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Appsawg                                                          D. Otis
Internet-Draft                                               Trend Micro
Intended status: Experimental                           October 13, 2015
Expires: April 15, 2016


                    Third-Party Authorization Label
                        draft-otis-tpa-label-08

Abstract

   This experimental specification proposes a Third-Party Authorization
   Label (TPA-Label) as a DNS-based method that allows Trusted Domains
   an efficient means to authorize acceptable Third-Party Domains.  This
   method permits autonomous unilateral authorizations and uses scalable
   individual DNS transactions.

   A TPA-Label Resource Record transaction asserts an alignment
   exception to convey informally Federated Domains.  It affords
   recipients a practical and safe means to extend Domain Alignment.
   Exceptions are managed by either the Trusted Domain, or their agent,
   seeking to avoid disruption of informal services enjoyed by their
   users.  Third-Party Authorization of a Federated Domain eliminates a
   need to share private credentials.


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
   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 April 15, 2016.



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Copyright Notice

   Copyright (c) 2015 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
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Domain Validation Issues . . . . . . . . . . . . . . . . . . .  6
   4.  Complaince with Non-Transactional Messages . . . . . . . . . .  7
   5.  TPA-Label Listed Domain, TPA-LLD,  . . . . . . . . . . . . . .  7
   6.  TPA-Label Resource Record Authorization Considerations . . . .  8
   7.  Evaluating the Third-Party Domain  . . . . . . . . . . . . . .  9
     7.1.  Third Party Authorization - Closed Mailing List Example  . 10
     7.2.  Third Party Authorization - Open Mailing List Example  . . 10
     7.3.  Third Party Authorization Example - Sender Header Field  . 10
     7.4.  Services Lacking DKIM Signatures . . . . . . . . . . . . . 11
       7.4.1.  Abuse and DSN Reporting  . . . . . . . . . . . . . . . 11
       7.4.2.  Third Party Authorization Example - SMTP Host  . . . . 11
       7.4.3.  Third Party Authorization Example - Return Path  . . . 11
       7.4.4.  Use of Path Authorization  . . . . . . . . . . . . . . 11
   8.  DNS Representation . . . . . . . . . . . . . . . . . . . . . . 12
   9.  TPA-Label and Tag Syntax Definitions . . . . . . . . . . . . . 13
   10. TPA-Label Generation . . . . . . . . . . . . . . . . . . . . . 13
   11. TPA-Label TXT Resource Record Structure  . . . . . . . . . . . 14
   12. TPA-Label Resource Record Definition . . . . . . . . . . . . . 15
   13. TPA-Label Resource Record Version  . . . . . . . . . . . . . . 15
   14. Authorized Validated Domains . . . . . . . . . . . . . . . . . 15
     14.1. TPA-Label Resource Record Param Syntax . . . . . . . . . . 16
     14.2. Header Dependent Authorizations  . . . . . . . . . . . . . 16
       14.2.1. List-ID Header Field . . . . . . . . . . . . . . . . . 16
       14.2.2. Sender Header Field  . . . . . . . . . . . . . . . . . 17
       14.2.3. OAR Header Field . . . . . . . . . . . . . . . . . . . 17
       14.2.4. Combined 'L' or 'S' Params . . . . . . . . . . . . . . 17
     14.3. DKIM signed domain . . . . . . . . . . . . . . . . . . . . 17
       14.3.1. DKIM signed  . . . . . . . . . . . . . . . . . . . . . 17



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     14.4. SMTP Host domains  . . . . . . . . . . . . . . . . . . . . 17
     14.5. SMTP Host domains  . . . . . . . . . . . . . . . . . . . . 17
     14.6. MailFrom Parameter . . . . . . . . . . . . . . . . . . . . 17
     14.7. Not Federated  . . . . . . . . . . . . . . . . . . . . . . 18
     14.8. SMTP Host domains  . . . . . . . . . . . . . . . . . . . . 18
   15. TPA-Label Resource Record Query Transactions . . . . . . . . . 18
   16. TPA-Label Resource Record Compliance Extension . . . . . . . . 19
   17. Alternative Mitigation Strategies  . . . . . . . . . . . . . . 20
     17.1. Proposed DKIM-Delegate Signature survives all Message
           modifications  . . . . . . . . . . . . . . . . . . . . . . 20
     17.2. Vouch-By-Reference . . . . . . . . . . . . . . . . . . . . 21
   18. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 21
   19. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     19.1. Moving RFC6541 to historic . . . . . . . . . . . . . . . . 22
     19.2. TPA-Label (TPA-LLD) Parameters . . . . . . . . . . . . . . 22
     19.3. Email Authentication Method Registry . . . . . . . . . . . 23
     19.4. Email Authentication Result Names Registry . . . . . . . . 23
     19.5. Third Party Authorizations Labels Registry . . . . . . . . 24
     19.6. Third Party Authorizations Param Registry  . . . . . . . . 24
   20. Security Considerations  . . . . . . . . . . . . . . . . . . . 25
     20.1. Benefits to Recipients . . . . . . . . . . . . . . . . . . 25
     20.2. Risks to Recipients  . . . . . . . . . . . . . . . . . . . 26
     20.3. Benefits to Trusted Domains  . . . . . . . . . . . . . . . 26
     20.4. Risks to Trusted Domains . . . . . . . . . . . . . . . . . 27
     20.5. Benefits to Third Party Signers  . . . . . . . . . . . . . 28
     20.6. Risks caused by Third Party Signers  . . . . . . . . . . . 28
     20.7. SHA-1 Collisions . . . . . . . . . . . . . . . . . . . . . 28
     20.8. DNS Limits . . . . . . . . . . . . . . . . . . . . . . . . 28
   21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 29
   22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     22.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     22.2. Informative References . . . . . . . . . . . . . . . . . . 31
   Appendix A.  DNS Example of TPA-Label Resource Record placement  . 33
   Appendix B.  C code for label generation . . . . . . . . . . . . . 34
   Appendix C.  History of Prior Efforts  . . . . . . . . . . . . . . 39
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41















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1.  Introduction

   A TPA-Label Resource Record supports an authorization of separately
   validated domains.  This added authorization step avoids a need to
   share private credentials.  Also, it ensures each domain remains
   apparent and open to validation when establishing an informal
   federation of domains protecting Federated Domain Identities.  To
   improve security, authorization records may also limit how they are
   to be applied.

   With TPA-Label Resource Records, mailing lists, among similar Third-
   Party Domain services, can indirectly assert protection of identities
   when the source domain is within an informally Federated Domain.
   Since mailing-lists receive differently formatted messages, a common
   practice is to convert multi-party conversations into consistent and
   compact formats facilitating the organization of the many multi-party
   conversations.  Such processing often breaks any meaningful message
   signature.  With the proposed scheme, trusting the federated message
   source can supersede otherwise broken alignment validation.

   Trusted Domains can seek to ensure the domains they federate protect
   the Federated Identity.  In situations where the Trusted Domain
   cannot be confirmed, TPA-Labels are able to signal which domains are
   within the Trusted Domain's informally established federation.  When
   a user wishes to utilize an informal Third-Party Domain service, it
   is both logical and desirable to retain the original Federated
   Identity to better convey who substantially created message content.
   Not retaining this identity would otherwise prohibit subsequent
   review of prior exchanges.  However, when recipients wish to
   determine whether to trust the Federated Identity, Domain Alignment
   with the validated source may not exist.  TPA-Labels can indicate
   whether the source domain has been informally federated by the
   Trusted Domain.

