Authenticated Received Chain (ARC) Protocol
draft-ietf-dmarc-arc-protocol-09

Abstract

The Authenticated Received Chain (ARC) protocol creates a mechanism whereby a series of handlers of an email message can conduct authentication of the email message as it passes among them on the way to its destination, and record the status of that authentication at each step along the handling path, for use by the final recipient in making choices about the disposition of the message. Changes in the message that might break DKIM or DMARC can be identified through the ARC set of header fields.

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/.

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This Internet-Draft will expire on March 10, 2018.

Copyright Notice

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Table of Contents

• 1. Introduction
• 2. Overview
• 3. Definitions and Terminology
• 3.1. Referenced Definitions
• 4. Instance (‘i=’) Tag
• 4.1. Valid Range for Instance Tags
• 5. The ARC Header Fields
• 5.1. ARC-Authentication-Results (AAR)
• 5.1.1. Additional Information for the AAR Header
• 5.2. ARC-Message-Signature (AMS)
• 5.3. ARC-Seal (AS)
• 5.3.1. The ‘cv’ Tag
• 5.3.2. Selected Header Fields
• 6. Verifier Actions
• 7. Signer Actions
• 8. Key Management
• 9. Usage of ARC and Chain Validity
• 9.1. Relationship between DKIM-Signature and AMS signing scopes
• 9.2. Assessing Chain Validity Violations
• 9.3. Marking and Sealing “cv=fail” (Invalid) Chains
• 9.4. Handling DNS Problems While Validating ARC
• 9.5. Responding to ARC Validity Violations
• 10. Recording and Reporting the Results of ARC Evaluation
• 10.1. Information from an ARC Evaluation
• 10.2. Recording (local) ARC Evaluation Results
• 10.3. DMARC Reporting of ARC Findings - Interim
• 11. Supporting Alternate Signing Algorithms
• 11.1. Introductory Period
• 11.2. Co-Existence Period
• 11.3. Deprecation Period
• 11.4. Obsolescence Period
• 12. Privacy Considerations
• 13. IANA Considerations
• 13.1. Authentication-Results Method Registry Update
• 13.2. Definitions of the ARC header fields
• 14. Security Considerations
• 14.1. Message Content Suspicion
• 15. Implementation Status
• 15.1. GMail test reflector and incoming validation
• 15.2. AOL test reflector and internal tagging
• 15.3. dkimpy
• 15.4. OpenARC
• 15.5. Mailman 3.1+ patch
• 15.6. Copernica/MailerQ web-based validation
• 15.7. Rspamd
• 15.8. PERL Mail::Milter::Authentication module
• 16. References
• 16.1. Normative References
• 16.2. Informative References
• Appendix A. Appendix A - Design Requirements
• A.1. Primary Design Criteria
• A.2. Out of Scope
• Appendix B. Appendix B - Example Usage
• B.1. Example 1: Simple mailing list
• B.1.1. Here’s the message as it exits the Origin:
• B.1.2. Message is then received at example.org
• B.1.3. Example 1: Message received by Recipient
• B.2. Example 2: Mailing list to forwarded mailbox
• B.2.1. Here’s the message as it exits the Origin:
• B.2.2. Message is then received at example.org
• B.2.3. Example 2: Message received by Recipient
• B.3. Example 3: Mailing list to forwarded mailbox with source
• B.3.1. Here’s the message as it exits the Origin:
• B.3.2. Message is then received at example.org
• B.3.3. Example 3: Message received by Recipient
• Appendix C. Acknowledgements
• Appendix D. Comments and Feedback
• Authors' Addresses

1. Introduction

Modern email authentication techniques such as the Sender Policy Framework (SPF) [RFC7208] and DomainKeys Identified Mail (DKIM) [RFC6376] have become common.
However, their end-to-end utility is limited by the effects of intermediaries along the transmission path, which either are not listed (for SPF) or which break digital signatures (for DKIM). These issues are described in substantial detail in those protocols’ defining documents as well as in [RFC6377] and [RFC7960].

Technologies that build upon the use of SPF and DKIM can reduce the success of fraudulent email campaigns. To this end, Domain-based Mail Authentication, Reporting and Compliance (DMARC) [RFC7489], validates the domain of the RFC5322.From author header field. However its use along email transmission paths that have independent intermediaries, such as some forwarders and essentially all mailing list services, produces false positive rejections that are problematic, both for the message authors, the intermediary service(s), and for those they are interacting with.

What is needed is a mechanism by which legitimate alteration of a message, which invalidates associated SPF and DKIM information, does not ultimately result in a rejection of an email message on delivery.
Authenticated Receive Chain (ARC) builds upon DKIM mechanisms to provide a sequence of signatures that are more survivable than DKIM’s and that provide a view of the handling sequence for a message, especially the points where alterations of the content might have occurred. Equipped with this more complete information, the recipient system(s) can make a more informed handling choice, reducing or eliminating the false negatives inherent in use of DKIM and/or SPF themselves.

2. Overview

In DKIM, every participating signing agent attaches a signature that is based on the some of the content of the message, local policy, and the domain name of the participating Administrative Management Domain (ADMD). Any verifier can process such a signature; a verified signature means that the domain referenced in the DKIM-Signture’s “d=” parameter has some responsibility for handling the message. An artifact of using digital signature technology for this means that verification also ensures that the message content that was “covered” by the signature has not been altered since the signature was applied. The signatures themselves are generally independent of one another.

By contrast, an ARC signature conveys the following pieces of information:

1. An assertion that, at the time that the intermediary ADMD processed the message, the various assertions (DKIM-Signature(s) and/or ARC sets) already attached to the message by other ADMDs were or were not valid;
2. As with DKIM, an assertion that, for a validated signature, the domain name in the signature takes some responsibility for handling of the message and that the message is unchanged since that signature was applied;
3. A further assertion that binds the ARC evaluation results into the ARC chain sequence.

