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DMARC Working Group                                          K. Andersen
Internet-Draft                                                  LinkedIn
Intended status: Experimental                               B. Long, Ed.
Expires: January 18, 2019                                         Google
                                                           S. Blank, Ed.
                                                                Valimail
                                                       M. Kucherawy, Ed.
                                                                     TDP
                                                         T. Draegen, Ed.
                                                                dmarcian
                                                           July 17, 2018


              Authenticated Received Chain (ARC) Protocol
                    draft-ietf-dmarc-arc-protocol-16

Abstract

   The Authenticated Received Chain (ARC) protocol allows Internet Mail
   Handlers to attach assertions of message authentication state to
   individual messages.  As messages traverse ARC-enabled Internet Mail
   Handlers, additional ARC assertions can be attached to messages to
   form ordered sets of ARC assertions that represent authentication
   state along each step of message handling paths.

   ARC-enabled Internet Mail Handlers can process sets of ARC assertions
   to inform message disposition decisions, to identify Internet Mail
   Handlers that might break existing authentication mechanisms, and to
   convey original authentication state across trust boundaries.

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 https://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 January 18, 2019.





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Copyright Notice

   Copyright (c) 2018 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
   (https://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.  General Concepts  . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Evidence  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Custody . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Chain of Custody  . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Validation of Chain of Custody  . . . . . . . . . . . . .   5
   3.  Terminology and Definitions . . . . . . . . . . . . . . . . .   5
     3.1.  ARC Set . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Authenticated Received Chain (ARC)  . . . . . . . . . . .   6
     3.3.  Sealer  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Validator . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Imported ABNF Tokens  . . . . . . . . . . . . . . . . . .   7
     3.6.  Common ABNF Tokens  . . . . . . . . . . . . . . . . . . .   7
   4.  Protocol Elements . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  ARC Headers . . . . . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  ARC-Authentication-Results (AAR)  . . . . . . . . . .   8
       4.1.2.  ARC-Message-Signature (AMS) . . . . . . . . . . . . .   8
       4.1.3.  ARC-Seal (AS) . . . . . . . . . . . . . . . . . . . .   9
     4.2.  ARC Set . . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Instance Tags . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Authenticated Received Chain  . . . . . . . . . . . . . .  11
     4.4.  Chain Validation Status . . . . . . . . . . . . . . . . .  11
   5.  Protocol Actions  . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  Sealer Actions  . . . . . . . . . . . . . . . . . . . . .  12
       5.1.1.  Header Fields To Include In ARC-Seal Signatures . . .  13
       5.1.2.  Marking and Sealing "cv=fail" (Invalid) Chains  . . .  13
       5.1.3.  Only One Authenticated Received Chain Per Message . .  13
       5.1.4.  Broad Ability to Seal . . . . . . . . . . . . . . . .  14
       5.1.5.  Sealing is Always Safe  . . . . . . . . . . . . . . .  14
       5.1.6.  Signing vs Sealing  . . . . . . . . . . . . . . . . .  14
     5.2.  Validator Actions . . . . . . . . . . . . . . . . . . . .  14



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       5.2.1.  All Failures Are Permanent  . . . . . . . . . . . . .  16
       5.2.2.  Responding to ARC Validation Failures During the SMTP
               Transaction . . . . . . . . . . . . . . . . . . . . .  16
     5.3.  Result of Validation  . . . . . . . . . . . . . . . . . .  16
   6.  Communication of Validation Results . . . . . . . . . . . . .  17
   7.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Communicate Authentication Results Across Trust
           Boundaries  . . . . . . . . . . . . . . . . . . . . . . .  17
       7.1.1.  Message Scanning Services . . . . . . . . . . . . . .  17
       7.1.2.  Multi-tier MTA Processing . . . . . . . . . . . . . .  18
       7.1.3.  Mailing Lists . . . . . . . . . . . . . . . . . . . .  18
     7.2.  Inform Message Disposition Decisions  . . . . . . . . . .  18
       7.2.1.  DMARC Local Policy Overrides  . . . . . . . . . . . .  19
       7.2.2.  DMARC Reporting . . . . . . . . . . . . . . . . . . .  19
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  20
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     9.1.  Increased Header Size . . . . . . . . . . . . . . . . . .  20
     9.2.  DNS Operations  . . . . . . . . . . . . . . . . . . . . .  21
     9.3.  Message Content Suspicion . . . . . . . . . . . . . . . .  21
     9.4.  Message Sealer Suspicion  . . . . . . . . . . . . . . . .  21
     9.5.  Replay Attacks  . . . . . . . . . . . . . . . . . . . . .  22
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Email Authentication Results Names Registry Update . . .  22
     10.2.  Email Authentication Methods Registry Update . . . . . .  22
     10.3.  Definitions of the ARC header fields . . . . . . . . . .  23
   11. Experimental Considerations . . . . . . . . . . . . . . . . .  23
     11.1.  Success Consideration  . . . . . . . . . . . . . . . . .  24
     11.2.  Failure Considerations . . . . . . . . . . . . . . . . .  24
     11.3.  Open Questions . . . . . . . . . . . . . . . . . . . . .  24
       11.3.1.  Value of the ARC-Seal (AS) Header  . . . . . . . . .  24
       11.3.2.  DNS Overhead . . . . . . . . . . . . . . . . . . . .  24
       11.3.3.  What Trace Information is Valuable . . . . . . . . .  25
   12. Implementation Status . . . . . . . . . . . . . . . . . . . .  25
     12.1.  GMail test reflector and incoming validation . . . . . .  26
     12.2.  AOL test reflector and internal tagging  . . . . . . . .  26
     12.3.  dkimpy . . . . . . . . . . . . . . . . . . . . . . . . .  27
     12.4.  OpenARC  . . . . . . . . . . . . . . . . . . . . . . . .  27
     12.5.  Mailman 3.x patch  . . . . . . . . . . . . . . . . . . .  27
     12.6.  Copernica/MailerQ web-based validation . . . . . . . . .  28
     12.7.  Rspamd . . . . . . . . . . . . . . . . . . . . . . . . .  28
     12.8.  PERL MAIL::DKIM module . . . . . . . . . . . . . . . . .  29
     12.9.  PERL Mail::Milter::Authentication module . . . . . . . .  29
     12.10. Sympa List Manager . . . . . . . . . . . . . . . . . . .  29
     12.11. Oracle Messaging Server  . . . . . . . . . . . . . . . .  30
     12.12. MessageSystems Momentum and PowerMTA platforms . . . . .  30
     12.13. Exim . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  31



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     13.2.  Informative References . . . . . . . . . . . . . . . . .  32
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   Appendix A.  Appendix A - Design Requirements . . . . . . . . . .  33
     A.1.  Primary Design Criteria . . . . . . . . . . . . . . . . .  34
     A.2.  Out of Scope  . . . . . . . . . . . . . . . . . . . . . .  34
   Appendix B.  Appendix B - Example Usage . . . . . . . . . . . . .  34
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  34
   Appendix D.  Comments and Feedback  . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   The utility of widely deployed email authentication technologies such
   as Sender Policy Framework (SPF) [RFC7208] and DomainKeys Identified
   Mail (DKIM) [RFC6376] is impacted by the processing of Internet Mail
   by intermediate handlers.  This impact is thoroughly documented in
   the defining documents for SPF and DKIM and further discussed in
   [RFC6377] and [RFC7960].