   Federating a domain also protects this domain's messages from being
   returned in DMARC feedback and possibly inadvertently exposing
   members in conversation with the Trusted Domain.  Such exposure might
   be used to facilitate convincing phishing attacks, for example.


2.  Terminology

   Please see [RFC5598] for general email terminology.

   The following additional terms are used:

   Transparent Domain Authorization:  Third-Party Domains validating as
      the Trusted Domain represent a type of transparent authorization.



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      This method normally depends on securely sharing private details
      between domain owners and providers.  However, private sharing
      between different administrative domains is expensive and carries
      some risk a security breach may result in the wrong administration
      being held accountable or more resources being placed in peril.


   Trusted Domain:  Often a visible domain that acts as a basis for
      acceptance and/or subsequent actions.  For DMARC, this is the
      fully qualified domain name found in the From header field.


   Author Domain:  See Section 3 of [DMARC] specifying the domain of the
      From header field which represents the Trusted Domain.


   Federated Domain:  A domain (among possibly others) working in
      concert with the Trusted Domain authorized as protecting Federated
      Identifiers.


   Federated Identity:  Identity protected by a Federated Domain.  In
      the case of DMARC, this identity is contained in the From header
      field.


   Domain Alignment:  Strict alignment requires matching the Trusted
      Domain.  Relaxed alignment allows source domains to be a sub-
      domain of the Trusted Domain.


   Third-Party Authorization:  A different domain authorized by the
      Trusted Domain.


   Informal Third-Party Service:  A Service not by the Trusted Domain
      that does not require administrative cooperation for users to
      independently establish their own access credentials.


   Third-Party Service:  A Service not by the Trusted Domain.


   Third-Party Domain:  A domain that is not the Trusted Domain.


   TPA-Label Authorization:  The referencing domain which meets the
      validation and header field content requirements of the resolved



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      TPA-Label Resource Record is thereby authorized by the Trusted
      Domain.


   TPA-Label Listed Domain, TPA-LLD:  TPA-Label Listed Domain, TPA-LLD,
      is a TXT Resource Record referenced with the hash value of the
      domain being authorized.  This Resource Record is published within
      a Trusted Domain.  When a "tpa" tag exists, the referencing domain
      (the domain used to generate the label) must be within the listed
      domains.  When the "tpa" tag does not exist, the referenced domain
      is presumed.  The "param" tag may stipulate a required existence
      of additional header fields, or indicate alternate domain
      validation methods to be applied against specific elements.  The
      "param" tag may also indicate that the domain matching in the
      prior "tpa" list validated according to "param" associated listed
      methods, is specifically not federated by the Trusted Domain.




3.  Domain Validation Issues

   Changing the validated domain, the one referenced by SPF [RFC7208],
   or the one adding a DKIM [RFC6376] signature, is not a problem since
   it is rare for acceptance to be based on From header field Domain
   Alignment.  However, when acceptance is based on the From header
   field alignment, as in the case of [DMARC] using either SPF or DKIM,
   this can disrupt many Third-Party Services.  The disruption becomes
   egregious when messages from the domain's own users are rejected
   based on the level of this domain's asserted alignment practices.  At
   the strictest alignment level, an erroneous assertion not only
   disrupts messages from their users, it can also affect subscriptions
   or services for other users of the Third-Party Service.

   DKIM, unlike SPF, permits better retention of From/signature
   alignment where only the From header field could be signed.
   Nevertheless, the integrity of the original DKIM signature is likely
   affected by message flattening, inclusion of Subject tags, or
   appended list footer information.  Just signing the From header field
   is not a practical solution, because it would expose this message
   fragment to replay abuse, even when given short signature expiry.

   TPA-Label authorization may individually authorize domain validation
   methods.  This may either increase or decrease the number of
   validation methods normally used.  For example, a virtual server may
   share an IP address with thousands of different domains.  Its
   authorization may need to exclude IP addresses as a basis for
   validation.  In the TPA-Label scheme, unless a validation method is



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   asserted, no changes to the domain validation process should be
   assumed.

   TPA-Labels can minimize the number of systems involved and the
   related deployment time before disruption of legitimate messaging is
   avoided.  TPA-Labels should also ensure greater cooperation to
   sustain desired protections.  These types of restrictions are
   increasingly likely to be relied upon to help mitigate harm caused by
   future breaches.


4.  Complaince with Non-Transactional Messages

   Administrative domains, that assert all of their outbound message
   sources can be validated as having aligned domains, offer significant
   forensic value.  However, messages where domains are not in alignment
   remain a potential issue.  Only domains offering messages of a
   transactional nature are unlikely to benefit from the use of TPA-
   Labels.

   This document describes how any Trusted Domain publishing DMARC
   records can autonomously authorize other validated domains.  TPA-
   Labels offer secondary compliance options whenever authorized
   exceptions are needed to permit the use of Third-Party Domains.  The
   intended purpose of TPA-Label Resource Records is to improve
   acceptance rates of genuine messages, to minimize DNS use, to
   minimize success rates for phishing, to improve sorting protections,
   and to minimize a recipient's administrative costs.

   TPA-Label Resource Records authorize Third-Party Domains and services
   to extend compliance options for asserted practices defined by
   [DMARC].  Domains, that both reference and are listed, and also
   comply with a TPA-Label resource record, should be considered
   equivalent to the authorizing Trusted Domain when assessing
   compliance with DMARC asserted practices.  Otherwise, DMARC may offer
   non-compliant messages as feedback to foreign domains which may
   unintentionally expose private exchanges on behalf of the Author
   domain's users or those of the Federated domain.


5.  TPA-Label Listed Domain, TPA-LLD,

   TPA-Label Listed Domain, TPA-LLD, is a TXT resource record referenced
   with a TPA-Label published within a Trusted Domain.  When a "tpa" tag
   exists within the TXT resource record located at the TPA-Label, the
   referencing domain (the domain used to generate the label) must be
   within the listed domain.  When the "tpa" tag does not exist, the
   referenced domain is presumed listed.  The "param" tag may stipulate



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   existence of additional header fields, or indicate alternate
   validation methods applied against specific email elements.

   Third-Party Domain validation might use a DKIM signature or confirm
   the Authorized Domain using specific methods with various path
   related email elements.  The default assertion for param is 'd' and
   'm', indicating DKIM or the Mail From parameter processed by SPF
   confirms the Authorized Domain when no other method is specified.
   The 'S' and 'L' param do not confirm the domain, but requires at
   least one Sender or List-ID header field to hold a TPA-LLD
   respectively.  The 'O' param can be stipulated when the Authorized
   Domain does not offer DMARC acceptance or validate access in
   association with From header fields.  The 'e', 'h', and 't' indicate
   specific alternative methods using message elements to confirm the
   Authorized Domain.

   When any param method is asserted (denoted by a lower case letter),
   methods not listed should not be considered to provide valid results.
   The 'n' assertion indicates that the prior domain listed in the same
   TPA-LLD is specifically not federated as determined by either the
   default or listed methods.  Being compliant with TPA-LLD allows the
   referencing domain to informally act on behalf of the Trusted Domain.
   Indicating domains as not federated necessitates the use of sequence
   sensitive "tpa" and "param" pairs within the TPA-LLD.  Per [RFC5321],
   domain name comparisons, as well as TPA-Labels, are case insensitive.