This protocol accomplishes each of these by adding a new header field to the message for each of these pieces of information, as follows:

• ARC-Authentication-Results (referred to below as “AAR”): virtually identical in syntax to an Authentication-Results field [RFC7601], this field records the results of all message authentication checks done by the recording ADMD at the time the message arrived. Additional information is placed in this field compared to a standard Authentication-Results field in order to support a more complete DMARC report (see Section 5.1);
• ARC-Message-Signature (referred to below as “AMS”): virtually identical in syntax to DKIM-Signature, this field contains the signature about the message header and body as they existed at the time of handling by the ADMD adding it; and
• ARC-Seal (referred to below as “AS”): highly similar in structure and format to a DKIM-Signature, this field applies a digital signature that protects the integrity of all three of these new fields when they are added by an ADMD, plus all instances of these fields added by prior ADMDs.

A distinguishing feature of all of these is that an ARC participant always adds all of them before relaying a message to the next handling agent en route to its destination. Moreover, as described in Section 4, they each have an “instance” number that increases with each ADMD in the handling chain so that their original order can be preserved and the three related header fields can be processed as a group.

3. Definitions and Terminology

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

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].

Because many of the core concepts and definitions are found in [RFC5598], readers SHOULD to be familiar with the contents of [RFC5598], and in particular, the potential roles of intermediaries in the delivery of email.

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

• “ARC set” - A single group of the header fields introduced in Section 2 is called an “ARC set”.
• “ARC chain” - The complete sequence of these groups (ARC sets) is called an “Authenticated Received Chain” or merely an “ARC chain”. Although the “Received” header field is typically not included in the signed content, the name is based on the notion that this is in essence a cryptographically signed series of header fields that attest to the handling chain of a message much as Received fields always have.

3.1. Referenced Definitions

The following terms are defined in other RFCs. Those definitions can be found as follows:

• ADMD - [RFC5598], Section 2.3
• MTA - [RFC5598], Section 4.3.2
• MSA - [RFC5598], Section 4.3.1
• MDA - [RFC5598], Section 4.3.3

The three header fields that are part of this specification borrow heavily from existing specifications. Rather than repeating all of the formal definitions that are being reused in ARC, this document only describes and specifies changes in syntax and semantics.

Language, syntax, and other details are imported from DKIM [RFC6376]. Specific references can be found below.

4. Instance (‘i=’) Tag

The header fields comprising a single ARC set are identified by the presence of a string in the value portion of the header field that complies with the “tag-spec” ABNF found in Section 3.2 of [RFC6376]. The tag-name is “i” and the value is the text representation of a positive integer, indicating the position in the ARC sequence this set occupies, where the first ARC set is numbered 1. In ABNF terms:

   instance = [FWS] %x69 [FWS] "=" [FWS] position [FWS] ";"
   position = 1*DIGIT

Valid ARC sets must have exactly one instance of each header field for a given position value. (Note that when multiple algorithms are supported, there is some nuance to this statement - see Section 11.)

Because the AMS and AS header field values are made up of tag-spec constructs, the i= tag may be found anywhere within the header field value, but is represented throughout this spec in the initial position for convenience. Implementers are encouraged to place the i= tag at the beginning of the field value to facilitate human inspection of the headers.

4.1. Valid Range for Instance Tags

The ‘i’ tag value can range from 1-1024 (inclusive).

ARC implementations MUST support at least ten (10) ARC sets.

An effective operational maximum will have to be developed through deployment experience in the field and will be documented within [ARC-USAGE] once determined.

ARC chains with more than the defined operational maximum count MAY be marked with “cv=fail”.

5. The ARC Header Fields

5.1. ARC-Authentication-Results (AAR)

The ARC-Authentication-Results header field is syntactically and semantically identical to an Authentication-Results header field (defined in Section 2.2 of [RFC7601] (A-R)), except for one mandatory addition and several optional data fields. These deviations are:

• There is an “i” tag, as described in Section 4; and
• Two (or more) additional pieces of information MAY be added (see Section 5.1.1).

The instance identifier MUST be separated from the rest of the Authentication-Results value contents with a semi-colon (‘;’, 0x3b).

The purpose of this header field is to incorporate into the record the success or failure of any authentication done on the message upstream of the participating ADMD that is validating and continuing the authentication chain.

The AAR MUST contain all A-R results from within the participating ADMD, regardless of how many A-R headers are on the message.

5.1.1. Additional Information for the AAR Header

An ARC signer generates this field in the same way that a conventional A-R field would be generated. Because the AAR is designed for machine-based consumption over the course of a message’s transit through a series of mediators and to facilitate troubleshooting of problematic sources by sending organizations, three additional fields of data SHOULD be added to the normal A-R content, dependent on the presence of DKIM-Signature and/or ARC set(s) and if available to the ADMD which is recording the A-R:

• smtp.client_id - The connecting client IP address from which the message was received;
• header.ds - The domain/selector pair for each dkim signature on the message (header.ds=example.com,selector)
• arc.closest_fail - The hop number of the most recent AMS that fails to validate, or 0 if all hops pass.

5.2. ARC-Message-Signature (AMS)

The ARC-Message-Signature header field is syntactically and semantically identical to a DKIM-Signature header field [RFC6376], with the following exceptions:

• There is an “i” tag, as described in Section 4.
• There is no “v” tag.

ARC-Seal header fields MUST NOT be included in the content covered by the signature in this header field.

The AMS SHOULD include any DKIM-Signature header fields already present on the message in the header fields covered by this signature.

The AMS header field MAY include (sign) the AAR header field(s).

Authentication-Results header fields SHOULD NOT be included since they are likely to be deleted by downstream ADMDs (per Section XXX of [RFC7601]), thereby breaking the AMS signature.

As with a DKIM-Signature, the purpose of this header field is to allow the ADMD generating it to take some responsibility for handling this message as it progresses toward delivery.