   The utility of technologies that build upon SPF and DKIM (such as
   DMARC [RFC7489]) is similarly impacted by intermediate handlers.  The
   disruption of authentication mechanisms for legitimate messages by
   intermediate handlers can impact all aspects of Internet Mail -
   message authors, message recipients, and even the intermediary
   handler itself.

   Authenticated Received Chain (ARC) creates a mechanism for individual
   Internet Mail Handlers to add their authentication processing results
   to a message's ordered set of processing results.  ARC encapsulates
   processing results in a DKIM signature derivative to grant other
   handlers the ability to verify the authenticity of the individual
   processing results as well as the aggregate set and sequence of
   results.

   Ordered sets of processing results can be used by ARC-enabled
   Internet Mail Handlers to inform message handling disposition, to
   identify where alteration of message content might have occurred, and
   to provide additional trace information for use in understanding
   message handling paths.

2.  General Concepts

   ARC is loosely based on concepts from evidence collection.  Evidence
   is usually collected, labeled, stored, and transported in specific
   ways to preserve the state of evidence and to document all processing
   steps.





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2.1.  Evidence

   In ARC's situation, the "evidence" is a message's authentication
   state at any point along the delivery path between origination and
   final delivery.  Authentication state can change when intermediate
   handlers modify message content (headers and/or body content), route
   messages through unforeseen paths, or change envelope information.

   The authentication state of a message is determined upon receipt of a
   message and documented in the Authentication-Results header field(s).
   ARC extends this mechanism to survive transit through intermediary
   ADMDs.

2.2.  Custody

   "Custody" refers to when an ARC-enabled Internet Mail Handler
   processes a message.  When a handler takes custody of a message, the
   handler becomes a Custodian and attaches their own evidence
   (authentication state upon receipt) to the message.  Evidence is
   added in such a way so that future handlers can verify the
   authenticity of both evidence and custody.

2.3.  Chain of Custody

   The "chain of custody" of ARC is the entire set of evidence and
   custody that travels with a message.

2.4.  Validation of Chain of Custody

   Any ARC-enabled Internet Mail Handler can validate the entire set of
   evidence and custody to yield a valid Chain of Custody.  If the
   evidence-supplying Custodians can be trusted, then the validated
   Chain of Custody describes the (possibly changing) authentication
   state as the message traveled through various Custodians.

   Even though a message's authentication state might have changed, the
   validated chain of custody can be used to determine if the changes
   (and the Custodians responsible for the changes) can be tolerated.

3.  Terminology and Definitions

   This section defines terms used in the rest of the document.

   Readers should to be familiar with the contents, core concepts, and
   definitions found in [RFC5598].  The potential roles of
   intermediaries in the delivery of email are directly relevant.





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   Language, syntax (including some ABNF constructs), and concepts are
   imported from DKIM [RFC6376].  Specific references to DKIM are made
   throughout this document.  The following terms are imported from
   [RFC5598]:

   o  ADministrative Management Domain (ADMD), Section 2.3

   o  Message Transfer Agents (MTA), Section 4.3.2

   o  Message Submission Agent (MSA), Section 4.3.1

   o  Message Delivery Agent (MDA), Section 4.3.3

   Internet Mail Handlers process and deliver messages across the
   Internet and include MSAs, MTAs, MDAs, gateways, and mailing lists.

   Syntax descriptions use Augmented BNF (ABNF) [RFC5234] and [RFC7405].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

3.1.  ARC Set

   Section 4.1 introduces three (3) ARC header fields.  Together, the 3
   header fields compose a single "ARC Set".  An ARC Set provides the
   means for an Internet Mail Handler to attach authentication state to
   a message in a manner that can be verified by future handlers.  A
   single message can contain multiple ARC Sets.

   In General Concept terms, an ARC Set represents Evidence and Custody.

3.2.  Authenticated Received Chain (ARC)

   The complete sequence of ARC Sets attached to a message is called the
   Authenticated Received Chain.  An Authenticated Received Chain is a
   recording of individual authentication states as a message traverses
   through ARC-participating ADMDs.

   The first attachment of an ARC Set to a message causes an
   Authenticated Received Chain to be created.  Additional attachments
   of ARC Sets cause the Authenticated Received Chain to be extended.





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   In General Concept terms, an Authenticated Received Chain represents
   Chain of Custody.

3.3.  Sealer

   A Sealer is an Internet Mail Handler that attaches a complete and
   valid ARC Set to a message.

   In General Concept terms, a Sealer adds Evidence and proof of Custody
   to the Chain of Custody.

3.4.  Validator

   A Validator is an ARC-enabled Internet Mail Handler that evaluates an
   Authenticated Received Chain for validity and content.  The process
   of evaluation of the individual ARC Sets that compose an
   Authenticated Received Chain is described in Section 5.2.

   In General Concept terms, a Validator inspects the Chain of Custody
   to determine the content and validity of individual Evidence supplied
   by Custodians.

3.5.  Imported ABNF Tokens

   The following ABNF tokens are imported:

   o  tag-list ([RFC6376] section 3.2)

   o  authres-payload ([I-D-7601bis] section 2.2)

   o  cfws ([RFC5322] section 3.2.2)

3.6.  Common ABNF Tokens

   The following ABNF tokens are used elsewhere in this document:

   position = 1*2DIGIT ; 1 - 50
   instance = [CFWS] %s"i" [CFWS] "=" [CFWS] position [CFWS] ";"
   chain-status = ("none" / "fail" / "pass")
   seal-cv-tag = %s"cv" [CFWS] "=" [CFWS] chain-status

4.  Protocol Elements

4.1.  ARC Headers

   ARC introduces three new header fields.  Syntax for new header fields
   borrows heavily from existing specifications.  This document only




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   describes where ARC-specific changes in syntax and semantics differ
   from existing specifications.

4.1.1.  ARC-Authentication-Results (AAR)

   The ARC-Authentication-Results (AAR) header field records the message
   authentication state as processed by an ARC-participating ADMD at
   message arrival time.

   In General Concept terms, the AAR header field is where Evidence is
   recorded by a Custodian.

   The AAR header field is similar in syntax and semantics to an
   Authentication-Results field [I-D-7601bis], with two (2) differences:

   o  the name of the header field itself;

   o  the presence of the "instance tag".  Additional information on the
      "instance tag" can be found in Section 4.2.1.

   The formal ABNF for the AAR header field is:

   arc-info = instance [CFWS] ";" authres-payload
   arc-authres-header = "ARC-Authentication-Results:" [CFWS] arc-info

   Because there is only one AAR allowed per ARC set, the AAR MUST
   contain all authentication results from within the participating
   ADMD, regardless of how many Authentication-Results headers are
   attached to the message.