6.  TPA-Label Resource Record Authorization Considerations

   When a Trusted Domain is not within a DKIM or SPF validated domain,
   the TPA-LLD scheme can extend Domain Alignment compliance.  The TPA-
   LLD scheme with an 'S', or 'L' param requires the respective Sender
   header field or a List-ID identifier of the List-ID header field to
   exist for at least one of the params, and to contain a domain within
   the TPA-LLD for authorization to be valid.  The 'd' param permits
   validations based upon the DKIM signing domain.  The 'm' param
   permits validations based upon the return path (Mail From) domain.
   The 'e', 'h', and 't' params permit acceptance based upon validation
   of the client hostname (EHLO/HELO).

   The 'S' and 'L' params support message sorting.  Any matching header
   field with a domain within the TPA-LLD allows recipients to
   differentiate sources, which satisfies requirements for any other 'S'
   or 'L' param.  The 'S' and 'L' params provide Trusted Domains a means
   to limit domain authorizations.

   The TPA-LLD scheme plays the role of only qualifying acceptable
   domains with the goal of improving delivery acceptance, such as



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   messages from specific mailing-lists.  The TPA-LLD authorization
   scheme only requires that DNS publications be made by the Trusted
   Domain, even when the sending domains and the Trusted Domain differ.
   This approach eliminates a need to exchange private information thus
   protecting the domain's integrity.  Before TPA-LLD authorization is
   deployed, the Trusted Domain should be assured by domains being
   authorized that appropriate measures are in place to validate those
   submitting messages and ensure the Federated Identity is protected.

   Retaining validation and authorization for the From, Sender, and
   List-ID header fields, and being able to ensure Third-party inclusion
   of a Sender or List-ID header fields, enhances protections afforded
   by message sorting.  This protection reduces susceptibility to
   deceptive look-alike phishing attempts.  Use of subdomains that
   assert less stringent practices might inadvertently combine with
   those having more stringent practices when sorting is based upon
   parent domains.  Consistently using the same domain prevents possible
   confusion that could be exploited to deceive recipients.

   TPA-Label authorization will not ensure all possible spoofing is
   prevented.  However, by permitting broader use of strict alignment
   practices, 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
   validated identities.  The TPA-LLD param SHOULD include the 'S' or
   'L' param where appropriate to allow recipients a means to isolate
   and distinguish different message sources.



7.  Evaluating the Third-Party Domain

   A Trusted 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 DMARC records while also
   utilizing Third-Party Services.

   When an authorized Third Party domain does not employ DKIM or SPF or
   does not include Authentication-Results header fields [RFC7001] or
   perhaps [I-D.kucherawy-original-authres] (OAR) or its "X-" version
   could allow authorizations to be exploited.  For Third Party domains
   not applying DMARC but capture the OAR, past compliance with DMARC
   based on the OAR can be made a requirement for authorization.

   While conceivably Domain Alignment might just rely on the content of
   the Original-Authentication-Results header, whether to trust this, or
   any other message content can not be based on the mere acceptance of
   the message alone.  Whether false content even effects message



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   acceptance would be difficult to determine.  Only the Trusted Domain
   is able to make this type of determination based on their knowledge
   of outbound messages and corrections needed based on DMARC feedback
   which they then share in the form of a TPA-Label.

7.1.  Third Party Authorization - Closed Mailing List Example

   The Trusted Domain wants to deploy a TPA-Label Resource Record for a
   mailing list with a closed posting policy.  The mailing list
   redistributes email which breaks the Trusted Domain Alignment, and
   the mailing list offers a means to validate the mailing list domain
   and includes an Authentication-Results header field for posted
   messages.  The closed posting policy can be enforced by requiring
   subscribers to validate control of their Author Addresses by
   responding to encoded "pingback" email sent to these addresses.

   Since the mailing list validates their domain as indicated in the
   TPA-Label, and validates control of the posted message Author
   Address, and includes Authentication-Results header fields, and
   includes a List-ID header field, the referenced TPA-Label Resource
   Record can include an 'L' param value to stipulate that the Third-
   Party Domain messages contain an authorized List-ID domain.

7.2.  Third Party Authorization - Open Mailing List Example

   The Trusted Domain wants to deploy a TPA-Label Resource Record for a
   mailing list with an open posting policy.  The mailing list
   redistributes email in a way that breaks Trusted Domain alignment,
   does not post from an Author Address not in compliance with DMARC,
   offers a means to validate the mailing list domain, and it includes
   an Authentication-Results header field for posted messages.

   Since the mailing list validates the domain as indicated in the TPA-
   Label, and is configured to include Authentication-Results header
   fields and possibly the Original-Authentication-Results
   [I-D.kucherawy-original-authres], and includes a List-ID header
   field, the referenced TPA-Label Resource Record can include an 'L'
   param value to stipulate the Third-Party Domain messages contain an
   authorized List-ID domain.

7.3.  Third Party Authorization Example - Sender Header Field

   Trusted 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.




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   Since "temp.example.org" 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' and 'd' param
   to specifically authorize DKIM signed messages containing the Sender
   header field, to help ensure these messages are not handled as
   phishing attempts.

7.4.  Services Lacking DKIM Signatures

7.4.1.  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
   the Trusted Domain is confirmed as having handled or authorized the
   handling of the message, only statistics and samples should be
   reported to the associated Autonomous System [RFC1930], and perhaps
   to the Trusted Domain when interest is expressed.

   The 'd', 'e', 'h', 'm', and 't' param options within the TPA-LLD
   records allow the Trusted Domain to be associated through various
   methods.  In this case, appropriate DSN or abuse reporting to the
   Trusted Domain is better assured as well.

7.4.2.  Third Party Authorization Example - SMTP Host

   Trusted 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 Trusted Domain can authorize
   the domain "invite.example.net" or "example.net" with the param of
   'e' to improve acceptance of messages that are sent on behalf of
   "example.com" from this outbound server.


7.4.3.  Third Party Authorization Example - Return Path

   Trusted Domain "example.com" makes use of tell-a-friend services.
   This service does not utilize DKIM with its own return path as
   "customer@taf.example.net" in the SMTP exchange.  The Trusted Domain
   can authorize the domain "taf.example.net" with the param of 'm' to
   improve acceptance of messages that are sent on behalf of
   "example.com" from this outbound server.


7.4.4.  Use of Path Authorization

   Those using validations related to 'e', 'h', 'm' param options should



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   not authorize domains requiring more than an average number of
   network transactions.  Those implementing DMARC should also limit the
   number of DNS transactions attempted, otherwise this could negatively
   impact unrelated domains when evaluating path related validation.

      Methods that create subsequent transactions based upon the macro
      expansion of email-address local-parts should not be used.
      Libraries that process SPF [RFC7208] record scripts may invoke a
      large number of DNS transactions from cached records, and target
      unrelated domains with queries modulated by the local-part
      component through receiver macro expansion.


8.  DNS Representation

   The receiver obtains domain authorizations with a DNS query for an IN
   class TXT TPA-Label Resource Record located below the
   "_smtp._tpa.<Trusted-Domain>" location.  The TPA-Label itself is
   generated by processing the domain in question, which normally
   matches the DKIM signature's "d=" parameter.  The Trusted Domain
   provides authorization for other domains with the existence of a TPA-
   Label TXT resource record.  When a "tpa" tag value exists, it MUST
   include the referenced domain before authorization is valid.  This
   represents an informal authorization on behalf of the Trusted Domain
   which can be limited by the "param" tag value for specific message
   elements.

   A Trusted 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.<Trusted-Domain> zone to the
   administrative entity handling publication of TPA-Label Resource
   Records.  This delegation could also be done unilaterally with a
   DNAME [RFC6672] resource record published at _smtp._tpa.<Trusted-
   Domain>.