5.3. ARC-Seal (AS)

The ARC-Seal header field is syntactically and semantically similar to a DKIM-Signature field, with the following exceptions:

• There is an “i” tag, as described in Section 4.
• The ARC-Seal covers none of the body content of the message. It only covers specific header fields. (See below: Section 5.3.2.) As a result, no body canonicalization is done. Further, only “relaxed” header canonicalization (Section 3.4.2 of [RFC6376]) is used.
• The only supported tags are “i” (Section 4 supercedes the [RFC6376] definition), and “a”, “b”, “d, “s”, “t”. The latter 5 tag definitions are copied from Section 3.5 of [RFC6376].
• An additional tag, “cv” is defined. (See below: Section 5.3.1)

5.3.1. The ‘cv’ Tag

A new tag “cv” (chain validation) indicates the the outcome of evaluating the existing ARC chain upon arrival at the ADMD that is adding this header field. It accepts one of three possible values:

• none: There was no chain on the message when it arrived for validation; typically occurs when the message arrives at a Message Transfer Agent (MTA) from a Message Submission Agent (MSA) or when any upstream MTAs may not be participating in ARC handling;
• fail: The message has a chain whose validation failed;
• pass: The message has a chain whose validation succeeded.

In ABNF terms:

 seal-cv-tag = %x63.76 [FWS] "=" [FWS] ("none" / "fail" / "pass")

5.3.2. Selected Header Fields

[[ Note: reword sentence 1 per Dave’s comments ]]

The ARC-Seal signature is a signature of the hash of the concatenation of the canonicalized form of the ARC sets present on the message at the time of sealing, in increasing instance order, starting at 1, including the one being added at the time of sealing the message.

Within a set, the header fields are listed in the following order:

1. ARC-Authentication-Results
2. ARC-Message-Signature
3. ARC-Seal

Where the ARC-Seal is the one being generated, it is input to the hash function in its final form except with an empty “b=” value, in the same manner by which a DKIM-Signature signs itself.

Note that the signing scope for the ARC-Seal is modified in the situation where a chain has failed validation (see Section 9.3).

6. Verifier Actions

The verifier takes the following steps to determine the current state of the ARC chain on the message. Canonicalization, hash functions, and signature validation methods are imported from Section 5 of [RFC6376].

[[ Note: need markdown flag to have subordinate numbering distinction ]]

1. Collect all ARC sets currently on the message. If there were none, the ARC state is “none” and the algorithm stops here.
2. If the form of any ARC set is invalid (e.g., does not contain exactly one of each of the three ARC-specific header fields), then the chain state is “fail” and the algorithm stops here.
   1. To avoid the overhead of unnecessary computation and delay from crypto and DNS operations, the cv value for all ARC-Seal(s) MAY be checked at this point. If any of the values are “fail”, then the overall state of the chain is “fail” and the algorithm stops here.
3. Conduct verification of the ARC-Message-Signature header field bearing the highest instance number. If this verification fails, then the chain state is “fail” and the algorithm stops here.
4. For each ARC-Seal from the “N”th instance to the first, apply the following logic:
   1. If the value of the “cv” tag on that seal is “fail”, the chain state is “fail” and the algorithm stops here. (This step SHOULD be skipped if the earlier step (2.1) was performed)
   2. In Boolean nomenclature: if ((i == 1 && cv != “none”) or (cv == “none” && i != 1)) then the chain state is “fail” and the algorithm stops here (note that the ordering of the logic is structured for short-circuit evaluation).
   3. Initialize a hash function corresponding to the “a” tag of the ARC-Seal.
   4. Compute the canonicalized form of the ARC header fields, in the order described in Section 5.3.2, using the “relaxed” header canonicalization defined in Section 3.4.2 of [RFC6376]. Pass the canonicalized result to the hash function.
   5. Retrieve the final digest from the hash function.
   6. Retrieve the public key identified by the “s” and “d” tags in the ARC-Seal, as described in Section 8.
   7. Determine whether the signature portion (“b” tag) of the ARC-Seal and the digest computed above are valid according to the public key. (See also Section Section 9.4 for failure case handling)
   8. If the signature is not valid, the chain state is “fail” and the algorithm stops here.
5. If all seals pass validation, then the chain state is “pass”, and the algorithm is complete.

[[ Note from Dave: possibly delete the following paragraph as it is more usage/procedural than specification guidance. KA: It was added to clarify the separation of the verification and signing steps as some of the initial implementations failed to realize that they were not necessarily done in one fell swoop. ]]

The verifier should save the cv state for subsequent use by any sealing which may be done later (potentially after message modification) within the same trust boundary. The cv state may be recorded by sealing at the time of verification in an initial ARC set (for the ADMD) or may be recorded out of band depending on the architecture of the ADMD.

7. Signer Actions

[[ Note from Dave: This seems more like implementation guidance than specification detail. KA: see explanation just above referring to the previous note. ]]

This section includes a specification of the actions an ARC signer takes when presented with a message.

The signer MUST undertake the following steps:

1. Before creating an ARC signature, perform any other, normal authentication and/or signing, so that the ARC signature can cover those results.
2. Build and attach the new ARC set:
   1. If an ARC chain exists on the message, then set “N” equal to the highest instance number found on the chain (i=); otherwise set “N” equal to zero for the following steps.
   2. Generate and attach to the message an ARC-Authentication-Results header field using instance number N+1 and the same content from the previous step.
   3. Generate and attach to the message an ARC-Message-Signature header field as defined in Section 5.2 above, using instance number N+1.
   4. Generate and attach to the message an ARC-Seal header field using the general algorithm described in Section 5.3 above, the chain validation status as determined in Section 6, and instance number N+1.

8. Key Management

The public keys for ARC header fields follow the same requirements, syntax and semantics as those for DKIM signatures, described in Section 3.6 of [RFC6376]. For operational convenience, signers MAY choose to use selectors and/or domains for the ARC header field signatures that are distinct from those used in DKIM signing.

9. Usage of ARC and Chain Validity

9.1. Relationship between DKIM-Signature and AMS signing scopes

DKIM-Signatures SHOULD never sign any ARC header fields.