4.1.2.  ARC-Message-Signature (AMS)

   The ARC-Message-Signature (AMS) header field allows an ARC-
   participating ADMD to convey some responsibility (custodianship) for
   a message and possible message modifications to future ARC-
   participating Custodians.

   In General Concept terms, the AMS header field identifies a
   Custodian.

   The AMS header field is similar in syntax and semantics to a DKIM-
   Signature field [RFC6376], with three (3) differences:

   o  the name of the header field itself;

   o  no version tag ("v") is defined for the AMS header field.  As
      required for undefined tags (in [RFC6376]), if seen, a version tag
      MUST be ignored;



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   o  the presence of the "instance tag".  Additional information on the
      "instance tag" can be found in Section 4.2.1.  The instance tag
      replaces the DKIM "AUID" tag;

   o  when building the header field list to be signed, ARC-related
      headers MUST be submitted to the hash function in increasing
      instance order.

   ARC places no requirements on the selectors and/or domains used for
   the AMS header field signatures.

   The formal ABNF for the AMS header field is:

   arc-ams-info = instance [CFWS] ";" tag-list
   arc-message-signature = "ARC-Message-Signature:" [CFWS] arc-ams-info

   To avoid unwanted invalidation of AMS signatures:

   o  AMS header fields are added by ARC-participating ADMDs as messages
      exit the ADMD.  AMS header fields SHOULD be attached so that any
      modifications made by the ADMD are included in the signature of
      the AMS header field.

   o  Authentication-Results header fields MUST NOT be included in AMS
      signatures as they are likely to be deleted by downstream ADMDs
      (per [I-D-7601bis] Section 5).

   o  ARC-related header fields (ARC-Authentication-Results, ARC-
      Message-Signature, ARC-Seal) MUST NOT be included in the list of
      header fields covered by the signature of the AMS header field.

   To preserve the ability to verify the integrity of a message, the
   signature of the AMS header field SHOULD include any DKIM-Signature
   header fields already present in the message.

4.1.3.  ARC-Seal (AS)

   The ARC-Seal (AS) header field is the mechanism by which ARC-
   participating ADMDs can verify the integrity of AAR header fields and
   corresponding AMS header fields.

   In General Concept terms, the AS header field is how Custodians bind
   Evidence into a Chain of Custody so that Validators can inspect
   individual Evidence and Custodians.

   The AS header field is similar in syntax and semantics to DKIM-
   Signatures [RFC6376], with the following differences:




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   o  the presence of the "instance tag".  Additional information on the
      "instance tag" can be found in Section 4.2.1.

   o  the signature of the AS header field does not cover the body of
      the message and therefore there is no 'bh' tag.  The signature of
      the AS header field only covers specific header fields as defined
      in Section 5.1.1.

   o  no body canonicalization is performed as the AS signature does not
      cover the body of a message.

   o  only "relaxed" header canonicalization ([RFC6376] section 3.4.2)
      is used.

   o  the only supported tags are "i" (from Section 4.2.1 of this
      document), and "a", "b", "d, "s", "t" from [RFC6376] Section 3.5.
      Note especially that the DKIM "h" tag is NOT allowed and if found,
      MUST result in a cv status of "fail" (for more information see
      Section 5.1.1);

   o  an additional tag, "cv" ("seal-cv-tag" in the ARC-Seal ABNF
      definition) is used to communicate Chain Validation Status to
      subsequent ADMDs.

   ARC places no requirements on the selectors and/or domains used for
   the AS header field signatures.

   The formal ABNF for the AS header field is:

   arc-as-info = instance [CFWS] ";" tag-list
   arc-seal = "ARC-Seal:" [CFWS] arc-as-info

4.2.  ARC Set

   An "ARC Set" is a single collection of three ARC Headers (AAR, AMS,
   and AS).  ARC Headers of an ARC Set share the same "instance" value.

   By adding all ARC Headers to a message, an ARC Sealer adds an ARC Set
   to a message.  A description of how Sealers add an ARC Set to a
   message is found in Section 5.1.

4.2.1.  Instance Tags

   Instance tags describe which ARC Headers belong to an ARC Set. Each
   ARC Header of an ARC Set shares the same instance tag value.

   Instance tag values are integers that begin at 1 and are incremented
   by each addition of an ARC Set. Through the incremental values of



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   instance tags, an ARC Validator can determine the order in which ARC
   Sets were added to a message.

   Instance tag values can range from 1-50 (inclusive).

   _INFORMATIONAL:_ The upper limit of 50 was picked based on some
   initial observations reported by early working group members with a
   safety margin multiple added on top to support the vast majority of
   all intermediary mail flows.

   Valid ARC Sets MUST have exactly one instance of each ARC Header
   field (AAR, AMS, and AS) for a given instance value and signing
   algorithm.

   _INFORMATIONAL:_ Initial development of ARC is only being done with a
   single allowed signing algorithm, but parallel work in the DCRUP
   working group is expanding that.  For handling multiple signing
   algorithms, see [ARC-MULTI].

4.3.  Authenticated Received Chain

   An Authenticated Received Chain is an ordered collection of ARC Sets.
   As ARC Sets are enumerated sets of ARC Headers, an Authenticated
   Received Chain represents the output of message authentication state
   along the handling path of ARC-enabled processors.

   Results of message authentication processing along each step of the
   ARC-enabled handling path is present in an Authenticated Received
   Chain in the form of AAR header fields.  The ability to verify the
   identity of message handlers and the integrity of message content is
   provided by AMS header fields.  AS header fields allow messages
   handlers to validate the assertions, order and sequence of the
   Authenticated Received Chain itself.

   In General Concept terms, an Authenticated Received Chain represents
   a message's Chain of Custody.  Validators can consult a message's
   Chain of Custody to gain insight regarding each Custodian of a
   message and the Evidence collected by each Custodian.

4.4.  Chain Validation Status

   The state of the Authenticated Received Chain at a specific
   processing step is called the "Chain Validation Status".  Chain
   Validation Status information is communicated in several ways:

   o  the AS header field in the "cv" tag, and

   o  as part of Authentication-Results and AAR headers.



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   Chain Validation Status has one of three possible values:

   o  none: There was no Authenticated Received Chain on the message
      when it arrived for validation.  Typically this occurs when a
      message is received directly from a message's original Message
      Transfer Agent (MTA) or Message Submission Agent (MSA), or from an
      upstream Internet Mail Handler that is not participating in ARC
      handling.

   o  fail: The message contains an Authenticated Received Chain whose
      validation failed.

   o  pass: The message contains an Authenticated Received Chain whose
      validation succeeded.

5.  Protocol Actions

   ARC-enabled Internet Mail Handlers generally act as both ARC Sealers
   (when sending messages) and ARC Validators (when receiving messages).

5.1.  Sealer Actions

   To "seal" a message, an ARC Sealer adds an ARC Set (the three ARC
   header fields AAR, AMS, and AS) to a message.  All ARC header fields
   in an ARC Set share the same instance tag value.

