DKIM Third-Party Authorization Label

A third party authorization label (TPA-Label) is a DNS-based extension for DKIM ADSP records that allows domains in the From header field to authorize acceptable third-party signatures. This approach allows autonomous and unilateral authorizations for third-party domains using scalable, individual DNS transactions. The extended scope of DKIM signing practice assertions introduced here supplants transparent authorization schemes that are more difficult to administer. Alternatives for facilitating third-party authorizations currently necessitate coordination between two or more domains to synchronously set up selector/key DNS records, DNS zone delegations, and/or a regular exchange of public/private keys.

Checking TPA-Label Resource Records for signing practices might occur infrequently when a message is not compliant with restrictive ADSP practices, where an Author Domain Signature is either missing or invalid. When a third-party signature is found, TPA-Label Resource Record transactions offer an efficient means for Author Domains to authorize specific third-party signing domains. Recipients are afforded a method to determine whether authorization exists in situations where other modes of authorization are impractical. TPA-Label Resource Records permit Author Domains a means to influence message handling selectively, for messages otherwise lacking valid Author Domain signatures.

Requirements Language

Status of this Memo

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1. Introduction
2. Language and Terminology
    2.1. Terms Imported from other DKIM Specifications:
    2.2. Terms Defined by this Specification:
        2.2.1. Third Party Signature
        2.2.2. Third Party Signer
        2.2.3. TPA-Label Listed Domain, TPA-LLD
        2.2.4. Author's Domain Acceptable Third-Party Signature
        2.2.5. Author's Domain Acceptable Third-Party Service
3. Combinatorial ADSP "dkim" tag Values.
    3.1. tpa-sig
    3.2. tpa-path
4. TPA-Label Resource Record Authorization Considerations
5. Evaluating the Third-party Signing Domain or Service
    5.1. Third Party Authentication
        5.1.1. Third Party Authentication - Web Email Provider with Subscriber Pingbacks
        5.1.2. Third Party Authentication - Closed Mailing List Example
        5.1.3. Third Party Authentication - Open Mailing List Example
        5.1.4. Third Party Authentication Example - Sender Header Field
        5.1.5. Services Lacking DKIM Signatures
6. DNS Representation
7. TPA-Label and Tag Syntax Definitions
8. TPA-Label Generation
9. TPA-Label TXT Resource Record Structure
10. TPA-Label Resource Record Definition
11. Outbound Extended Signing Practice
12. TPA-Label Resource Record Scope Syntax
    12.1. Authorized Signing Domain
    12.2. TPA-Label Listed Domain Authorization
        12.2.1. From (Author) Header Field
    12.3. Header Dependent Authorizations
        12.3.1. List-ID Header Field
        12.3.2. Sender Header Field
        12.3.3. Combined 'L' or 'S' Scopes
    12.4. SMTP Host domains
    12.5. SMTP Host domains
    12.6. MailFrom Parameter
    12.7. SMTP Host domains
13. TPA-Label Resource Record Query Transactions
14. TPA-Label Resource Record Compliance Assessment
15. IANA Considerations
    15.1. Author Domain Signing Practices (ADSP) Parameters
    15.2. Email Authentication Method Registry
    15.3. Email Authentication Result Names Registry
    15.4. Third Party Authorizations Labels Registry
    15.5. Third Party Authorizations Scope Registry
16. Security Considerations
    16.1. Benefits to Recipients
    16.2. Risks to Recipients
    16.3. Benefits to Author Domains
    16.4. Risks to Author Domains
    16.5. Benefits to Third Party Signers
    16.6. Risks caused by Third Party Signers
    16.7. SHA-1 Collisions
    16.8. DNS Limits
17. Acknowledgements
18. References
    18.1. Normative References
    18.2. Informative References
Appendix A. DNS Example of TPA-Label Resource Record placement
Appendix B. C code for label generation
§ Authors' Addresses

1. Introduction

A transparent method for DKIM authorization requires the sharing of a number of details between the domain owner, and one or more providers of email and DNS. Transparent authorization occurs when an Author Domain's DNS resolves a public DKIM key, with the corresponding private key being held by a third-party. With the many ways in which this might be achieved, it is unlikely that standardized formats will be developed to exchange necessary, and at times, sensitive information between various informal third-party services, such as mailing-lists. With transparent authorizations, when a security breach does occur, the wrong party might be held accountable for content it never saw nor logged. For informal authorizations, the TPA-Label Resource Record supports a simple authorization method that keeps visible which administrative entity signed a message, and whether an Author Domain authorized the signature. The authorization record may also impose additional header field requirements to better ensure protective message sorting.