[[ KA: Response to Dave’s concern: If DKIM covers ARC and ARC covers DKIM, which comes first? The chicken or the egg? I’m open to alternate ways to phrase this without opening the “modifying the DKIM spec” can of worms. ]]

9.2. Assessing Chain Validity Violations

Email transit can produce broken signatures for a wide variety of benign reasons. This includes possibly breaking one or more ARC signatures. Therefore, receivers need to be wary of ascribing motive to such breakage although patterns of common behaviour may provide some basis for adjusting local policy decisions.

ARC does not attempt to protect an entire message. There are various ways that a message can still be problematic, in spite of having a valid ARC chain. Consequently, all normal, content-based analysis SHOULD still be performed on any message having a valid chain of ARC header sets.

9.3. Marking and Sealing “cv=fail” (Invalid) Chains

The header fields signed by the AS header field b= value in the case of a chain failure MUST be only the matching ‘i=’ instance headers created by the MTA which detected the malformed chain, as if this newest ARC set was the only set present.

9.4. Handling DNS Problems While Validating ARC

DNS-based failures to verify a chain are treated no differently than any other ARC violation. They result in a “cv=fail” verdict.

9.5. Responding to ARC Validity Violations

If a receiver determines that the ARC chain has failed, the receiver MAY signal the breakage through the extended SMTP response code 5.7.7 [RFC3463] “message integrity failure” [ENHANCED-STATUS] and corresponding SMTP response code.

10. Recording and Reporting the Results of ARC Evaluation

The evaluation of an ARC chain provides information which will be useful to both the receiver (or intermediary) and to the initial sender of the message. This information should be preserved and reported as follows.

10.1. Information from an ARC Evaluation

The evaluation of an ARC chain produces a list of domain names for participating intermediaries which handled the message, to wit:

• A list of the “d=” domains found in the validated ARC-Seal header fields
• The “d=” domain found in the most recent (highest instance number) AMS header field (since that is the only one necessarily validated)

In the case of a failed chain, only the terminal ARC set is covered by the ARC-Seal so the reporting is limited to the findings in that terminal ARC set.

10.2. Recording (local) ARC Evaluation Results

Receivers MAY add an “arc=[pass|fail|policy]” method annotation into a locally-affixed Authentication-Results [RFC7601] header field along with any salient comment(s).

Details of the ARC chain which was evaluated should be included in the Authentication-Results and AAR headers per Section Section 5.1.1.

10.3. DMARC Reporting of ARC Findings - Interim

[[ Note: Discussion on the IETF DMARC-WG list has indicated some interest in more substantial reporting for analytic purposes. To support that effort, the following guidance is provided only as an interim, minimal data set. A more complete reporting construct will be specified in a related spec - TBD. (see the additional fields specified in Section 5.1.1) ]]

Receivers SHOULD indicate situations in which ARC evaluation influenced the results of their local policy determination. DMARC reporting of ARC-informed decisions can be accomplished by adding a local_policy comment explanation containing the list of data discovered in the ARC evaluation (Section 10.1 and Section 5.1.1):

<policy_evaluated>
  <disposition>delivered</disposition>
  <dkim>fail</dkim>
  <spf>fail <comment>source.ip=10.0.0.1</comment></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</comment>
  </reason>
</policy_evaluated>

In the suggested sample, d2.example is the sealing domain for ARC[2] and d1.example is the sealing domain for ARC[1].

Mediators SHOULD generate DMARC reports on messages which transit their system just like any other message which they receive. This will result in multiple reports for each mediated message as they transit the series of handlers. DMARC report consumers should be aware of this behaviour and make the necessary accommodations.

11. Supporting Alternate Signing Algorithms

[[ Note: Some additional development of this section is needed. ]]

In the following branch diagrams, each algorithm is represented by an ‘A’ or ‘B’ at each hop to depict the ARC chain that develops over a five hop scenario. ‘x’ represents a hop that does not support that algorithm.

Note that during a transitional period where multiple algorithms are allowed, all of the statements in this spec which refer to “exactly one set of ARC headers per instance” need to be understood as “at least one set per instance and no more than one instance-set per algorithm”.

11.1. Introductory Period

Intermediaries MUST be able to validate ARC chains built with either algorithm but MAY create ARC sets with either (or both) algorithm.

The introductory period should be at least six (6) months.

11.2. Co-Existence Period

Intermediaries MUST be able to validate ARC chains build with either algorithm and MUST create ARC sets with both algorithms. Chains ending with either algorithm may be used for the result.

11.3. Deprecation Period

ARC sets built with algorithms that are being deprecated MAY be considered valid within an ARC chain, however, intermediaries MUST NOT create additional sets with the deprecated algorithm.

The deprecation period should be at least two (2) years.

11.4. Obsolescence Period

ARC sets built with algorithms that are obsolete MUST NOT be considered valid within an ARC chain. Intermediaries MUST NOT create any sets with any obsoleted algorithm.

12. Privacy Considerations

The ARC chain provides a verifiable record of the handlers for a message. Anonymous remailers will probably not find this compatible with their operating goals.

13. IANA Considerations

This specification adds three new header fields as defined below.

13.1. Authentication-Results Method Registry Update

This draft adds one item to the IANA “Email Authentication Methods” registry:

• Method : arc
  
  Defined: [I-D.ARC]
  
  ptype: header
  
  Property: chain evaluation result
  
  Value: chain evaluation result status (see Section 5.3)
  
  Status: active
  
  Version: 1

13.2. Definitions of the ARC header fields

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

• Header field name: ARC-Seal
  
  Applicable protocol: mail
  
  Status: draft
  
  Author/Change controller: IETF
  
  Specification document(s): [I-D.ARC]
  
  Related information: [RFC6376]
• Header field name: ARC-Message-Signature
  
  Applicable protocol: mail
  
  Status: draft
  
  Author/Change controller: IETF
  
  Specification document(s): [I-D.ARC]
  
  Related information: [RFC6376]
• Header field name: ARC-Authentication-Results
  
  Applicable protocol: mail
  
  Status: standard
  
  Author/Change controller: IETF
  
  Specification document(s): [I-D.ARC]
  
  Related information: [RFC7601]

14. Security Considerations

The Security Considerations of [RFC6376] and [RFC7601] apply directly to this specification.