   To perform Sealing (aka to build and attach a new ARC Set), the
   following actions must be taken by an ARC Sealer when presented with
   a message:

   1.  All message modifications (including adding DKIM-Signatures) MUST
       be performed before Sealing.

   2.  Calculate the instance value: if the message contains an
       Authenticated Received Chain, the instance value is 1 more than
       the highest instance number found in the Authenticated Received
       Chain.  If no Authenticated Received Chain exists, the instance
       value is 1.

   3.  Using the calculated instance value, generate and attach to the
       message in the following order:

   4.  An ARC-Authentication-Results header field as defined in
       Section 4.1.1.

   5.  An ARC-Message-Signature header field as defined in
       Section 4.1.2.




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   6.  An ARC-Seal header field using the AS definition found in
       Section 4.1.3, the method described in Section 5.1.1, and the
       Chain Validation Status as determined during ARC validation.

5.1.1.  Header Fields To Include In ARC-Seal Signatures

   The signature of an AS header field signs a specific canonicalized
   form of the ARC Set header values.  The ARC set header values are
   supplied to the hash function in increasing instance order, starting
   at 1, and include the ARC Set being added at the time of Sealing the
   message.

   Within an ARC Set, header fields are supplied to the hash function in
   the following order:

   1.  ARC-Authentication-Results

   2.  ARC-Message-Signature

   3.  ARC-Seal

   The ARC-Seal is generated in a manner similar to when DKIM-Signatures
   are added to messages ([RFC6376], section 3.7).

   Note that when an Authenticated Received Chain has failed validation,
   the signing scope for the ARC-Seal is modified (see Section 5.1.2).

5.1.2.  Marking and Sealing "cv=fail" (Invalid) Chains

   In the case of a failed Authenticated Received Chain, the header
   fields included in the signature scope of the AS header field b=
   value MUST only include the ARC Set headers created by the MTA which
   detected the malformed chain, as if this newest ARC Set was the only
   set present.

   _INFORMATIONAL_: This approach is mandated to handle the case of a
   malformed or otherwise invalid Authenticated Received Chain.  There
   is no way to generate a deterministic set of AS header fields
   (Section 5.1.1) in most cases of invalid chains.

5.1.3.  Only One Authenticated Received Chain Per Message

   A message can have only one Authenticated Received Chain on it at a
   time.  Once broken, the chain cannot be continued, as the chain of
   custody is no longer valid and responsibility for the message has
   been lost.  For further discussion of this topic and the designed
   restriction which prevents chain continuation or re-establishment,
   see [ARC-USAGE].



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5.1.4.  Broad Ability to Seal

   ARC is not solely intended for perimeter MTAs.  Any mediator
   ([RFC5598], section 5) that modifies a message may Seal its own
   changes.  For additional information, see Section 7.1.

5.1.5.  Sealing is Always Safe

   The utility of an Authenticated Received Chain is limited to very
   specific cases.  Authenticated Received Chains are designed to
   provide additional information to an Internet Mail Handler when
   evaluating messages for delivery in the context of authentication
   failures.  Specifically:

   o  Properly adding an ARC Set to a message does not damage or
      invalidate an existing Authenticated Received Chain.

   o  Sealing an Authenticated Received Chain when a message has not
      been modified does not negatively affect the chain.

   o  Validating a message exposes no new threat vectors (see
      Section 9).

   o  An ADMD may choose to Seal all inbound messages whether or not a
      message has been modified or will be retransmitted.

5.1.6.  Signing vs Sealing

   Signing is the process of affixing a digital signature to a message
   as a header, such as when a DKIM-Signature (as in [RFC6376] section
   2.1), or an AMS or AS is added.  Sealing is when an ADMD affixes a
   complete and valid ARC Set to a message creating or continuing an
   Authenticated Received Chain.

5.2.  Validator Actions

   A validator performs the following steps, in sequence, to process an
   Authenticated Received Chain.  Canonicalization, hash functions, and
   signature validation methods are imported from [RFC6376] section 5.

   1.  Collect all ARC Sets currently attached to the message.  If there
       are none, the Chain Validation Status is "none" and the algorithm
       stops here.  The maximum number of ARC Sets that can be attached
       to a message is 50.  If more than the maximum number exist the
       Chain Validation Status is "fail" and the algorithm stops here.
       In the following algorithm, the maximum ARC instance value is
       referred to as "N".




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   2.  If the Chain Validation Status of the highest instance value ARC
       Set is "fail", then the Chain Validation status is "fail" and the
       algorithm stops here.

   3.  Validate the structure of the Authenticated Received Chain.  A
       valid ARC has the following conditions:

       1.  Each ARC Set MUST contain exactly one each of the three ARC
           header fields (AAR, AMS, and AS).

       2.  The instance values of the ARC Sets MUST form a continuous
           sequence from 1..N with no gaps or repetition.

       3.  The "cv" value for all ARC-Seal header fields must be non-
           failing.  For instance values > 1, the value must be "pass".
           For instance value = 1, the value must be "none".

       *  If any of these conditions are not met, the Chain Validation
          Status is "fail" and the algorithm stops here.

   4.  Validate the AMS with the greatest instance value (most recent).
       If validation fails, then the Chain Validation Status is "fail"
       and the algorithm stops here.

   5.  _OPTIONAL:_ Determine the "oldest-pass" value from the ARC Set by
       validating each prior AMS beginning with the N-1 and proceeding
       in decreasing order to the AMS with the instance value of 1:

   6.  If an AMS fails to validate (for instance value "M"), then set
       the oldest-pass value to the lowest AMS instance value which
       passed (M+1) and go to the next step (there is no need to check
       any other (older) AMS headers).  This does not affect the
       validity of the Authenticated Received Chain.

   7.  If all AMS headers verify, set the oldest-pass value to zero (0).

   8.  Validate each AS beginning with the greatest instance value and
       proceeding in decreasing order to the AS with the instance value
       of 1.  If any AS fails to validate, the Chain Validation Status
       is "fail" and the algorithm stops here.

   9.  If the algorithm reaches this step, then the Chain Validation
       Status is "pass", and the algorithm is complete.

   The end result of this Validation algorithm is added into the
   Authentication-Results header for the ADMD.





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   As with a failing DKIM signature ([RFC6376] section 6.3), a message
   with a failing Authenticated Received Chain MUST be treated the same
   as a message with no Authenticated Received Chain.

   _INFORMATIONAL_: Recipients of an invalid or failing Authenticated
   Received Chain can use that information as part of a wider handling
   context.  ARC adoption cannot be assumed by intermediaries; many
   intermediaries will continue to modify messages without adding ARC
   Seals.

5.2.1.  All Failures Are Permanent

   Authenticated Received Chains represent the traversal of messages
   through one or more intermediaries.  All errors, including DNS
   failures, become unrecoverable and are considered permanent.

   Any error Validating an Authenticated Received Chain results in a
   failed Chain Validation Status.  For further discussion of this topic
   and the design restriction which prevents chain continuation or re-
   establishment, see [ARC-USAGE].

5.2.2.  Responding to ARC Validation Failures During the SMTP
        Transaction

   If an ARC Validator determines that the incoming message fails
   authentication checks (potentially including ARC validation), the
   Validator MAY signal the breakage through the extended SMTP response
   code 5.7.7 [RFC3463] "message integrity failure" [ENHANCED-STATUS]
   and corresponding SMTP response code.