Tens of thousands of domains of various financial institutions are frequently being phished. Phishing creates a nuisance for those who aren't expecting these messages, and a threat for those who then interact with them. Whenever institutions employ DKIM and wish to utilize various informal third-party services, the integrity of their Author Domain Signatures might be affected. Some respond to this by implementing less stringent Author Domain Signing Practices on subdomains to accommodate the impact of these third-party services, as suggested by [I‑D.ietf‑dkim‑mailinglists] (Kucherawy, M., “DKIM And Mailing Lists,” July 2010.) Section 4.1, that recommends use of subdomains to assert less restrictive ADSP practices.

As currently structured, ADSP does not offer an alternative to using more domains, where only some are protected by a restrictive practice. As such, it lacks a method to retain an authentication & authorization as an acceptance condition when using informal third party services. Loss of authentication will lead to an increase of those being deceived by phishing attempts. This is because people often do not understand the significance of URI hierarchy, and become confused or insensitive to domain changes. APWG phishing trends, [apwg‑globalphishingsurvey‑2H2009] (Anti-Phishing Working Group, “Global Phishing Survey: Trends and Domain Name Use 2H2009,” May 2009.) page 18, indicates phishing commonly uses subdomains in a URL to fool potential victims.

Deterrents based upon reputation and/or path based strategies that utilize a variety of originating header fields are ineffective. These header fields often remain invisible to recipients, and contain domains being exploited for periods measured in hours, in an avoidance of any Whack-A-Mole like response. Even long term reputations are increasingly problematic due to an intermix of messages from compromised accounts. Few recipients will inspect the stack of message header fields, or be able to draw useful conclusions from a profusion of unfriendly information. However, many recipients deal with abuse by sorting messages into groups based upon an assumed source found in a few originating header fields.

ADSP represents an open registry that offers domain specific guidance for DKIM acceptance criteria when determining whether messages should be delivered, refused, or discarded. However, appropriate actions become unclear whenever third-party services are involved. For example, it is not clear whether ADSP "dkim=all" assertions include third-party services that could potentially damage Author-Domain signatures. Although ADSP warns of a potential for disruption, specific handling recommendations are limited to "dkim=discardable". Administrative domains that assert all of their outbound messages are signed offer significant forensic value. However, the handling for their messages lacking an Author Domain Signature with an ADSP "dkim=all" assertion remains unclear.

2. Language and Terminology

2.1. Terms Imported from other DKIM Specifications:

2.2. Terms Defined by this Specification:

2.2.1. Third Party Signature

A "Third Party Signature" is a Valid Signature that does not qualify as an Author Domain Signature.

2.2.2. Third Party Signer

A "Third Party Signer" is a signer that adds a valid DKIM signature that references a domain with the 'd=' tag in the DKIM-Signature header field that is not the Author Domain.

2.2.3. TPA-Label Listed Domain, TPA-LLD

TPA-Label Listed Domain, TPA-LLD, is a TXT resource record referenced with a TPA-Label published within an Author Domain. When a "tpa" tag exists within the TXT resource record located at the TPA-Label, the referencing domain (the domain used to generate the label) must be within the listed domain. When the "tpa" tag does not exist, the referenced domain is presumed listed. The "scope" tag may stipulate existence of additional header fields, or indicate alternate confirmation methods applied against specific email elements. Third-party domain confirmation might use a DKIM signature or confirm the authorized domain using specific methods with various path related email elements. 'F' is the default confirmation scope assumed, indicating DKIM confirms the authorized domain when no other method is specified. The 'S' and 'L' scope do not confirm the domain, but require at least one Sender or List-ID header field to hold the identity of the authorized domain respectively. The 'e', 'h', 'm', and 't' indicate specific alternative methods using message elements to confirm the authorized domain. Being compliant with TPA-LLD allows the referencing domain to informally act on behalf of the Author Domain. Following [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.), domain name comparisons, as well as TPA-Labels, are case insensitive.