Inclusion of ARC sets in the header of emails may cause problems for some older or more constrained MTAs if they are unable to accept the greater size of the header.

Operators who receive a message bearing N ARC sets have to complete up to N+1 DNS queries to evaluate the chain (barring DNS redirection mechanisms which can increase the lookups for a given target value). This has at least two effects:

1. An attacker can send a message to an ARC partipant 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.
2. DKIM only does one DNS check per signature, while this one can do many (per chain). Absent caching, slow DNS responses can cause SMTP timeouts; and backlogged delivery queues on mediating systems. This could be exploited as a DoS attack.

14.1. Message Content Suspicion

Recipients are cautioned to treat messages bearing ARC sets with the same suspicion that they apply to all other email messages. This includes appropriate content scanning and other checks for potentially malicious content. The handlers which are identified within the ARC chain may be used to provide input to local policy engines in cases where DMARC validation fails (due to mediation impacting SPF attribution, DKIM validity or alignment).

15. Implementation Status

[[ Note: For minimizing section number references when the RFC editor removes this section, it has been moved to be the last section of the document before the Appendicies. ]]

[[ Note to the RFC Editor: Please remove this section before publication along with the reference to [RFC6982]. ]]

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 [RFC6982]. 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 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 seventh interoperability test event which was held on 2017-07-15 & 16 at IETF99.

15.1. 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-06]

Licensing: Proprietary - Internal only

Implementation Notes:

• Full functionality was demonstrated during the interop testing on 2017-07-15.

Contact Info: arc-discuss@dmarc.org

15.2. 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-06]

Licensing: Proprietary - Internal only

Implementation Notes:

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

Contact Info: arc-discuss@dmarc.org

15.3. dkimpy

Organization: dkimpy developers/Scott Kitterman

Description: Python DKIM package

Status of Operation: Production

Coverage:

• 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] (https://github.com/ValiMail/arc_test_suite) with both python and python3.

Licensing: Open/Other (same as dkimpy package = BCD version 2)

Contact Info: https://launchpad.net/dkimpy

15.4. 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-06]

Licensing: Open/Other (same as OpenDKIM and OpenDMARC packages)

Implementation Notes:

• The build is FreeBSD oriented but some packages have been built for easier deployment on RedHat-based Linux platforms.
• 2017-07-15: Testing showed problems with the hash calculation for the AMS header b= field. Several other bugs were discovered and were either fixed during the following week of IETF meetings or are under active repair.
• 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.

Contact Info: arc-discuss@dmarc.org

15.5. Mailman 3.1+ patch

Organization: Mailman development team

Description: Integrated ARC capabilities within the Mailman 3.1+ package

Status of Operation: Patch submitted

Coverage: Unknown

Licensing: Same as mailman package - GPL

Implementation Notes:

• 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

15.6. 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:

• Released 2016-10-24
• Requires full message content to be pasted into a web form found at http://arc.mailerq.com/ (warning - https is not supported).
• An additional instance of an ARC signature can be added if one is willing to paste a private key into an unsecured web form.
• 2017-07-15: Testing shows that results match the other implementations listed in this section.

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

15.7. 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-06]

Licensing: Open source

Implementation Notes:

• 2017-06-12: Released with version 1.6.0
• 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

15.8. PERL Mail::Milter::Authentication module

Organization: FastMail

Description: Email domain authentication milter, previously included SPF / DKIM / DMARC, now has ARC added

Status of Operation: Intial validation completed during IETF99 hackathon with some follow-on work during the week

Coverage: Built to support [I-D.ARC]

Licensing: Open Source

Implementation Notes:

• 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
• 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

16. References

16.1. Normative References

16.2. Informative References

Appendix A. Appendix A - Design Requirements

(This section is re-inserted for background information from [ARC-DRAFT-06] and earlier versions.)

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

• Provide a verifiable “chain of custody” for email messages;
• Not require changes for originators of email;
• Support the verification of the ARC header field set by each hop in the handling chain;
• Work at Internet scale; and
• 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. ]]

(Obsolete but retained for illustrative purposes)

B.1. Example 1: Simple mailing list

B.1.1. Here’s the message as it exits the Origin:

Return-Path: <jqd@d1.example>
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijrvQw
     bv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4Gd3TRJl
     gotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@dmarc.org
Subject: Example 1
 
Hey gang,
This is a test message.
--J.

B.1.2. Message is then received at example.org

B.1.2.1. Example 1, Step A: Message forwarded to list members

Processing at example.org:

• example.org performs authentication checks
• No previous Authentication-Results or ARC-Seal headers are present
• example.org adds ARC-Authentication-Results header
• example.org adds Received: header
• example.org adds a ARC-Seal header

Here’s the message as it exits example.org:

Return-Path: <jqd@d1.example>
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF1F5
     vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3A+m4bw
     a6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=1; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijr
     vQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4G
     d3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.1.3. Example 1: Message received by Recipient

Let’s say that the Recipient is example.com

Processing at example.com:

• example.com performs usual authentication checks
• example.com adds Authentication-Results: header, Received header
• Determines that message fails DMARC
• Checks for ARC-Seal: header; finds one
• Validates the signature in the ARC-Seal: header, which covers the ARC-Authentication-Results: header
• example.com can use the ARC-Authentication-Results values or verify the DKIM-Signature from lists.example.org

Here’s what the message looks like at this point:

Return-Path: <jqd@d1.example>
Received: from example.org (example.org [208.69.40.157])
    by clothilde.example.com with ESMTP id 
    d200mr22663000ykb.93.1421363207
    for <fmartin@example.com>; Thu, 14 Jan 2015 15:02:40 -0800 (PST)
Authentication-Results: clothilde.example.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@example.org; dmarc=fail; arc=pass
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF
     1F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3
     A+m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=1; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijrvQw
     bv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4Gd3TRJl
     gotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.2. Example 2: Mailing list to forwarded mailbox

B.2.1. Here’s the message as it exits the Origin:

Return-Path: <jqd@d1.example>
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijrvQw
     bv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4Gd3TRJl
     gotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: Example 1
 
Hey gang,
This is a test message.
--J.