5.3.  Result of Validation

   An Authenticated Received Chain with a Chain Validation Status of
   "pass" allows Internet Mail Handlers to ascertain:

   o  all ARC-participating ADMDs that claim responsibility for handling
      (and possibly modifying) the message in transit;

   o  the authentication state of the message as perceived by each ADMD
      (from Authentication-Results header fields).

   Given this information, handlers can inform local policy decisions
   regarding disposition of messages that experience authentication
   failure due to intermediate processing.







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6.  Communication of Validation Results

   Chain Validation Status (described in Section 4.4) is communicated
   via Authentication-Results (and AAR) headers using the auth method
   "arc".  This auth method is described in Section 10.1.

   If necessary data is available, the ptypes and properties defined in
   Section 10.2 SHOULD be recorded in an Authentication-Results header
   field:

   o  smtp.client-ip - The connecting client IP address from which the
      message is received.

   o  header.oldest-pass - The instance number of the oldest AMS that
      still validates, or 0 if all pass.

   Upon Sealing of a message, this Authentication-Results information
   along with all other Authentications-Results added by the ADMD will
   be recorded into the AAR as defined in section Section 4.1.1.

   In General Concept terms, the information recorded in the ARC-
   Authentication-Results header field is the Evidence that gets
   attached to a message.

7.  Use Cases

   This section explores several messaging handling use cases that are
   addressed by ARC.

7.1.  Communicate Authentication Results Across Trust Boundaries

   When an intermediary ADMD adds an ARC Set to a message's
   Authenticated Received Chain (or creates the initial ARC Set), the
   ADMD communicates authentication state to the next ADMD in the
   message handling path.

   If ARC-enabled ADMDs are trusted, Authenticated Received Chains can
   be used to bridge administrative boundaries.

7.1.1.  Message Scanning Services

   Message services are available to perform anti-spam, anti-malware,
   and anti-phishing scanning.  Such services typically remove malicious
   content, replace HTTP links in messages with sanitized links, and/or
   attach footers to messages advertising the abilities of the message
   scanning service.  These modifications almost always break signature-
   based authentication (such as DKIM).




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   Scanning services typically require clients to point MX records of an
   Internet domain to the scanning service.  Messages destined for the
   Internet domain are initially delivered to the scanning service.
   Once scanning is performed, messages are then routed to the client's
   own mail handling infrastructure.  Re-routing messages in this way
   almost always breaks path-based authentication (such as SPF).

   Message scanning services can attach Authenticated Received Chains to
   messages to communicate authentication results into client ADMDs.
   Clients can then benefit from the message scanning service while
   processing messages as if the client's infrastructure were the
   original destination of the Internet domain's MX record.

7.1.2.  Multi-tier MTA Processing

   Large message processing infrastructure is often divided into several
   processing tiers that can break authentication information between
   tiers.  For example, a large site may maintain a cluster of MTAs
   dedicated to connection handling and enforcement of IP-based
   reputation filtering.  A secondary cluster of MTAs may be dedicated
   and optimized for content-based processing of messages.

   Authenticated Received Chains can be used to communicated
   authentication state between processing tiers.

7.1.3.  Mailing Lists

   Mailing lists resend posted messages to subscribers.  A full
   description of authentication-related mailing list issues can be
   found in [RFC7960] Section 3.2.3.

   Mailing list services can implement ARC to convey the original
   authentication state of posted messages sent to the list's subscriber
   base.  The ADMDs of the mailing list subscribers can then use the
   Authenticated Received Chain to determine the authentication state of
   the original message before mailing list handling.

7.2.  Inform Message Disposition Decisions

   ARC functionality allows Internet Mail Handlers to reliably identify
   intermediary ADMDs and for ADMDs to expose authentication state that
   can survive additional intermediary handling.

   Intermediaries often break authentication through content
   modification, interfere with path-based authentication (such as SPF),
   and strip authentication results (if an MTA removes Authentication-
   Results headers).




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   Authenticated Received Chains allow ARC Validators to:

   1.  identify ARC-enabled ADMDs that break authentication while
       processing messages;

   2.  gain extended visibility into the authentication-preserving
       abilities of ADMDs that relay messages into ARC-enabled ADMDs.

   Through the collection of ARC-related data, an ADMD can identify
   handling paths that have broken authentication.

   An Authenticated Received Chain allows an Internet Mail Handler to
   potentially base decisions of message disposition on authentication
   state provided by different ADMDs.

7.2.1.  DMARC Local Policy Overrides

   DMARC introduces a policy model where Domain Owners can request email
   receivers to reject or quarantine messages that fail DMARC alignment.
   Interoperability issues between DMARC and indirect email flows are
   documented in [RFC7960].

   Authenticated Received Chains allow DMARC processors to consider
   authentication states provided by other ADMDs.  As a matter of local
   policy, a DMARC processor may choose to accept the authentication
   state provided by an Authenticated Received Chain when determining if
   a message is DMARC compliant.

   When an Authenticated Received Chain is used to determine message
   disposition, the DMARC processor can communicate this local policy
   decision to Domain Owners as described in Section 7.2.2.

7.2.2.  DMARC Reporting

   DMARC-enabled receivers indicate when ARC Validation influences
   DMARC-related local policy decisions.  DMARC reporting of ARC-
   influenced decisions is accomplished by adding a local_policy comment
   containing a list of data discovered during ARC Validation, which at
   a minimum includes:

   o  the Chain Validation Status,

   o  the domain and selector for each AS,

   o  the originating IP address from the first ARC Set:






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   EXAMPLE:

   <policy_evaluated>
     <disposition>none</disposition>
     <dkim>fail</dkim>
     <spf>fail</spf>
     <reason>
      <type>local_policy</type>
      <comment>arc=pass ams[2].d=d2.example ams[2].s=s1
        as[2].d=d2.example as[2].s=s2 as[1].d=d1.example
        as[1].s=s3 client-ip[1]=10.10.10.13</comment>
     </reason>
   </policy_evaluated>

   In the above example DMARC XML reporting fragment, data relating to
   specific validated ARC Sets are enumerated using array syntax (eg,
   "ams[2]" means AMS header field with instance value of 2). d2.example
   is the Sealing domain for ARC Set #2 (i=2) and d1.example is the
   Sealing domain for ARC Set #1 (i=1).

   Depending on the reporting practices of intermediate message
   handlers, Domain Owners may receive multiple DMARC reports for a
   single message.  DMARC report processors should be aware of this
   behaviour and make the necessary accommodations.

8.  Privacy Considerations

   The Authenticated Received Chain provides a verifiable record of the
   handlers for a message.  This record may include Personally
   Identifiable Information such as IP address and domain names.  Such
   information is also including in existing header fields such as the
   "Received" header field.

9.  Security Considerations

   The Security Considerations of [RFC6376] and [I-D-7601bis] apply
   directly to this specification.

   As with other domain authentication technologies (such as SPF, DKIM,
   and DMARC), ARC makes no claims about the semantic content of
   messages.