2.2.4. Author's Domain Acceptable Third-Party Signature

An "Author's Domain Acceptable Third-Party Signature" is a Valid Signature in which the domain name of the DKIM signing entity, i.e., the 'd=' tag in the DKIM-Signature header field, is the domain name referenced in the TPA-Label Resource Record published by the Author Domain with a scope of 'F'. The 'F' scope indicates this third-party domain implements DKIM signing. For 'S' and 'L' scopes, the respective Sender header field or a List-ID identifier of the List-ID header field must exist for either scope and contain a domain within the TPA-LLD for authorization to be valid.

2.2.5. Author's Domain Acceptable Third-Party Service

An "Author's Domain Acceptable Third-Party Service" is a service that confirms the administrative domain through the message element determined by the method specified by the scope of 'e', 'h', 'm', or 't'. For 'S' or 'L' scopes, the respective Sender header field or a List-ID identifier of the List-ID header field must exist for either scope and contain a domain within the TPA-LLD for authorization to be valid.

3. Combinatorial ADSP "dkim" tag Values.

This document defines new values listed with the ADSP "dkim" tag in addition to those defined in [RFC5617] (Allman, E., Fenton, J., Delany, M., and J. Levine, “DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP),” August 2009.) Section 4.2.1. These values can be appended to those currently defined or used separately. When used separately, the value "all" is to be assumed to prefix the new values when recognized, otherwise the value "unknown" will be assumed. It is not recommended to use any new value in conjunction with "discardable", because when not understood, a message that depends upon different handling might become lost.

3.1. tpa-sig

The ADSP dkim tag value "tpa-sig" indicates that TPA-Labels will offer a comprehensive list of Author's Domain Acceptable Third-Party Signatures that may also include header field requirements. When there is no valid Author Domain Signature or Author's Domain Acceptable Third-Party Signature, the Author Domain recommends these messages be refused.

3.2. tpa-path

The ADSP dkim tag value "tpa-path" indicates that TPA-Labels will offer a comprehensive list of Author's Domain Acceptable Third-Party Signatures and Authorized Third-Party Services that may also include header field requirements.

The "tpa-path" allows alternative path verification methods, to accommodate third party services lacking DKIM signatures. The message element is determined by the method specified by the scope. When the path domain is confirmed by the specified method, and the message is compliant with TPA-LLD requirements, then it is also compliant with the Author Domain's asserted signing practices. The leaf of the hostname (left most label) may need to be omitted when checking for TPA-Label Resource Record authorization.

When there is no valid Author Domain Signature, or Author's Domain Acceptable Third-Party Signature, or Author's Domain Acceptable Third-Party Service, the Author Domain recommends these messages be refused.

ADSP defends domains against spoofing. Any subdomain of a domain publishing an ADSP with the "dkim" tag value containing "tpa-sig" or "tpa-path" not also publishing an MX resource record should be assumed to have published the same ADSP records there as well.

4. TPA-Label Resource Record Authorization Considerations

When an Author Domain is not within the DKIM signing domain, the TPA-LLD scheme can extend ADSP signing practice compliance. The TPA-LLD scheme with an 'F', 'S', or 'L' scope permits a contained Third Party Signature to be treated as an Author Domain Signature. The 'e', 'h', and 't' scopes permit acceptance based upon confirmation of the client hostname (EHLO/HELO). The 'm' scope permits confirmations based upon the return path (Mail From) domain. For 'S' and 'L' scopes, a respective Sender header field or a List-ID identifier of the List-ID header field must exist for at least one of the scopes, and contain a domain within the TPA-LLD for authorization to be valid.

The 'S' and 'L' scopes support message sorting. Any matching header field with a domain within the TPA-LLD allows recipients to differentiate sources, which satisfies requirements for any other 'S' or 'L' scope. The 'S' and 'L' scopes provide Author Domains a means to extend restrictive policy compliance.