B.2.2. Message is then received at example.org

B.2.2.1. Example 2, Step A: Message forwarded to list members

Processing at example.org:

• example.org performs authentication checks
• example.org applies standard DKIM signature
• No previous Authentication-Results or ARC-Seal headers are present
• example.org adds ARC-Authentication-Results header
• example.org adds usual Received: header
• example.org adds a ARC-Seal header

Here’s the message as it exits Step A:

Return-Path: <jqd@d1.example>
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz6
     1TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L
     69EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF
     1F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3
     A+m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=1; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijr
     vQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4G
     d3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.2.2.2. Example 2, Step B: Message from list forwarded

The message is delivered to a mailbox at gmail.com
Processing at gmail.com:

• gmail.com performs usual authentication checks
• gmail.com adds Authentication-Results: and Received: header
• Determines that message fails DMARC
• Checks for ARC-Seal: header; finds one
• Validates the signature in the ARC-Seal: header, which covers the ARC-Authentication-Results: header
• Uses the ARC-Authentication-Results: values, but:
• Instead of delivering message, prepares to forward message per user settings
• Applies usual DKIM signature
• gmail.com adds it’s own ARC-Seal: header, contents of which are
  • version
  • sequence number (“i=2”)
  • hash algorithm (SHA256 as example)
  • timestamp (“t=”)
  • selector for key (“s=notary01”)
  • domain for key (“d=gmail.com”)
  • headers included in hash (“h=ARC-Authentication-Results:ARC-Seal”)
  • Note: algorithm requires only ARC-Seals with lower sequence # be included, in ascending order
  • signature of the header hash

Here’s what the message looks like at this point:

Return-Path: <jqd@d1.example>
ARC-Seal: i=2; a=rsa-sha256; t=1421363253;
    s=notary01; d=gmail.com; cv=pass; 
    b=sjHDMriRZ0Mui5eVEOGscRHWbQHcy97lvrduHQ8h+f2CfIrxUiKOE44x3LQwDWR
     YbDjf5fcM9MdcIahC+cP59BQ9Y9DHwMDzwRTnM7NVb4kY+tSaVnLoIOaP9lF/sut
     txO+RRNr0fCFw==
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed;
    d=gmail.com; s=20120806;
    h=mime-version:content-type:x-original-sender:
     x-original-authentication-results:precedence:mailing-list:
     list-id:list-post:list-help:list-archive:sender:reply-to:
     list-unsubscribe:DKIM-Signature;
    bh=2+gZwZhUK2V7JbpoO2MTrU19WvhcA4JnjiohFm9ZZ/g=;
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0Ab8Oi1ebYV/hIBmfhS
     LF1E80hMPcMijONfTQB6g5Hoh/kE6N2fgp6aSngL/WA3+g3Id8ElhXHvIGcJRFeM
     KdJqiW5cxdqPTRW+BnR5ee6Tzg06kr265NTDIAU8p8fQNuLfZj49MMA+QwDBJtXw
     bQoZyRtb6X6q0mYaszUB8kw==
Received: by mail-yk0-f179.google.com with SMTP id 19so2728865ykq.10
    for <mailbox@gmail.com>; Thu, 14 Jan 2015 15:02:45 -0800 (PST)
Authentication-Results: i=2; gmail.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@example.org; dmarc=fail; arc=pass
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=none:
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF
     1F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3
     A+m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=1; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijr
     vQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4G
     d3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.2.3. Example 2: Message received by Recipient

Let’s say that the Recipient is example.com
Processing at example.com:

• example.com performs usual authentication checks
• example.com adds Authentication-Results: header, Received header
• Determines that message fails DMARC
• Checks for ARC-Seal: header; finds two
• Validates the signature in the highest numbered (“i=2”) ARC-Seal: header, which covers all previous ARC-Seal: and ARC-Authentication-Results: headers
• Validates the other ARC-Seal header (“i=1”), which covers the ARC-Authentication-Results: header
• example.com uses the ARC-Authentication-Results: values

Here’s what the message looks like at this point:

Return-Path: <jqd@d1.example>
Received: from mail-ob0-f188.google.com (mail-ob0-f188.google.com
    [208.69.40.157]) by clothilde.example.com with ESMTP id
    d200mr22663000ykb.93.1421363268
    for <fmartin@example.com>; Thu, 14 Jan 2015 15:03:15 -0800 (PST)
Authentication-Results: clothilde.example.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@gmail.com; dmarc=fail; arc=pass
ARC-Seal: i=2; a=rsa-sha256; t=1421363253;
    s=notary01; d=gmail.com; cv=pass; 
    b=sjHDMriRZ0Mui5eVEOGscRHWbQHcy97lvrduHQ8h+f2CfIrxUiKOE44x3LQwDWR
     YbDjf5fcM9MdcIahC+cP59BQ9Y9DHwMDzwRTnM7NVb4kY+tSaVnLoIOaP9lF/sut
     txO+RRNr0fCFw==
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed;
    d=gmail.com; s=20120806;
    h=mime-version:content-type:x-original-sender:
     x-original-authentication-results:precedence:mailing-list:
     list-id:list-post:list-help:list-archive:sender:reply-to:
     :list-unsubscribe:DKIM-Signature;
    bh=2+gZwZhUK2V7JbpoO2MTrU19WvhcA4JnjiohFm9ZZ/g=;
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0Ab8Oi1ebYV/hIBmfhS
     LF1E80hMPcMijONfTQB6g5Hoh/kE6N2fgp6aSngL/WA3+g3Id8ElhXHvIGcJRFeM
     KdJqiW5cxdqPTRW+BnR5ee6Tzg06kr265NTDIAU8p8fQNuLfZj49MMA+QwDBJtXw
     bQoZyRtb6X6q0mYaszUB8kw==
Received: by mail-yk0-f179.google.com with SMTP id 19so2728865ykq.10
    for <mailbox@gmail.com>; Thu, 14 Jan 2015 15:02:45 -0800 (PST)
Authentication-Results: i=2; gmail.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@example.org; dmarc=fail; arc=pass
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF
     1F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3
     A+m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=1; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=d1.example;
    s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=Message-ID:Date:From:MIME-Version:To:CC:Subject:Content-Type:
     Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijr
     vQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4G
     d3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.3. Example 3: Mailing list to forwarded mailbox with source