   Character-strings contained within the TXT resource record are
   concatenated into forming a single string.  A character-string, as
   defined in [RFC1035] Section 3.3 for resource records, 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>._smtp._tpa.<Trusted-Domain>.


9.  TPA-Label and Tag Syntax Definitions



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   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

           FWS       =  ([*WSP CRLF] 1*WSP) ; omits RFC5322 obs-FWS
           tag-sep   =  %x3B ; "%"
           tag-list  =  tag-spec 0*( tag-sep tag-spec ) [ tag-sep ]
           tag-spec  =  [FWS] tag-name [FWS] "=" [FWS] tag-value [FWS]
           tag-name  =  ALPHA 0*ALNUMPUNC / "v" / [["tpa"] / ["param"]]
           tag-value =  [ tval 0*( 1*(WSP / FWS) tval ) ]
                           ; WSP and FWS prohibited at beginning and end
           tval      =  1*VALCHAR
           VALCHAR   =  %x21-3A / %x3C-7E
                           ; EXCLAMATION to TILDE except SEMICOLON
           ALNUMPUNC =  ALPHA / DIGIT / "_"


10.  TPA-Label Generation

   The TPA-Label is generated by nesting functions as follows:

      "base32" function is defined in [RFC4648].

      "sha1" function is defined in [FIPS.180-2.2002].

      "lcase" converts upper-case ALPHA characters to lower-case.

      "tpa-domain" is normally the "d=" tag value defined in Section 3.5
      of [RFC6376].

      (underscore) base32( sha1( lcase(tpa-domain)))

   The TPA-Label is created from the hash value returned by the "sha1"
   function of the tpa-domain expressed in lower case ASCII.  Any
   terminating period is not included with the tpa-domain, as indicated
   by the ABNF definition.

      Note: No newline character, 0x0A, is to be appended to the end of



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      the domain name, as might occur with the command line generation
      of sha1 values.  For example, these command line appended newlines
      can be avoided by using the 'echo -n" option.

   The label encoding process inputs the hash as a byte stream of four
   40-bit data blocks where each data block outputs 8 encoded
   characters.  Proceeding from left to right, a 40-bit input group is
   formed by concatenating 5 bytes.  The 40-bit input is then treated as
   8 concatenated 5-bit groups, each of which is translated into a
   single digit of the base32 alphabet.  The bit stream is ordered with
   the most-significant-bit first, being the high-order bit of the first
   byte.  The entire output is then concatenated first to last, left to
   right, into 32 characters prefixed with an underscore.


11.  TPA-Label TXT Resource Record Structure

   Every TPA-Label TXT resource record MUST start with the version tag,
   so the first six characters of the record are lowercase "v=tpa1",
   TPA-Label syntax descriptions for additional tags follow the tag-
   value syntax described in the ABNF below, the WSP token is inherited
   from [RFC5322].  The ALPHA and DIGIT tokens are imported from
   [RFC5234].  The "param" values refer only to domains previously
   listed in the TPA-LLD which makes the "tpa" and "param" tags
   sensitive to their sequence.

   The tags used in TPA-Label Resource Records are as follows:

           +------------+--------------------------------------+
           |     Tag    | Function                             |
           +------------+--------------------------------------+
           |      v     | Label Version (version-tag)          |
           |     tpa    | Authorized Domains List (tpa-tag)    |
           |    param   | Authorization Param List (param-tag) |
           +------------+--------------------------------------+

                              TPA-Label Tags














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   +--------+------------------------------------+---------------------+
   |  param | Field or Parameter                 | Method              |
   | values |                                    |                     |
   +--------+------------------------------------+---------------------+
   |    L   | List-ID Header Field               | Match List-ID       |
   |        |                                    | Identifier          |
   |    S   | Sender Header Field                | Match Address       |
   |        |                                    | Domain              |
   |    O   | Original Authentication Results    | Match Address       |
   |        | Header Field                       | Domain              |
   |    d   | DKIM Signature                     | Match Signature     |
   |        |                                    | Domain              |
   |    e   | SMTP Hostname                      | Resolve Hostname IP |
   |        |                                    | Addr                |
   |    h   | SMTP Hostname                      | Pass SPF with       |
   |        |                                    | Hostname            |
   |    n   | Not Federated                      | See Other Params    |
   |    m   | MailFrom                           | Pass SPF with       |
   |        |                                    | MailFrom            |
   |    t   | SMTP Hostname                      | Cert of Hostname    |
   +--------+------------------------------------+---------------------+

                          TPA-Label Param Values


12.  TPA-Label Resource Record Definition

   Tags in the TPA-Label Resourse Record are shown below.  The ver-tag
   MUST be present as the left most tag.  Unrecognized tags MUST be
   ignored.

   TPA-Label Resource Record Definition
      tpalabelrr = v-tag [tag-sep] 0*( 1*(WSP) tag-list) ]


13.  TPA-Label Resource Record Version

   Label Version (Required).  This tag defines the version of the TPA-
   Label.  Only recognized tpa:param values offer DMARC authorizations
   (except for param "n" which excludes domains validated per other
   param values).

   "v" tag
      v-tag = %76.3d.74.70.61.31 ; "v=tpa1"


14.  Authorized Validated Domains




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   Authorized validated domain list. (optional) This tag, when present,
   MUST contain a domain that repeats all or right-most 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.  The purpose
   of the ANY label is to reduce the size of the resource records.
   Containing the entire string to confirm hostnames or List-ID content
   is unnecessary.  The hash label must still be an exact match of the
   domain authorized.  Additional domains may be included as optional
   Sender or List-ID comparison options.  The tpa list is optionally
   followed by param list.  There can be multiple tpa:param sets.

   Use of the ANY label is not intended to support wildcards for
   referencing hash labels.  No wildcard labels are to be used below the
   "_tpa." label to access DNS resources.

   "tpa" tag
      ad_val = [ANY] domain
      ad-list = %x74.70.61 *WSP "=" [ ad_val 0*( 1*(WSP) ad_val )]

14.1.  TPA-Label Resource Record Param Syntax

   Authorization Param List (Optional).  This tag defines a list of
   assertions for the preceding (if listed) domains which indicate
   various email-address locations within the message and authorized
   validation methods.  Only recognized param values offer any form of
   DMARC authorization.  The "n" param however excludes the prior tpa
   list as not being within the federation.

   "param" tag
      pa_val = "L" / "S" / "O" / "d" / "e" / "h" / "m" / "n" / "t"
      pa-list = %x73.63.6f.70.65 *WSP "=" [ pa_val 0*( 1*(WSP) pa_val )]

14.2.  Header Dependent Authorizations

14.2.1.  List-ID Header Field

   The "L" param asserts that authorization is valid only when a List-ID
   identifier of the List-ID header field [RFC2919] contains a domain
   that is within a domain listed in the TPA-LLD "tpa" tag.

   The syntax of the List-Id header field is as follows:
      list-id-header = "List-ID:" [phrase] "<"identifier">"CRLF





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14.2.2.  Sender Header Field

   The "S" param asserts that authorization is valid only when the
   domain in the Sender header field is within the TPA-LLD.


14.2.3.  OAR Header Field

   The "O" param asserts that authorization is valid only when the
   domain in an Original Authentication Results header field indicates
   it passed based on a domain within the TPA-LLD or the Trusted Domain
   itself.


14.2.4.  Combined 'L' or 'S' Params

   When combined, the params 'L' and 'S' require that either a List-ID
   identifier of the List-ID header field or the Sender header field
   must contain a domain within the TPA-LLD for the authorization to be
   valid.