9.1.  Increased Header Size

   Inclusion of Authenticated Received Chains into messages may cause
   issues for older or constrained MTAs due to increased total header
   size.  Large header blocks, in general, may cause failures to deliver




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   or other outage scenarios for such MTAs.  ARC itself would not cause
   problems.

9.2.  DNS Operations

   The validation of an Authenticated Received Chain composed of N ARC
   Sets can require up to 2*N DNS queries (not including any DNS
   redirection mechanisms which can increase the total number of
   queries).  This leads to two considerations:

   1.  An attacker can send a message to an ARC participant with a
       concocted sequence of ARC Sets bearing the domains of intended
       victims, and all of them will be queried by the participant until
       a failure is discovered.  The difficulty of forging the signature
       values should limit the extent of this load to domains under
       control of the attacker.  Query traffic pattern analysis may
       expose information about downstream validating ADMD
       infrastructure.

   2.  DKIM only performs one DNS query per signature, while ARC can
       introduce many (per chain).  Absent caching, slow DNS responses
       can cause SMTP timeouts; and backlogged delivery queues on
       Validating systems.  This could be exploited as a DoS attack.

9.3.  Message Content Suspicion

   Recipients are cautioned to treat messages bearing Authenticated
   Received Chains with the same suspicion applied to all other
   messages.  This includes appropriate content scanning and other
   checks for potentially malicious content.

   Just as passing message authentication is not an indication of
   message safety, forwarding that information through the mechanism of
   ARC is also not an indication of message safety.  Even if all ARC-
   enabled ADMDs are trusted, ADMDs may have become compromised, may
   miss unsafe content, or may not properly authenticate messages.

9.4.  Message Sealer Suspicion

   Recipients are cautioned to treat every Sealer of the ARC Chain with
   suspicion.  Just as with a validated DKIM signature, responsibility
   for message handling is attributed to the signing domain, but whether
   or not that signer is a malicious actor is out of scope of the
   authentication mechanism.  Since ARC aids message delivery in the
   event of an authentication failure, ARC Sealers should be treated
   with suspicion, so that a malicious actor cannot Seal spam or other
   fraudulent messages to aid their delivery, too.




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9.5.  Replay Attacks

   Since ARC inherits heavily from DKIM, it has similar attack vectors.
   In particular, the Replay Attack described in [RFC6376] section 8.6
   is potentially amplified by ARC's chained statuses.  In an ARC replay
   attack, a malicious actor would take an intact and passing ARC Chain,
   and then resend it to many recipients without making any
   modifications that invalidate the latest AMS or AS.  The impact to a
   receiver would be more DNS lookups and signature evaluations.  This
   scope of this attack can be limited by caching DNS queries and
   following the same signing scope guidance from [RFC6376] section
   5.4.1.

10.  IANA Considerations

   [[ *Note to the RFC Editors:* "dkim - header - s" is defined both
   here and in [I-D-7601bis].  Please delete the overlap from whichever
   document goes through the publication process after the other. ]]

   This draft introduces three new headers fields and updates the Email
   Authentication Parameters registry with one new authentication method
   and several status codes.

10.1.  Email Authentication Results Names Registry Update

   This draft adds one Auth Method with three Codes to the IANA "Email
   Authentication Result Names" registry:

   o  Auth Method : arc
      Code: "none", "pass", "fail"
      Specification: [I-D.ARC] 2.2
      Status: active

10.2.  Email Authentication Methods Registry Update

   This draft adds several new items to the Email Authentication Methods
   registry, most recently defined in [I-D-7601bis]:

   o  Method: arc
      Definition: [I-D.ARC]
      ptype: smtp
      Property: client-ip
      Value: IP address of originating SMTP connection
      Status: active
      Version: 1

   o  Method: arc
      Definition: [I-D.ARC]



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      ptype: header
      Property: oldest-pass
      Value: The instance id of the oldest validating AMS, or 0 if they
      all pass (see Section 5.2)
      Status: active
      Version: 1

   o  Method: dkim
      Definition: [RFC6376]
      ptype: header
      Property: s
      Value: value of signature "s" tag
      Status: active
      Version: 1

10.3.  Definitions of the ARC header fields

   This specification adds three new header fields to the "Permanent
   Message Header Field Registry", as follows:

   o  Header field name: ARC-Seal
      Applicable protocol: mail
      Status: draft
      Author/Change controller: IETF
      Specification document(s): [I-D.ARC]
      Related information: [RFC6376]

   o  Header field name: ARC-Message-Signature
      Applicable protocol: mail
      Status: draft
      Author/Change controller: IETF
      Specification document(s): [I-D.ARC]
      Related information: [RFC6376]

   o  Header field name: ARC-Authentication-Results
      Applicable protocol: mail
      Status: standard
      Author/Change controller: IETF
      Specification document(s): [I-D.ARC]
      Related information: [I-D-7601bis]

11.  Experimental Considerations

   The ARC protocol is designed to address common interoperability
   issues introduced by intermediate message handlers.  Interoperability
   issues are described in [RFC6377] and [RFC7960].





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   As the ARC protocol is implemented by intermediary handlers over
   time, the following should be evaluated in order to determine the
   success of the protocol in accomplishing the intended benefits.

11.1.  Success Consideration

   In an attempt to deliver legitimate messages that users desire, many
   receivers use heuristic-based methods to identify messages that
   arrive via indirect delivery paths.

   ARC will be a success if the presence of Authenticated Received
   Chains allows for improved decision making when processing legitimate
   messages.

11.2.  Failure Considerations

   ARC should function without introducing significant new vectors for
   abuse (see Section 9).  If unforseen vectors are enabled by ARC, then
   this protocol will be a failure.  Note that weaknesses inherent in
   the mail protocols ARC is built upon (such as DKIM replay attacks and
   other known issues) are not new vectors which can be attributed to
   this specification.

11.3.  Open Questions

   The following open questions are academic and have no clear answer at
   the time of the development of the protocol.  However, additional
   deployment should be able to gather the necessary data to answer some
   or all of them.

11.3.1.  Value of the ARC-Seal (AS) Header

   Data should be collected to show if the ARC-Seal (AS) provides value
   beyond the ARC Message Signature (AMS) for either making delivery
   decisions or catching malicious actors trying to craft or replay
   malicious chains.

11.3.2.  DNS Overhead

   Longer Authenticated Received Chains will require more queries to
   retrieve the keys for validating the chain.  While this is not
   believed to be a security issue (see Section 9.2), it is unclear how
   much overhead will truly be added.  This is similar to some of the
   initial processing and query load concerns which were debated at the
   time of the DKIM specification development.

   Data should be collected to better understand usable length and
   distribution of lengths found in valid Authenticated Received Chains



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   along with the the DNS impact of processing Authenticated Received
   Chains.

   An effective operational maximum will have to be developed through
   deployment experience in the field.

11.3.3.  What Trace Information is Valuable

   There are several edge cases where the information in the AAR can
   make the difference between message delivery or rejection.  For
   example, if there is a well known mailing list that seals with ARC
   but doesn't do its own initial DMARC enforcement, an Internet Mail
   Handler with this knowledge could make a delivery decision based upon
   the authentication information it sees in the corresponding AAR
   header.