B.3.1. Here’s the message as it exits the Origin:

Return-Path: <jqd@d1.example>
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=origin2015; d=d1.example; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61T
     X6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69EU
     8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=d1.example; s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=MIME-Version:CC:Content-Type:Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijrv
     Qwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4Gd3
     TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: Example 1
 
Hey gang,
This is a test message.
--J.

B.3.2. Message is then received at example.org

B.3.2.1. Example 3, Step A: Message forwarded to list members with source

Processing at example.org:

• example.org performs authentication checks
• example.org applies standard DKIM signature
• Checks for ARC-Seal: header; finds one (i=1)
• Validates the signature in the ARC-Seal (i=1): header, which covers the d1.example ARC-Message-Signature: header
• example.org adds ARC-Authentication-Results header
• example.org adds usual Received: header
• example.org adds a DKIM-Signature
• example.org adds a ARC-Seal header, contents of which are
  • sequence number (“i=2”)
  • hash algorithm (SHA256 as example)
  • timestamp (“t=”)
  • chain validity (“cv=”)
  • selector for key (“s=seal2015”)
  • domain for key (“d=example.org”)
  • signature (“b=”)

Here’s the message as it exits Step A:

Return-Path: <jqd@d1.example>
ARC-Seal: i=2; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=pass; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz6
     1TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L
     69EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:From:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF
     1F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3
     A+m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=2; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=origin2015; d=d1.example; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=d1.example; s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=MIME-Version:CC:Content-Type:Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijr
     vQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4G
     d3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.3.2.2. Example 3, Step B: Message from list forwarded with source

The message is delivered to a mailbox at gmail.com
Processing at gmail.com:

• gmail.com performs usual authentication checks
• gmail.com adds Authentication-Results: and Received: header
• Determines that message fails DMARC
• Checks for ARC-Seal: header; finds two
• Validates the signature in the ARC-Seal (i=2): header, which covers the ARC-Authentication-Results: header
• Validates the signature in the ARC-Seal (i=1): header, which covers the d1.example ARC-Message-Signature: header
• Uses the ARC-Authentication-Results: values, but:
• Instead of delivering message, prepares to forward message per user settings
• Applies usual DKIM signature
• gmail.com adds it’s own ARC-Seal: header, contents of which are
  • version
  • sequence number (“i=2”)
  • hash algorithm (SHA256 as example)
  • timestamp (“t=”)
  • selector for key (“s=notary01”)
  • domain for key (“d=gmail.com”)
  • Note: algorithm requires only ARC-Seals with lower sequence # be included, in ascending order
  • signature of the chain

Here’s what the message looks like at this point:

Return-Path: <jqd@d1.example>
ARC-Seal: i=3; a=rsa-sha256; t=1421363253;
    s=notary01; d=gmail.com; cv=pass; 
    b=sjHDMriRZ0Mui5eVEOGscRHWbQHcy97lvrduHQ8h+f2CfIrxUiKOE44x3LQwD
     WRYbDjf5fcM9MdcIahC+cP59BQ9Y9DHwMDzwRTnM7NVb4kY+tSaVnLoIOaP9lF
     /suttxO+RRNr0fCFw==
ARC-Message-Signature: i=3; a=rsa-sha256; c=relaxed/relaxed;
    d=gmail.com; s=20120806;
    h=mime-version:content-type:x-original-sender
     :x-original-authentication-results:precedence:mailing-list
     :list-id:list-post:list-help:list-archive:sender
     :list-unsubscribe:reply-to;
    bh=2+gZwZhUK2V7JbpoO2MTrU19WvhcA4JnjiohFm9ZZ/g=;
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz6
     1TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L
     69EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0Ab8Oi1ebYV/hIBm
     fhSLF1E80hMPcMijONfTQB6g5Hoh/kE6N2fgp6aSngL/WA3+g3Id8ElhXHvIGcJ
     RFeMKdJqiW5cxdqPTRW+BnR5ee6Tzg06kr265NTDIAU8p8fQNuLfZj49MMA+QwD
     BJtXwbQoZyRtb6X6q0mYaszUB8kw==
Received: by mail-yk0-f179.google.com with SMTP id 19so2728865ykq.10
    for <mailbox@gmail.com>; Thu, 14 Jan 2015 15:02:45 -0800 (PST)
Authentication-Results: i=3; gmail.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@example.org; dmarc=fail; arc=pass
ARC-Seal: i=2; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=pass; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF1
     F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3A+
     m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=2; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=origin2015; d=d1.example; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=d1.example; s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=MIME-Version:CC:Content-Type:Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYij
     rvQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD
     4Gd3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

B.3.3. Example 3: Message received by Recipient

Let’s say that the Recipient is example.com
Processing at example.com:

• example.com performs usual authentication checks
• example.com adds Authentication-Results: header, Received header
• Determines that message fails DMARC
• Checks for ARC-Seal: header; finds three
• Validates the signature in the highest numbered (“i=2”) ARC-Seal: header, which covers all previous ARC-Seal: and ARC-Authentication-Results: headers
• Validates the other ARC-Seal header (“i=2”), which covers the ARC-Authentication-Results: header
• Validates the other ARC-Seal header (“i=1”), which covers the d1.example ARC-Message-Signature: header
• example.com uses the ARC-Authentication-Results: values

