Domain
Acceptable Third-Party Service might not implement DKIM, the TPA-
Label is still able to authorize any domain name with a valid length.
As a result, the maximum allowable Author Domain is increased by 83
bytes or 68% over simple name concatenation.
Normally, DNS messages should not exceed 512 bytes as per Section
2.3.4 of [RFC1035]. Using TPA-Label Resource Records in the DNS, as
described by this document, consumes a consistent 50 bytes, in
addition to the domain name publishing the TPA-Labels. With this
being constant, a limit can be determined as a constraint to resource
record size, to ensure a response does not exceed the maximum DNS
message size. DNS servers that add additional resource records, for
nameservers as an example, will further reduce available resource
record capacity. Domains publishing TPA-Labels exceeding the DNS
message limit will need to rely on recipients using TCP for DNS
retrieval, or EDNS0 [RFC2671] for extended DNS lengths.
15. Acknowledgements
Frank Ellermann, Michael Deutschmann, Jeff MacDonald, and Wietse
Otis & Black Expires December 29, 2010 [Page 26]
Internet-Draft TPA-Label June 2010
Venema.
16. References
16.1. Normative References
[FIPS.180-2.2002]
National Institute of Standards and Technology, "Secure
Hash Standard", FIPS PUB 180-2, August 2002, <http://
csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
[RFC5617] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", RFC 5617, August 2009.
16.2. Informative References
Otis & Black Expires December 29, 2010 [Page 27]
Internet-Draft TPA-Label June 2010
[I-D.ietf-dkim-mailinglists]
Kucherawy, M., "DKIM And Mailing Lists",
draft-ietf-dkim-mailinglists-00 (work in progress),
June 2010.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016,
October 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5672] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM)
Signatures -- Update", RFC 5672, August 2009.
[RFC5863] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
"DomainKeys Identified Mail (DKIM) Development,
Deployment, and Operations", RFC 5863, May 2010.
[apwg-globalphishingsurvey-2H2009]
Anti-Phishing Working Group, "Global Phishing Survey:
Trends and Domain Name Use 2H2009", May 2009, <http://
www.antiphishing.org/reports/
APWG_GlobalPhishingSurvey_2H2009.pdf>.
Appendix A. DNS Example of TPA-Label Resource Record placement
####
# Practices for Example.com email domain using example.com, isp.com,
# and example.com.isp.com as signing domains.
####
#### 5322.From authorization for 3P domains ####
Otis & Black Expires December 29, 2010 [Page 28]
Internet-Draft TPA-Label June 2010
## "isp.com" TPA-Label Resource Record ##
_HTIE4SWL3L7G4TKAFAUA7UYJSS2BTEOV._tpa._domainkey.example.com. IN TXT
"dkim=all tpa-sig; tpa=isp.com; scope=F;"
#### 5322.Sender/List-ID authorization for 3P domains ####
## "example.com.isp.com" TPA-Label Resource Record ##
_6MEHLQLKWAL5HQREXWDN2TBXAJ6VZ44B._tpa._domainkey.example.com. IN TXT
"dkim=all tpa-sig; tpa=*.isp.com; scope=L:S;"
Otis & Black Expires December 29, 2010 [Page 29]
Internet-Draft TPA-Label June 2010
Appendix B. C code for label generation
The following utility can be compiled as tpa-label.c using the
following:
gcc -lcrypto tpa-label.c -o tpa-label
/*
* TPA-Label generation utility
* Copyright (C) 2010 The IETF Trust & and the persons identified as
* the document authors. All rights reserved.
* Redistributions of source code must retain the above copyright
* notice and the following disclaimer.
*
* This document is subject to the rights, licenses and restrictions
* contained in BCP 78, and except as set forth therein, the authors
* retain all their rights.