   Certain trace information in the AAR is useful/necessary in the
   construction of DMARC reports.

   Certain receivers believe the entire set of trace information would
   be valuable to feed into machine learning systems to identify fraud
   and/or provide other signals related to message delivery.

   It is unclear what trace information will be valuable for all
   receivers, regardless of size.

   Data should be collected on what trace information receivers are
   using that provides useful signals that affect deliverability, and
   what portions of the trace data are left untouched or provide no
   useful information.

   Since many such systems are intentionally proprietary or confidential
   to prevent gaming by abusers, it may not be viable to reliably answer
   this particular question.  The evolving nature of attacks can also
   shift the landscape of "useful" information over time.

12.  Implementation Status

   [[ Note to the RFC Editor: Please remove this section before
   publication along with the reference to [RFC7942]. ]]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort



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   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   This information is known to be correct as of the eighth
   interoperability test event which was held on 2018-03-17 at IETF101.

   For a few of the implementations, later status information was
   available as of June 2018.

12.1.  GMail test reflector and incoming validation

   Organization: Google
   Description: Internal production implementation with both debug
   analysis and validating + sealing pass-through function
   Status of Operation: Production - Incoming Validation
   Coverage: Full spec implemented as of [ARC-DRAFT-14]
   Licensing: Proprietary - Internal only
   Implementation Notes:

   o  Full functionality was demonstrated during the interop testing on
      2018-03-17.

   Contact Info: arc-discuss@dmarc.org [1]

12.2.  AOL test reflector and internal tagging

   Organization: AOL
   Description: Internal prototype implementation with both debug
   analysis and validating + sealing pass-through function
   Status of Operation: Beta
   Coverage: ARC Chain validity status checking is operational, but only
   applied to email addresses enrolled in the test program.  This system
   conforms to [ARC-DRAFT-05]
   Licensing: Proprietary - Internal only
   Implementation Notes:

   o  2017-07-15: Full functionality verified during the interop
      testing.

   o  2018-06: Partially retired but still accessible by special request
      due to the in process evolution of the AOL mail infrastructure to
      the integrated OATH environment.  The implementation was based on
      the Apache James DKIM code base and may be contributed back to
      that project in the future.




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   Contact Info: arc-discuss@dmarc.org [2]

12.3.  dkimpy

   Organization: dkimpy developers/Scott Kitterman
   Description: Python DKIM package
   Status of Operation: Production
   Coverage:

   o  2017-07-15: The internal test suite is incomplete, but the command
      line developmental version of validator was demonstrated to
      interoperate with the Google and AOL implementations during the
      interop on 2017-07-15 and the released version passes the tests in
      [ARC-TEST] arc_test_suite [3] with both python and python3.

   Licensing: Open/Other (same as dkimpy package = BCD version 2)
   Contact Info: https://launchpad.net/dkimpy

12.4.  OpenARC

   Organization: TDP/Murray Kucherawy
   Description: Implemention of milter functionality related to the
   OpenDKIM and OpenDMARC packages
   Status of Operation: Beta
   Coverage: Built to support [ARC-DRAFT-14]
   Licensing: Open/Other (same as OpenDKIM and OpenDMARC packages)
   Implementation Notes:

   o  The build is FreeBSD oriented but some packages have been built
      for easier deployment on RedHat-based Linux platforms.

   o  Some issues still exist when deploying in a chained milter
      arrangement (such as OpenSPF -> OpenDKIM -> OpenDMARC -> OpenARC)
      with coordination between the stages.  When deployed in a
      "sandwich" configuration around an MLM, there is no effective
      mechanism to convey trust from the ingress (validator) to egress
      (signer) instances.  (_NOTE_: this is expected to resolved with a
      new release of OpenDMARC expected in mid-2018.)

   Contact Info: arc-discuss@dmarc.org [4]

12.5.  Mailman 3.x patch

   Organization: Mailman development team
   Description: Integrated ARC capabilities within the Mailman 3.2
   package
   Status of Operation: Patch submitted
   Coverage: Based on OpenARC



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   Licensing: Same as mailman package - GPL
   Implementation Notes:

   o  Appears to work properly in at least one beta deployment, but
      waiting on acceptance of the pull request into the mainline of
      mailman development

   Contact Info: https://www.gnu.org/software/mailman/contact.html

12.6.  Copernica/MailerQ web-based validation

   Organization: Copernica
   Description: Web-based validation of ARC-signed messages
   Status of Operation: Beta
   Coverage: Built to support [ARC-DRAFT-05]
   Licensing: On-line usage only
   Implementation Notes:

   o  Released 2016-10-24

   o  Requires full message content to be pasted into a web form found
      at http://arc.mailerq.com/ (warning - https is not supported).

   o  An additional instance of an ARC signature can be added if one is
      willing to paste a private key into an unsecured web form.

   o  2017-07-15: Testing shows that results match the other
      implementations listed in this section.

   Contact Info: https://www.copernica.com/

12.7.  Rspamd

   Organization: Rspamd community
   Description: ARC signing and verification module
   Status of Operation: Production, though deployment usage is unknown
   Coverage: Built to support [ARC-DRAFT-14]
   Licensing: Open source
   Implementation Notes:

   o  2017-06-12: Released with version 1.6.0

   o  2017-07-15: Testing during the interop showed that the validation
      functionality interoperated with the Google, AOL, dkimpy and
      MailerQ implementations

   Contact Info: https://rspamd.com/doc/modules/arc.html and
   https://github.com/vstakhov/rspamd



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12.8.  PERL MAIL::DKIM module

   Organization: FastMail
   Description: Email domain authentication (sign and/or verify) module,
   previously included SPF / DKIM / DMARC, now has ARC added
   Status of Operation: Production, deployment usage unknown
   Coverage: Built to support [ARC-DRAFT-10]
   Licensing: Open Source
   Implementation Notes:

   o  2017-12-15: v0.50 released with full test set passing for ARC

   Contact Info: http://search.cpan.org/~mbradshaw/Mail-DKIM-0.50/

12.9.  PERL Mail::Milter::Authentication module

   Organization: FastMail
   Description: Email domain authentication milter, uses MAIL::DKIM (see
   above)
   Status of Operation: Intial validation completed during IETF99
   hackathon with some follow-on work during the week
   Coverage: Built to support [ARC-DRAFT-14]
   Licensing: Open Source
   Implementation Notes:

   o  2017-07-15: Validation functionality which interoperates with
      Gmail, AOL, dkimpy was demonstrated; later in the week of IETF99,
      the signing functionality was reported to be working

   o  2017-07-20: ARC functionality has not yet been pushed back to the
      github repo but should be showing up soon

   Contact Info: https://github.com/fastmail/authentication_milter

12.10.  Sympa List Manager

   Organization: Sympa Dev Community
   Description: Work in progress
   Status of Operation: Work in progress
   Coverage: unknown
   Licensing: open source
   Implementation Notes:

   o  2018-01-05: Tracked as https://github.com/sympa-community/sympa/
      issues/153

   Contact Info: https://github.com/sympa-community




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12.11.  Oracle Messaging Server

   Organization: Oracle
   Description:
   Status of Operation: Intial development work during IETF99 hackathon.
   Framework code is complete, crypto functionality requires integration
   with libsodium
   Coverage: Work in progress
   Licensing: Unknown
   Implementation Notes:

   o  2018-03: Protocol handling components are completed, but crypto is
      not yet functional.