Here’s what the message looks like at this point:

Return-Path: <jqd@d1.example>
Received: from mail-ob0-f188.google.com (mail-ob0-f188.google.com
    [208.69.40.157]) by clothilde.example.com with ESMTP id
    d200mr22663000ykb.93.1421363268
    for <fmartin@example.com>; Thu, 14 Jan 2015 15:03:15 -0800 (PST)
Authentication-Results: clothilde.example.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@gmail.com; dmarc=fail; arc=pass
ARC-Seal: i=3; a=rsa-sha256; t=1421363253;
    s=notary01; d=gmail.com; cv=pass; 
    b=sjHDMriRZ0Mui5eVEOGscRHWbQHcy97lvrduHQ8h+f2CfIrxUiKOE44x3LQwDW
     RYbDjf5fcM9MdcIahC+cP59BQ9Y9DHwMDzwRTnM7NVb4kY+tSaVnLoIOaP9lF/s
     uttxO+RRNr0fCFw==
ARC-Message-Signature: i=3; a=rsa-sha256; c=relaxed/relaxed;
    d=gmail.com; s=20120806;
    h=mime-version:content-type:x-original-sender
     :x-original-authentication-results:precedence
     :mailing-list:list-id:list-post:list-help:list-archive:sender
     :list-unsubscribe:reply-to;
    bh=2+gZwZhUK2V7JbpoO2MTrU19WvhcA4JnjiohFm9ZZ/g=;
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz6
     1TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L
     69EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0Ab8Oi1ebYV/hIBm
     fhSLF1E80hMPcMijONfTQB6g5Hoh/kE6N2fgp6aSngL/WA3+g3Id8ElhXHvIGcJ
     RFeMKdJqiW5cxdqPTRW+BnR5ee6Tzg06kr265NTDIAU8p8fQNuLfZj49MMA+QwD
     BJtXwbQoZyRtb6X6q0mYaszUB8kw==
Received: by mail-yk0-f179.google.com with SMTP id 19so2728865ykq.10
    for <mailbox@gmail.com>; Thu, 14 Jan 2015 15:02:45 -0800 (PST)
Authentication-Results: i=3; gmail.com; spf=fail
    smtp.from=jqd@d1.example; dkim=pass (1024-bit key) 
    header.i=@example.org; dmarc=fail; arc=pass
ARC-Seal: i=2; a=rsa-sha256; t=1421363107;
    s=seal2015; d=example.org; cv=pass; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz6
     1TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L
     69EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed;
    d=example.org; s=clochette; t=1421363105;
    bh=FjQYm3HhXStuzauzV4Uc02o55EzATNfL4uBvEoy7k3s=;
    h=List-Id:List-Unsubscribe:List-Archive:List-Post:
     List-Help:List-Subscribe:Reply-To:DKIM-Signature;
    b=Wb4EiVANwAX8obWwrRWpmlhxmdIvj0dv0psIkiaGOOug32iTAcc74/iWvlPXpF1
     F5vYVF0mw5cmKOa824tKkUOOE3yinTAekqnly7GJuFCDeSA1fQHhStVV7BzAr3A+
     m4bwa6RIDgr3rOPJil678dZTHfztFWyjwIUxB5Ajxj/M=
Received: from segv.d1.example (segv.d1.example [72.52.75.15])
    by lists.example.org (8.14.5/8.14.5) with ESMTP id t0EKaNU9010123
    for <arc@example.org>; Thu, 14 Jan 2015 15:01:30 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Authentication-Results: i=2; lists.example.org;
    spf=pass smtp.mfrom=jqd@d1.example;
    dkim=pass (1024-bit key) header.i=@d1.example; 
    dmarc=pass
Received: from [10.10.10.131] (w-x-y-z.dsl.static.isp.com [w.x.y.z])
    (authenticated bits=0)
    by segv.d1.example with ESMTP id t0FN4a8O084569;
    Thu, 14 Jan 2015 15:00:01 -0800 (PST)
    (envelope-from jqd@d1.example)
ARC-Seal: i=1; a=rsa-sha256; t=1421363107;
    s=origin2015; d=d1.example; cv=none; 
    b=pCw3Qxgfs9E1qnyNZ+cTTF3KHgAjWwZz++Rju0BceSiuwIg0Pkk+3RZH/kaiz61
     TX6RVT6E4gs49Sstp41K7muj1OR5R6Q6llahLlQJZ/YfDZ3NImCU52gFWLUD7L69
     EU8TzypfkUhscqXjOJgDwjIceBNNOfh3Jy+V8hQZrVFCw0A=
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed;
    d=d1.example; s=20130426; t=1421363082;
    bh=EoJqaaRvhrngQxmQ3VnRIIMRBgecuKf1pdkxtfGyWaU=;
    h=MIME-Version:To:CC:Subject:Content-Type:Content-Transfer-Encoding;
    b=HxsvPubDE+R96v9dM9Y7V3dJUXvajd6rvF5ec5BPe/vpVBRJnD4I2weEIyYijr
     vQwbv9uUA1t94kMN0Q+haFo6hiQPnkuDxku5+oxyZWOqtNH7CTMgcBWWTp4QD4G
     d3TRJlgotsX4RkbNcUhlfnoQ0p+CywWjieI8aR6eof6WDQ=
Message-ID: <54B84785.1060301@d1.example>
Date: Thu, 14 Jan 2015 15:00:01 -0800
From: John Q Doe <jqd@d1.example>
To: arc@example.org
Subject: [Lists] Example 1
 
Hey gang,
This is a test message.
--J.

Appendix C. Acknowledgements

This draft is 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.

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. Earlier discussions can be found at arc-discuss@dmarc.org.

Authors' Addresses