The TPA-LLD scheme plays the role of only providing acceptable signatures or services which might be suitable for non-critical messages, with the goal of improving delivery acceptance, such as those from specific mailing-lists. The TPA-LLD authorization scheme only requires that DNS publications be made by the Author Domain, even when signing domains and the Author Domain differ. This approach avoids a need to exchange DKIM key related information and better protects private key information. Before TPA-LLD authorization is deployed, the Author Domain should be assured by domains being authorized that appropriate measures are in place to authenticate those who are submitting messages.

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

At this time, it is not practical for large ISPs to make restrictive ADSP assertions. This would impair the utility of their service by excluding all third-party services lacking suitable source authentication. However, institutions can benefit by making restrictive ADSP assertions that better retain recipient trust in their email notifications, without forgoing use of all informal third-party services. Rather than using subdomains lacking ADSP restrictions, suitable third-party services can be authorized by using TPA-Labels instead. This approach also offers a proactive means for recipients to filter phishing attempts.

TPA-Label authorization will not ensure all possible spoofing is prevented. However, by permitting broader use of restrictive practices, this should generally reduce the level of spoofing over what might be otherwise allowed. Authorized third party messages SHOULD NOT receive annotations that indicate the message contains authenticated identities. The TPA-LLD scope SHOULD include the 'S' or 'L' scope where appropriate to allow recipients a means to isolate and distinguish different message sources.

The "dkim" tag within the TPA-Label Resource Record is normally expected to contain a copy of the value asserted by the ADSP Resource Record "dkim" tag. When the TPA-Label Resource Record "dkim" tag value differs, and the message is compliant with the "scope" and "tpa" tag values, the TPA-Label Resource Record "dkim" tag value overrides the ADSP Resource Record "dkim" tag value. When the TPA-Label Resource Record "dkim" tag value contains "discardable", compliance with the "scope" tag values is not required to override the ADSP Resource Record. Use of "tpa-path" SHOULD selectively override the ADSP "tpa-sig" only where needed.

5. Evaluating the Third-party Signing Domain or Service

An Author Domain deploying a TPA-Label Resource Record does so on a trust basis. Reasons for deploying TPA-Label Resource Records might be to allow deployment of more stringent ADSP records while also utilizing third-party signatures or services.

5.1. Third Party Authentication

The Author Domain SHOULD ensure the Authorization Scope of the TPA-Label Resource Record is authenticated. There are a number of ways email can be authenticated, and different authentication mechanisms validate different parts of the email. The following are examples of how authorization might work.

5.1.1. Third Party Authentication - Web Email Provider with Subscriber Pingbacks

The Author Domain "example.com" wants to deploy a TPA-Label Resource Record to permit its traveling agents the use of "webmail.example.net" services. This email provider has a closed user policy and adds DKIM signatures to messages on behalf of the "webmail.example.net" domain.

The closed user policy of "webmail.example.net" permits subscribers to post messages with Author Domains that are not "webmail.example.net" in the From header fields, only when control of the Author Address has been validated by a response to an encoded "pingback" email. The "webmail.example.net" service also establishes accounts to authenticate all users sending messages through its service. Therefore, the referenced TPA-Label Resource Record can include an 'F' scope value to authorize Author Domain messages signed by this Third-Party Signer.

5.1.2. Third Party Authentication - Closed Mailing List Example

The Author Domain wants to deploy a TPA-Label Resource Record for a mailing list with a closed posting policy that redistributes email in a way that breaks Author Domain Signatures, but adds a DKIM signature on behalf of the mailing list domain and includes an Authentication-Results header field for posted messages. The closed posting policy is enforced by requiring subscribers to validate their control of their Author Addresses by responding to encoded "pingback" email sent to these addresses.

Since the mailing list management always verifies control of the Author Address, and is configured to include Authentication-Results header fields, and includes a List-ID header field, the referenced TPA-Label Resource Record can include an 'L' scope value to permit Author Domain messages containing an authorized List-ID domain to be signed by this Third-Party Signer.