   Contact Info: Chris Newman, Oracle

12.12.  MessageSystems Momentum and PowerMTA platforms

   Organization: MessageSystems/SparkPost
   Description: OpenARC integration into the LUA-enabled Momentum
   processing space
   Status of Operation: Beta
   Coverage: Same as OpenARC
   Licensing: Unknown
   Implementation Notes:

   o  Initial deployments for validation expected in mid-2018.

   Contact Info: TBD

12.13.  Exim

   Organization: Exim developers
   Status of Operation: Operational; requires specific enabling for
   compile.
   Coverage: Full spec implemented as of [ARC-DRAFT-13]
   Licensing: GPL
   Contact Info: exim-users@exim.org
   Implementation notes:

   o  Implemented as of Exim 4.91

13.  References








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13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3463]  Vaudreuil, G., "Enhanced Mail System Status Codes",
              RFC 3463, DOI 10.17487/RFC3463, January 2003,
              <https://www.rfc-editor.org/info/rfc3463>.

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

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

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

   [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,
              <https://www.rfc-editor.org/info/rfc6376>.

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

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

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC7601]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7601,
              DOI 10.17487/RFC7601, August 2015,
              <https://www.rfc-editor.org/info/rfc7601>.





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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

13.2.  Informative References

   [ARC-DRAFT-05]
              Andersen, K., "Authenticated Received Chain (ARC) Protocol
              (I-D-05)", n.d., <https://tools.ietf.org/html/
              draft-ietf-dmarc-arc-protocol-05>.

   [ARC-DRAFT-10]
              Andersen, K., "Authenticated Received Chain (ARC) Protocol
              (I-D-10)", n.d., <https://tools.ietf.org/html/
              draft-ietf-dmarc-arc-protocol-10>.

   [ARC-DRAFT-13]
              Andersen, K., "Authenticated Received Chain (ARC) Protocol
              (I-D-13)", n.d., <https://tools.ietf.org/html/
              draft-ietf-dmarc-arc-protocol-13>.

   [ARC-DRAFT-14]
              Andersen, K., "Authenticated Received Chain (ARC) Protocol
              (I-D-14)", n.d., <https://tools.ietf.org/html/
              draft-ietf-dmarc-arc-protocol-14>.

   [ARC-MULTI]
              Andersen, K., "Using Multiple Signing Algorithms with
              ARC", January 2018, <https://tools.ietf.org/html/
              draft-ietf-dmarc-arc-multi-01>.

   [ARC-TEST]
              Blank, S., "ARC Test Suite", January 2017,
              <https://github.com/Valimail/arc_test_suite>.

   [ARC-USAGE]
              Jones, S., Adams, T., Rae-Grant, J., and K. Andersen,
              "Recommended Usage of the ARC Headers", April 2018,
              <https://tools.ietf.org/html/
              draft-ietf-dmarc-arc-usage-05>.

   [ENHANCED-STATUS]
              "IANA SMTP Enhanced Status Codes", n.d.,
              <http://www.iana.org/assignments/smtp-enhanced-status-
              codes/smtp-enhanced-status-codes.xhtml>.






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   [I-D-7601bis]
              Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", February 2018,
              <https://datatracker.ietf.org/doc/
              draft-ietf-dmarc-rfc7601bis/>.

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

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC7960]  Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky,
              E., Ed., and K. Andersen, Ed., "Interoperability Issues
              between Domain-based Message Authentication, Reporting,
              and Conformance (DMARC) and Indirect Email Flows",
              RFC 7960, DOI 10.17487/RFC7960, September 2016,
              <https://www.rfc-editor.org/info/rfc7960>.

13.3.  URIs

   [1] mailto:arc-discuss@dmarc.org

   [2] mailto:arc-discuss@dmarc.org

   [3] https://github.com/Valimail/arc_test_suite

   [4] mailto:arc-discuss@dmarc.org

   [5] https://trac.ietf.org/trac/dmarc/ticket/17

   [6] mailto:dmarc@ietf.org

   [7] mailto:arc-discuss@dmarc.org

   [8] mailto:arc-interop@dmarc.org

   [9] https://arc-spec.org

Appendix A.  Appendix A - Design Requirements

   [[ Note: This section is re-inserted for background information from
   early versions of the spec. ]]




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   The specification of the ARC framework is driven by the following
   high-level goals, security considerations, and practical operational
   requirements.

A.1.  Primary Design Criteria

   o  Provide a verifiable "chain of custody" for email messages;

   o  Not require changes for originators of email;

   o  Support the verification of the ARC header field set by each hop
      in the handling chain;

   o  Work at Internet scale; and

   o  Provide a trustable mechanism for the communication of
      Authentication-Results across trust boundaries.

A.2.  Out of Scope

   ARC is not a trust framework.  Users of the ARC header fields are
   cautioned against making unsubstantiated conclusions when
   encountering a "broken" ARC sequence.

Appendix B.  Appendix B - Example Usage

   [[ Note: The following examples were mocked up early in the
   definition process for the spec.  They no longer reflect the current
   definition and need various updates which will be included in a
   future draft.  Issue 17 [5] ]]

   [[ Note: Need input from the WG as to what sort of sequence of
   examples would be considered useful - otherwise we'll just drop this
   section entirely. ]]

   <removed for now to reduce confusion>

Appendix C.  Acknowledgements

   This draft originated with the work of OAR-Dev Group.

   The authors thank all of the OAR-Dev group for the ongoing help and
   though-provoking discussions from all the participants, especially:
   Alex Brotman, Brandon Long, Dave Crocker, Elizabeth Zwicky, Franck
   Martin, Greg Colburn, J.  Trent Adams, John Rae-Grant, Mike Hammer,
   Mike Jones, Steve Jones, Terry Zink, Tim Draegen.





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   Grateful appreciation is extended to the people who provided feedback
   through the discuss mailing list.

Appendix D.  Comments and Feedback

   Please address all comments, discussions, and questions to
   dmarc@ietf.org [6].  Earlier discussions can be found at arc-
   discuss@dmarc.org [7].  Interop discussions planning can be found at
   arc-interop@dmarc.org [8].

   Some introductory material for less technical people can be found at
   https://arc-spec.org [9].

Authors' Addresses

   Kurt Andersen
   LinkedIn
   1000 West Maude Ave
   Sunnyvale, California  94085
   USA

   Email: kurta@linkedin.com


   Brandon Long (editor)
   Google

   Email: blong@google.com


   Seth Blank (editor)
   Valimail

   Email: seth@valimail.com


   Murray Kucherawy (editor)
   TDP

   Email: superuser@gmail.com


   Tim Draegen (editor)
   dmarcian

   Email: tim@dmarcian.com





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