14.3.  DKIM signed domain

14.3.1.  DKIM signed

   The "d" param asserts that messages carrying the Trusted Domain
   within the From header field are authorized to be signed by the TPA-
   LLD.


14.4.  SMTP Host domains

   The "e" param asserts that host names given in [RFC5321] EHLO or HELO
   commands within TPA-LLD is authorized when the hostname resolves the
   server's IP address.


14.5.  SMTP Host domains

   The "h" param asserts that host names given in [RFC5321] EHLO or HELO
   commands within TPA-LLD is authorized only when this hostname
   submitted to an SPF [RFC7208] process returns pass.


14.6.  MailFrom Parameter

   The "m" param asserts that an email-address domain in the [RFC5321]
   MAIL command within a TPA-LLD is authorized only when this email-



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   address submitted to an SPF [RFC7208] process returns pass.


14.7.  Not Federated

   The "n" param asserts that a previous "tpa" listed domain is
   specifically not federated and not authorized.  The use of this
   parameter is to suppress subsequent processing that may otherwise be
   needlessly repeated in the case where the third-party is being
   detected as conveying user messages but are not authorized for policy
   related reasons, such as not adequately protecting the Federated
   Identity.


14.8.  SMTP Host domains

   The "t" param asserts that host names given in [RFC5321] EHLO command
   after [RFC3207] negotiation where the Cert DNS-ID domain is within
   TPA-LLD is authorized.  It will also be interesting to see whether
   [I-D.ietf-dane-smtp-with-dane] establishes a way to authenticate
   sending domains.

   Note to RFC Editor: Remove this comment before publishing.
      Currently, no general practice employs certificates to confirm the
      domain of the client initiating a connection.  This may be needed
      for clients within IPv6 IP address space where tunneling, carrier
      grade NATs, and rapid space assignment without any practical
      reverse mapping reduces the effectiveness of IP address based
      reputations.
      There is an existing TLS option for SMTP and an ongoing effort to
      standardize automated server confirmation.  It might be possible
      to leverage this effort to establish practices used at the client.
      See conversations defined in [RFC4954] Section 4.  For information
      related to ongoing server related efforts see:
      [RFC6125] and [RFC6698]


15.  TPA-Label Resource Record Query Transactions

   The discovery of TPA-Label Resource Records need not be subsequent to
   the discovery of the DMARC record.  However, when no DMARC record is
   discovered which includes the tag value of "tpa", the verifier MAY
   assume no TPA-Label Resource Records have been published.  Otherwise,
   when there is no Trusted Domain validation, the discovery of TPA-
   Label Resource Records should be attempted.


16.  TPA-Label Resource Record Compliance Extension



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   The signing practice compliance assessment of Third Party Signatures
   is a discretionary operation performed by the verifier.  For messages
   that do not have valid Trusted Domain alignment, a verifier may
   decide to assess compliance for Third Party messages when there is a
   DMARC tag of "tpa".  Elements then referenced in the TPA-Label param
   values of "d", "m", "e", "h", "t" are to be checked in their listed
   succession.  One of the following sets of conditions MUST be met for
   the result to be considered a pass:

   For Third Party DKIM signatures, the following represents the set of
   conditions to be checked:

   o  The Third Party Signature MUST validate according to [RFC6376].
   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 Resource Record structure MUST be valid.
   o  The domain that created the TPA-Label MUST be within the TPA-LLD.
   o  Where a param of 'd' is specified, the Trusted Domain MUST have an
      authorized DKIM signature.
   o  Where a param of 'L' or 'S' is specified, a List-ID identifier in
      the List-ID header field or a Sender header field MUST contain a
      domain within the TPA-LLD.  This provides Third-Party services a
      reason to ensure their outbound messages do not spoof these
      associated header fields.
   o  Where a param of 'O' is specified, an Original Authentication
      Results header field MUST indicate a pass for the Trusted Domain
      or for a domain within the TPA-LLD.  This parameter requires the
      message was received from an approved Originating source.

   For non-DKIM validations, the TXT record discovery process continues
   until a TPA-Label Resource Record structure is found where:

   One of the three possible TXT resource records is checked in their
   listed succession.  Each would be referenced by an 'h' or 'e' or 't'
   related 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.


   o  The discovered TPA-Label 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 with a listed
      param of 'e', 'h' or 'm' or 't'.
   o  Where a param of 'L' or 'S' is specified, either the domain in
      List-ID given by [RFC2919] in the List-ID header field is within
      the TPA-LLD, or a Sender header field contains a domain within the
      TPA-LLD respectively.
   o  Once these four conditions have been met, for 'h' or 'm' params



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      the domain MUST be confirmed by submitting the domain to an SPF
      process that then returns pass.  The 'e' param MUST be confirmed
      by a forward DNS reference that resolves the address of the SMTP
      client.  The 't' param MUST be confirmed by the DNS-ID in the
      client certificate.

   When the TPA-Label Resource Record can not be retrieved due to some
   error that is likely transient in nature, such as "SERVFAIL" for
   example, the result of the TPA-Label Resource Record compliance
   assessment is "temperror".

   When the TPA-Label 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 Resource Record can not be retrieved with a DNS
   "NXDOMAIN" response, the result of the TPA-Label Resource Record
   compliance assessment is "nxdomain".



17.  Alternative Mitigation Strategies

17.1.  Proposed DKIM-Delegate Signature survives all Message
       modifications

   When a domain sends a message to services likely to invalidate DKIM
   signatures, such as that of a mailing-list, some envision use of a
   [I-D.kucherawy-dkim-delegate] header field as a type of DMARC
   disruption prophylactic conveying signed Third-Party Authorizations
   similar to that of TPA-LLD.

   The DKIM-Delegate header field is not a DKIM message signature nor
   can it generally authorize trustworthy sources.  This header field
   represents a signed domain authorization list placed in the header
   field's 't' tag.  To limit its duration, short expiry can be
   specified in the 'x' tag.  Nevertheless, validity of this header
   field is completely independent of the message body or any other
   message header field.

   Since DKIM-Delegate authorization is actually unrelated to any
   specific message, it offers less protection at the expense of a
   higher transactional overhead compared to that of the smaller and
   simpler TPA-LLD transactions.  The TPA-LLD resource also better
   facilitates timely authorization retractions.  Before DKIM-Delegate
   can be as effective, separate and much larger signature resources
   would be needed for each authorized domain to match the timely de-
   authorization selectivity the TPA-Label scheme permits.



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   Since either scheme is primarily aimed at circumventing DMARC
   compliance issues, once adopted, changes to DMARC related validation
   can quickly bolster DKIM/DMARC legacy shortfalls in a similar fashion
   to that offered by TPA-LLD.  It is even conceivable, that to obtain
   an automated authorization expiry, a TPA-LLD could signal when a
   DKIM-Delegate should be included.  The TPA-LLD can still reduce the
   authorization latitude offered by DKIM-Delegate.  Unlike TPA-LLD,
   DKIM-Delegate is unable to stipulate an additional set of required
   conditions.

   Since mailing-lists are normally fairly public, bad actors only need
   to subscribe to obtain a fresh set of DKIM-Delegate header fields as
   a means to circumvent its expiry feature.  Using freshly minted DKIM-
   Delegate header fields also avoids elements that might be identified
   as pertaining to a specific campaign at little cost to the
   malefactor.

17.2.  Vouch-By-Reference

   VBR uses a third-party domain referenced in a message header to vouch
   for the types of messaging expected from a domain verified as part of
   normal message handling.  Since any domain other than the DMARC
   domain attempting to guard against From header field spoofing is
   unable to make such assessments, for the purposes of Anti-Phishing,
   VBR lacks the necessary input and knowledge to offer timely and
   accurate advice.  The nature of Phishing abuse is often too low in
   frequency for typical Anti-Spam policies to be effective.