* This document and the information contained herein are provided on an
* "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
* OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
* THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
* OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
* THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
* WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
*/
#include <stdio.h>
#include <sys/types.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <openssl/sha.h>
#define TPA_LABEL_VERSION 102
#define MAX_DOMAIN_NAME 256
#define MAX_FILE_NAME 1024
static char base32[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
static char sign_on[] =
{"%s v%d.%02d Copyright (C) (2009) The IETF Trust & Douglas Otis\n"};
char err_cmd[] =\
"ERR: Command error with [%s]\n";
char use_txt[]=\
Otis & Black Expires December 29, 2010 [Page 30]
Internet-Draft TPA-Label June 2010
"Usage: tpa-label [-i domain_input_file] [-o label_output_file][-v]\n";
char help_txt[]=\
"The options are as follows:\n"\
"-i domain name input. Defaults to stdin. Removes trailing '.'\n"\
"-o TPA-Label output. Defaults to stdout.\n"\
"-v Specifies Verbose Mode.\n\n";
static void usage(void);
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
static void
usage(void)
{
(void) fprintf(stderr, "\n%s%s", use_txt, help_txt);
exit(1);
}
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
int
main (int argc, char * argv[])
{
int ret_val, in_mode, out_mode, verbose, done, i, j, k;
char ch;
unsigned int len;
unsigned long long b_5;
char in_fn[MAX_FILE_NAME], out_fn[MAX_FILE_NAME];
unsigned char in_buf[MAX_DOMAIN_NAME + 2];
unsigned char sha_res[20], tpa_label[33];
FILE *in_file, *out_file;
ret_val = in_mode = out_mode = verbose = done = 0;
len = 0;
while ((ch = getopt(argc, argv, "i:o:v")) != -1)
{
switch (ch)
{
case 'i':
in_mode = 1; /* input from file */
(void) strncpy(in_fn, optarg, sizeof(in_fn));
in_fn[sizeof(in_fn) - 1] = '\0';
break;
case 'o':
out_mode = 1; /* out to file */
(void) strncpy(out_fn, optarg, sizeof(out_fn));
out_fn[sizeof(out_fn) - 1] = '\0';
break;
case 'v':
Otis & Black Expires December 29, 2010 [Page 31]
Internet-Draft TPA-Label June 2010
verbose = 1;
break;
case '?':
default:
(void) usage();
break;
}
};
if (in_mode)
{
if ((in_file = fopen(in_fn, "r")) == NULL)
{
(void) fprintf(stderr,
"ERR: Error opening [%s] input file.\n",
in_fn);
exit(2);
}
}
else
{
in_file = stdin;
}
if (out_mode)
{
if ((out_file = fopen(out_fn, "w")) == NULL)
{
(void) fprintf(stderr,
"ERR: Error opening [%s] output file.\n",
out_fn);
exit(3);
}
}
else
{
out_file = stdout;
}
if (out_mode && verbose)
{
(void) printf(sign_on, "tpa-label utility",
TPA_LABEL_VERSION / 100,
TPA_LABEL_VERSION % 100);
}
for (i = 0; i < MAX_DOMAIN_NAME && !done; i++)
{
Otis & Black Expires December 29, 2010 [Page 32]
Internet-Draft TPA-Label June 2010
if ((ch = fgetc(in_file)) == EOF)
{
ch = 0;
}
else if (ch == '\n' || ch == '\r')
{
ch = 0;
}
in_buf[i] = tolower(ch);
if (ch == 0)
{
len = i; /* string length */
done = 1;
}
}
if (!done)
{
(void) fprintf(stderr, "ERR: Domain name too long.\n");
exit (4);
}
if (len && in_buf[len - 1] == '.') /* remove any trailing "." */
{
len--;
in_buf[len] = 0; /* replace trailing "." with 0 */
}
in_buf[len] = 0; /* terminate string */
if (len < 2)
{
(void)
fprintf(stderr,
"ERR: Domain name [%s] too short with %d length.\n",
in_buf,
len);
exit (5);
}
SHA1(in_buf, len, sha_res);
if (verbose)
{
printf("Normalized Domain = [%s] %d, SHA-1 = ", in_buf, len);
Otis & Black Expires December 29, 2010 [Page 33]
Internet-Draft TPA-Label June 2010
for (i = 0; i < 20; i++)
{
printf("%02x", sha_res[i]);
}
printf("\nTPA-Label: 5 bit intervals left to right.\n");
}
/* process sha1 results 4 times by 40 bits (0 to 160) */
for (i = 0, j = 0; i < 4 ; i++)
{
b_5 = (unsigned long long) sha_res[(i * 5)] << 32;
b_5 |= (unsigned long long) sha_res[(i * 5) + 1] << 24;
b_5 |= (unsigned long long) sha_res[(i * 5) + 2] << 16;
b_5 |= (unsigned long long) sha_res[(i * 5) + 3] << 8;
b_5 |= (unsigned long long) sha_res[(i * 5) + 4];
if (verbose)
{
printf(" {%010llX}->", b_5);
}
for (k = 35; k >= 0; k-= 5, j++) /* convert 40 bits (5x8) */
{
tpa_label[j] = base32[(b_5 >> k) & 0x1F];
if (verbose)
{
printf(" %02X:%c",
(unsigned int)(b_5 >> k) & 0x1F,
tpa_label[j]);
}
}
if (verbose)
{
printf ("\n");
}
}
if (verbose)
{
printf("\n");
}
tpa_label[j] = 0; /* terminate label string */
fprintf(out_file, "_%s", tpa_label);
printf("\n");
/* close */
Otis & Black Expires December 29, 2010 [Page 34]
Internet-Draft TPA-Label June 2010
if (out_mode)
{
if (fclose (out_file) != 0)
{
(void) fprintf(stderr,
"ERR: Unable to close %s output file.\n",
out_fn);
ret_val = 6;
}
}
if (in_mode)
{
if (fclose (in_file) != 0)
{
(void) fprintf(stderr,
"ERR: Unable to close %s input file.\n",
in_fn);
ret_val = 7;
}
}
return (ret_val);
}
Authors' Addresses
Douglas Otis
Trend Micro
10101 N. De Anza Blvd
Cupertino, CA 95014
USA
Phone: +1.408.257-1500
Email: doug_otis@trendmicro.com
Daniel Black
Canberra ACT
Australia
Email: daniel.subs@internode.on.net
Otis & Black Expires December 29, 2010 [Page 35]