18.  Privacy Considerations

   Unless all valid Third-Party Domains have been authorized, personally
   identifiable information will be exchanged within the DMARC feedback.
   This feedback can unintentionally expose private exchanges made on
   behalf or the DMARC domain's users.  To the greatest extent possible,
   this feedback information should not be shared with other domains not
   offering the information.  This feedback can even identify mailing-
   list subscribers that never sent any message to the list, or invoices
   made on behalf of an accountant's client.

   As with other authorization schemes that utilize DNS, relationships
   are publicly revealed.  This is the nature of SPF authorization which
   reveals first party services being used.  A TPA-Label on the other
   hand can resolve a hash obscured Third-Party Service.  Unlike SPF, a
   TPA-Label does not include any user identity related parameters and
   does not reveal any users specific relationships.  In addition, these
   relationships are accessed with a hash of the entire domain.  Use of
   a few random subdomains can inhibit discovery of these relationships.



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   However, the low latency of DNS means resource records can not be
   assumed to remain secret.

   Even so, disclosures of Third-Party Services might be justified by
   dissuading malefactors who have compromised the Trusted Domain and
   then are able to subsequently spoof the discovered personal
   relationships.  Such spoofing might be seen as causing greater harm
   than public knowledge of possible Third-Party Services used by the
   Trusted Domain's users.

   It seems this can not be overstated: The overhead associated with
   managing a "_tpa." zone is fairly small and is well offset by
   squelching DMARC feedback generation and the remediation of a loss of
   legitimate messages.  Alternatives to TPA-Labels are likely to be the
   dissemination of plaintext lists of domains known to cause alignment
   failures, although operating in full compliance with SMTP protocols
   and practices.  The dissemination of lists lessens the domain's
   privacy, and their ability to react to mitigate abuse.


19.  IANA Considerations

19.1.  Moving RFC6541 to historic

   This document is seeking to replace [RFC6541] and to move it to
   historic.

19.2.  TPA-Label (TPA-LLD) Parameters

   To accommodate the extensions to [RFC7001] needs the following
   elements to be added:

                        +------+-----------------+
                        | Type |    Reference    |
                        +------+-----------------+
                        |  tpa | (this document) |
                        +------+-----------------+

                TPA-Label Resource Record validation Method












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19.3.  Email Authentication Method Registry

   To accommodate the method derived from TPA-Label Resource Record
   processing, the IANA Registry "Email Authentication Method" defined
   by Section 6.2 of [RFC7001] needs the following elements to be added:

   +---------+-----------+--------+----------+-------------------------+
   | Method  |  Defined  |  ptype | property | value                   |
   +---------+-----------+--------+----------+-------------------------+
   | tpa-lld |   (this   | domain | 3p-dom   | Domain evaluated.  The  |
   |         | document) |        |          | method results from     |
   |         |           |        |          | [RFC7001] should also   |
   |         |           |        |          | be included in a        |
   |         |           |        |          | Authenticated Results   |
   |         |           |        |          | header field.           |
   |         |           |        | param    | value of param          |
   |         |           |        |          | (Section 19.6) tag.     |
   |         |           |        |          | (When 'param' contains  |
   |         |           |        |          | 'e', 'h' or 'm', the    |
   |         |           |        |          | iprev [RFC7001]         |
   |         |           |        |          | (Section 3) method      |
   |         |           |        |          | results should also be  |
   |         |           |        |          | included in the         |
   |         |           |        |          | Authenticated-Results   |
   |         |           |        |          | header field to capture |
   |         |           |        |          | the SMTP client IP      |
   |         |           |        |          | address.                |
   |         |           |        | ca-param | The params              |
   |         |           |        |          | (Section 19.6) with a   |
   |         |           |        |          | compliance assessment   |
   |         |           |        |          | as pass                 |
   |         |           |        | tpa      | Value of tpa            |
   |         |           |        |          | (Section 14) tag at     |
   |         |           |        |          | time of compliance      |
   |         |           |        |          | assessment              |
   +---------+-----------+--------+----------+-------------------------+

                TPA-Label Resource Record validation Method


19.4.  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 [RFC7001] needs the following elements added:






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   +----------+----------+---------------------------------------------+
   |   code   |  method  | meaning                                     |
   +----------+----------+---------------------------------------------+
   |   none   |  tpa-lld | No TPA-Label was published                  |
   |   pass   |  tpa-lld | Section 16                                  |
   | tempfail |  tpa-lld | Section 16                                  |
   | permfail |  tpa-lld | Section 16                                  |
   |  hdrfail |  tpa-lld | The TPA-Label Resource Record param values  |
   |          |          | of "S" or "L" failed to match.              |
   | nxdomain |  tpa-lld | When obtaining the TPA-Label Resource       |
   |          |          | Record, DNS indicated this domain does not  |
   |          |          | exist.                                      |
   +----------+----------+---------------------------------------------+

          TPA-Label Resource Record complaince assessment Results

19.5.  Third Party Authorizations Labels Registry

   Names of tags that are valid in TPA-Label Resource Records with the
   exception of experimental tags Section 11 MUST be registered in this
   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                       |
   +-----------------+--------------+----------------------------------+
   |        v        | Section 11   | Label Version                    |
   |       tpa       | Section 14   | List of Authorized Domains or    |
   |                 |              | Identifiers                      |
   | prior tpa param | Section 14.1 | Section 19.6                     |
   +-----------------+--------------+----------------------------------+

          TPA-Label Resource Record compliance assessment Results

19.6.  Third Party Authorizations Param Registry

   Values that correspond to Section 14.1 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



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   [RFC5226].

   Each value registered must correspond to a definition.

   The initial set of values for this registry is:

                      +------------+----------------+
                      |    value   | defined        |
                      +------------+----------------+
                      |      L     | Section 14.2.1 |
                      |      S     | Section 14.2.2 |
                      |      O     | Section 14.2.3 |
                      |      d     | Section 14.3   |
                      |      h     | Section 14.5   |
                      |      e     | Section 14.4   |
                      |      m     | Section 14.6   |
                      |      n     | Section 14.7   |
                      |      t     | Section 14.8   |
                      +------------+----------------+

          TPA-Label Resource Record compliance assessment Results


20.  Security Considerations

   This draft extends Domain Alignment validation practices that depend
   on DKIM [RFC6376] or SPF [RFC7208].  Most related security matters
   are discussed in those specifications.  Additional considerations are
   also included in [RFC6377].  Security considerations for the TPA-LLD
   scheme are mostly related to attempts on the part of malefactors to
   falsely represent themselves as others, often in an attempt to
   defraud either the recipient or the alleged originator.  Some
   receivers mistakenly bypass validation of the [RFC5322] header fields
   because a signature from a Trusted Domain had been confirmed as
   perhaps suggested in [RFC5863].  Do not omit the validation of header
   fields unless the message is not accepted for other reasons.

   Additional security considerations regarding DKIM signing practices
   may be found in the DKIM threat analysis [RFC4686].

20.1.  Benefits to Recipients

   The verifier, after validating a Federated Domain, will have
   significantly greater confidence in the Third-Party, than when no
   TPA-Label Resource Record is obtained.  This enhanced confidence may,
   at the receivers' discretion, cause a message to be delivered to the
   recipient with less stringent assessments.




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20.2.  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 validation methods
   determined by authorization param labels.  When the 'e', 'h', or 'm'
   param domain is not confirmed, or the Third-Party Domain does not
   validate the submitter, there is a risk of accepting potentially
   spoofed messages.  Authentication-Results header fields then play an
   important role when there is no out-of-band validations confirming
   the submitter.  Without proper Authentication-Results handling by the
   Third-Party, there is also risk of accepting potentially spoofed
   messages.

   With the TPA-Label specification, third party validation provides
   verifiable value.  Implementers should consider the possibility a
   malefactor will send a message having a large number of valid DKIM
   Signatures.  Verifying all the signatures may consume a large amount
   of processing resources.  As such, it might be worth checking for the
   existence of a TPA-Label Resource Record first to minimize network
   amplification concerns.  Section 15 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.

   Services that depend only upon path authorizations might permit the
   Trusted Domain to be spoofed and yet obtain acceptance.  During such
   events, the Trusted Domain might need to retract its authorization
   from the service.  For this reason, path related validation based on
   IP addresses should only be used as a carefully monitored interim
   solution.

20.3.  Benefits to Trusted 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 Trusted Domains an ability to limit the scope of their
   authorizations, while also not being mistaken for having validated
   the entity submitting the message.

   TPA-Label Resource Records convey which domains are authoritative
   even when they are not the Trusted Domain.  However, Authorized



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   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 Federated Identity of an email undergoes validation
   that offers acceptable protections for the Trusted Domain.  For
   example, such validation might ensure submitting entities have
   demonstrated receipt of "pingback" messages sent to the Federated
   Identity (Author's address) contained within the messages being
   signed.

   By deploying TPA-Label Resource Records, Trusted Domains benefit when
   recipients assess the senders' practice compliance by using the TPA-
   LLD scheme.  These recipients will be less likely to drop the Trusted
   Domain's genuine messages, whenever the Trusted Domain attempts to
   restrict acceptance.  Restricting acceptance of non-compliant
   messages is the basic motivation for publishing DMARC records.  In
   addition, recipients are more likely to validate messages by an
   Authorized Domain.

   Broader use of strict DMARC alignment assertions provides a greater
   likelihood of being able to eliminate a broader range of non-
   compliant messages, in addition to improving acceptance from
   authorized sources.  TPA-Labels also allow Trusted Domains to control
   message Sender and List-ID attributes, to exclude problematic
   validation methods or include others as they become available.

   Trusted Domains having good reputations might extend limited
   compliance assessment resources to otherwise unknown domains or SMTP
   Clients that are referenced by their TPA-LLD.  Conversely, TPA-LLD
   resources that assert a domain as not being federated can be used to
   suppress any processing that might be otherwise needlessly repeated.

   Privacy is better protected when messages from Federated domains
   won't contribute to a collection of messages as Feedback returned
   using less secure methods or that exposes the Federated members of a
   conversation.

20.4.  Risks to Trusted Domains

   As indicated in Section 7, it is ultimately an issue of trusting the
   Third Party Domain to do the right thing and not generate, or allow
   others to generate, messages that falsely appear to be from the
   Trusted Domain.  The validation methods in place for different email
   elements need to be carefully reflected in the "param" tag of the
   TPA-LLD.

   Authorization of mailing lists with TPA-LLD could cause a loss of
   confidentiality in mailing list participation by the Trusted Domain.



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   This might help malefactors deduce which subscription related email
   the Trusted 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
   domain.  In addition, service organizations or community groups are
   able to share comprehensive lists.  Such possible sharing means even
   though a domain has been authorized, that in itself does not mean the
   Trusted Domain is exchanging messages with the Authorized Domain.

20.5.  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.

20.6.  Risks caused by Third Party Signers

   As mentioned before, Authorized Third Party Signers need to validate
   messages from Trusted Domains.  This validation provides a safety
   mechanism for the Trusted Domain and their recipients.  The Third
   Party may not be aware of the validation value or the message
   elements involved, and as a result make changes without understanding
   the impact this may have on Trusted Domains and their recipients.
   For example, the Third Party might stop DKIM signing or stop applying
   Authentication-Results header fields.  The unexpected exposure that
   this might enable could allow abuse and prove detrimental for both
   the Trusted Domain and their recipients.

20.7.  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
   less than a 0.1 percent risk of any two domain labels colliding.

20.8.  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



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   encode international character sets.  Perhaps, soon there will be a
   further 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 adds an additional byte by having a right-most label that
   represents the root "." signified as a zero length label.  A labeling
   scheme that combines together a listed domain with the publishing
   domain separated by some label for this convention, reduces the
   maximal domain name in half, where the convention label reduces this
   further.

   If "_smtp._tpa." were used as the convention label with a simple
   listing method, the maximum domain name size this supports would be
   128 bytes.  The suffix for TPA-Labels is "_smtp._tpa." which consumes
   11 bytes.  The TPA-Label itself consumes 34 bytes.  A domain that
   publishes the TPA-Labels in its domain would then have 122 bytes
   available for their Trusted Domain.  This permits the authorization
   of any domain having a valid length with a deterministic amount of
   space available for resource records.

   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 [RFC6891] for extended DNS lengths.



21.  Acknowledgements

   Daniel Black, Jeff MacDonald, Michael Deutschmann, Frank Ellermann,
   Murray Kucherawy, Wietse Venema, Alessandro Vesely, and John Leslie.







22.  References



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22.1.  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, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2919]  Chandhok, R. and G. Wenger, "List-Id: A Structured Field
              and Namespace for the Identification of Mailing Lists",
              RFC 2919, DOI 10.17487/RFC2919, March 2001,
              <http://www.rfc-editor.org/info/rfc2919>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <http://www.rfc-editor.org/info/rfc3207>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
              RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [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>.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              DOI 10.17487/RFC5598, July 2009,
              <http://www.rfc-editor.org/info/rfc5598>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125,
              March 2011, <http://www.rfc-editor.org/info/rfc6125>.



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   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <http://www.rfc-editor.org/info/rfc6376>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
              RFC6891, April 2013,
              <http://www.rfc-editor.org/info/rfc6891>.

   [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7001, DOI 10.17487/
              RFC7001, September 2013,
              <http://www.rfc-editor.org/info/rfc7001>.

22.2.  Informative References

   [DMARC]    Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <http://www.rfc-editor.org/info/rfc7489>.

   [I-D.ietf-dane-smtp-with-dane]
              Dukhovni, V. and W. Hardaker, "SMTP security via
              opportunistic DANE TLS", draft-ietf-dane-smtp-with-dane-19
              (work in progress), May 2015.

   [I-D.kucherawy-dkim-delegate]
              Kucherawy, M. and d. dcrocker, "Delegating DKIM Signing
              Authority", draft-kucherawy-dkim-delegate-02 (work in
              progress), April 2015.

   [I-D.kucherawy-original-authres]
              Chew, M. and M. Kucherawy, "Original-Authentication-
              Results Header Field", draft-kucherawy-original-authres-00
              (work in progress), February 2012.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [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>.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, DOI 10.17487/RFC1930, March 1996,



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              <http://www.rfc-editor.org/info/rfc1930>.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686,
              September 2006, <http://www.rfc-editor.org/info/rfc4686>.

   [RFC4954]  Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service
              Extension for Authentication", RFC 4954, DOI 10.17487/
              RFC4954, July 2007,
              <http://www.rfc-editor.org/info/rfc4954>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5518]  Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
              Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009,
              <http://www.rfc-editor.org/info/rfc5518>.

   [RFC5863]  Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
              "DomainKeys Identified Mail (DKIM) Development,
              Deployment, and Operations", RFC 5863, DOI 10.17487/
              RFC5863, May 2010,
              <http://www.rfc-editor.org/info/rfc5863>.

   [RFC6377]  Kucherawy, M., "DomainKeys Identified Mail (DKIM) and
              Mailing Lists", BCP 167, RFC 6377, DOI 10.17487/RFC6377,
              September 2011, <http://www.rfc-editor.org/info/rfc6377>.

   [RFC6541]  Kucherawy, M., "DomainKeys Identified Mail (DKIM)
              Authorized Third-Party Signatures", RFC 6541,
              DOI 10.17487/RFC6541, February 2012,
              <http://www.rfc-editor.org/info/rfc6541>.

   [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>.

   [RFC7208]  Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,
              <http://www.rfc-editor.org/info/rfc7208>.



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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 ####

   ## "isp.com" TPA-Label Resource Record ##
   _HTIE4SWL3L7G4TKAFAUA7UYJSS2BTEOV._smtp._tpa.example.com. IN TXT
     "v=tpa1; tpa=isp.com; param=d;"

   #### 5322.Sender/List-ID authorization for 3P domains ####

   ## "example.com.isp.com" TPA-Label Resource Record ##
   _6MEHLQLKWAL5HQREXWDN2TBXAJ6VZ44B._smtp._tpa.example.com.  IN TXT
     "v=tpa1 tpa=*.isp.com; param=d L S;"

































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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

<CODE BEGINS>
/*
 * TPA-Label generation utility
 * 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.
 *
 * 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";



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static char sign_on[] =
{"%s v%d.%02d Copyright (C) (2014)  The IETF Trust\n"};
char err_cmd[] =\
 "ERR: Command error with [%s]\n";
char use_txt[]=\
 "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 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':



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                out_mode = 1;         /* out to file */
                (void) strncpy(out_fn, optarg, sizeof(out_fn));
                out_fn[sizeof(out_fn) - 1] = '\0';
                break;
            case 'v':
                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,



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                      TPA_LABEL_VERSION % 100);
    }

    for (i = 0; i < MAX_DOMAIN_NAME && !done; i++)
    {
        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);



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    if (verbose)
    {
        printf("Normalized Domain = [%s] %d, SHA-1 = ", in_buf, len);

        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 (160 bits) */
    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");



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    /* close */
    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);
 }
 <CODE ENDS>


Appendix C.  History of Prior Efforts

   To withstand asserting strict alignment practices, a scheme was
   devised that transferred the burden of a resulting disruption from
   receivers back to the Trusted Domains making the stringent requests.
   As such, a method to authorize other validated domains to establish
   informally Federated Third-Party Services, such as mailing-lists was
   developed.  This initial scheme was then modified and proposed by
   ATPS [RFC6541].  Unlike the initial scheme, ATPS required Third-Party
   Services to use specific non-standard DKIM signatures to signal use
   of the ATPS authorization strategy.  ATPS also required the DKIM
   signatures used by Third-Party Services to somehow determine the
   different label encoding employed by the many Trusted Domains without
   any defined discovery or exchange method.

   Both of these changes made deployment impractical by impacting
   systems not benefiting from additional alignment requirements.
   Third-parties have often been offering free services for decades.
   Even renaming From headers would impair normal handling.  Those
   offering these services should not be expected to carry the burden of
   enabling a new Trusted Domain compliance scheme.  Trusted Domains
   should offer the information needed to avoid disrupting these



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   services instead, which is the purpose of TPA-Labels.

   Rather, the Trusted Domain seeking cooperative handling and receiving
   receiver feedback necessary to mitigate disruption should handle this
   burden instead.  It is the Trusted Domain that directly benefits
   after all.  There should not be unnecessary and problematic encoding
   schemes or assertions of delivery chains being expected of any Third-
   Party Service.  Such matters are simply not their concern nor in
   their benefit.

   It seems the added complexities found in ATPS were to defend against
   a single DNS transaction.  However, before this transaction occurs,
   the Third-Party must permit the validation of their own domain.  Even
   then, a Third-Party checking transaction only occur after the domain
   is not within the Trusted Domain's alignment assertions.  An
   assertion that can always be removed at any point.  It is clearly in
   the interest of the Trusted Domain where the checking transaction
   represents a very minor contribution in support of desired receiver
   cooperation.

   Tailoring their TPA-Label list to suit their own users should
   discourage non-cooperative references to their domain.  As more
   domains reference a common "_tpa." zone, the clout of that zone
   increases at a very moderate cost.  This additional clout better
   ensures timely responses to abuse notifications.  In this manner,
   DMARC/TPA-Labels would be helping to improve anti-abuse cooperation.
   In that light, TPA-Labels should be considered a sound investment and
   not an unwanted burden.

   ATPS required new tags be included in Third-Party DKIM signatures.
   These were "atps" and "atpsh" to construct a chain of "d=" and
   "atps=".  This added complexity without any immediate benefit.
   Determining optional label encoding without any defined discovery
   method overlooks that authorization is only possible after the Third-
   Party Domain has been validated.  A complete lack of ATPS deployment
   should have been expected since necessary changes did not align with
   benefits.

   In contrast, TPA-Labels do not require ANY change be made by
   authorized third-parties.  Disrupting legitimate communications
   imposes inordinate support costs as a result of erroneously asserting
   strict alignment practices.  The resulting disruption will eventually
   cause the domain's assertions to be ignored.  If this disruption
   becomes endemic, assertions of other domains will become ignored as
   well.  Domains wishing to benefit from their handling advice being
   employed while sending legitimate messages that may not retain their
   asserted alignment practices, should be offering the needed TPA-Label
   exception information.  This information is essential and is only



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   known by the Trusted Domain through their DMARC feedback.

   At this time, it is not practical for large ISPs to make strict DMARC
   assertions.  Strict alignment assertions exclude normal Third-Party
   Services that modify the requisite alignment.  TPA-Label lists
   specifically tailored to handle their users' desired Third-Party
   Services will permit their users to have normal email use.  While
   entailing some administrative effort, TPA-Labels will generally be of
   benefit to their users by widely discouraging any spoofing of their
   messages.  This affords greater protection for the users and the
   user's recipients.

   SPF purported to provide an anti-spoofing feature for an unseen
   parameter.  Nevertheless, its strict IP address authorization causes
   problems and is largely disregarded for anything other than limiting
   the sending of DSNs or the scoring of messages.  Many institutions
   will benefit by ensuring their strict DMARC assertions are not
   disruptive.  Exercising this care will help retain recipient's trust
   in their assertions and the veracity of their messages.  TPA-Labels
   would allow these institutions a means to use informal Third-Party
   Services with minimal administrative effort.  Rather than using
   subdomains that lack DMARC restrictions, suitable Third-Party
   Services can be authorized by TPA-Labels.  This approach offers a
   proactive method for recipients to better filter possible phishing
   attempts by not exposing them to unrestricted subdomain abuse.

   TPA-Label publishing is similar to VBR ([RFC5518]).  However, it
   leverages Third-Party validation confirmed by labels held in the
   Trusted Domain.  DNS also permits the information to be transparently
   made available from other domains whenever desired.  TPA-Labels
   provide domains a means to protect their recipients while still
   permitting the use of legitimate SMTP exchanges.  By implementing
   DMARC/TPA-Labels, these domains should be better able to stand on
   their own merit.


Author's Address

   Douglas Otis
   Trend Micro
   10101 N. De Anza Blvd
   Cupertino, CA  95014
   USA

   Phone: +1.408.257-1500
   Email: doug_otis@trendmicro.com





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