5.1.3. Third Party Authentication - Open Mailing List Example

The Author Domain wants to deploy a TPA-Label Resource Record for a mailing list with an open posting policy that redistributes email in a way that breaks Author Domain Signatures, but that adds a DKIM signature on behalf of the mailing list domain and includes an Authentication-Results header field for posted messages. The open posting policy will refuse messages lacking Author Domain Signatures for domains that have deployed an ADSP signing practice of "dkim=all" or "dkim=discardable".

Since the list management always refuses to post an Author Address lacking a Author Domain Signature when the domain has deployed an ADSP record with an "dkim=all" or "dkim=discardable", and is configured to include Authentication-Results header fields, and includes a List-ID header field, the referenced TPA-Label Resource Record can include an 'L' scope value to permit Author Domain messages containing an authorized List-ID domain to be signed by this Third-Party Signer.

5.1.4. Third Party Authentication Example - Sender Header Field

Author Domain "example.com" wishes to temporarily employ the service agency "temp.example.org" to handle overflow secretarial support. The agency "temp.example.org" sends email on behalf of the executive staff of "example.com" and adds the Sender header field of "secretary@temp.example.org" in the email. Since "temp.example.org" only allows its own staff to email through its server which adds "temp.example.org" DKIM signatures, a TPA-LLD can include the "temp.example.org" domain with an 'S' scope to specifically authorize messages containing the Sender header field to help ensure these messages are not handled as phishing attempts.

5.1.5. Services Lacking DKIM Signatures

5.1.5.1. Abuse and DSN Reporting

There is likely little interest for an otherwise uninvolved domain to receive a massive number of bogus messages being returned as feedback. Often the purpose of feedback is to discover compromised systems or accounts actively being exploited in some manner. Unless the Author Domain is confirmed as having handled or authorized the handling of the message, only statistics and samples should be reported to the associated Autonomous System [RFC1930] (Hawkinson, J. and T. Bates, “Guidelines for creation, selection, and registration of an Autonomous System (AS),” March 1996.), and perhaps to the Author Domain when interest is expressed.

The 'e', 'h', 'm', and 't' scope options within the TPA-LLD records allow the Author Domain to be associated through various methods with path related domains, when DKIM is not employed by the third-party service. In this case, appropriate DSN or abuse reporting to the Author Domain is better assured as well.

5.1.5.2. Third Party Authentication Example - SMTP Host

Author Domain "example.com" makes use of invite services. This service does not utilize DKIM, where the host name given by the EHLO command is "invite.example.net". The Author Domain can authorize the domain "invite.example.net" or "example.net" with the scope of 'e' to improve acceptance of messages that are sent on behalf of "example.com" from this outbound server.

5.1.5.3. Third Party Authentication Example - Return Path

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

5.1.5.4. Use of Path Authorization

Those using the "tpa-path" value should not authorize domains requiring more than 5 network transactions to confirm their domain. Those implementing this ADSP extension should also limit the number of DNS transactions attempted, or this could negatively impact unrelated domains when evaluating path related protocols.

6. DNS Representation

An Author Domain may wish to delegate the listing of third-party services to a different administrative domain. Ideally, this would be accomplished by delegating the _tpa._domainkey.<Author-Domain> zone to the administrative entity handling publication of TPA-Label Resource Records. This delegation could also be done unilaterally with a [RFC2672] (Crawford, M., “Non-Terminal DNS Name Redirection,” August 1999.) DNAME resource record published at _tpa._domainkey.<Author-Domain>.

7. TPA-Label and Tag Syntax Definitions

8. TPA-Label Generation

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

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

9. TPA-Label TXT Resource Record Structure

10. TPA-Label Resource Record Definition

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

11. Outbound Extended Signing Practice

Outbound Signing Practice Extensions. This tag defines a list of extended outbound signing practices that combine with those for various email-address locations within the message. Only recognized scope values offer any form of ADSP authorization.

12. TPA-Label Resource Record Scope Syntax

Authorization Scope List (Optional). This tag defines a list of scoping assertions for various email-address locations within the message. Only recognized scope values offer any form of ADSP authorization.

12.1. Authorized Signing Domain

Authorized Signing Domain list. (optional) This tag, when present, MUST repeat all or portions of the domain encoded within the TPA-Label Resource Record. This option ensures the proper handling of possible hash collisions. When a domain is prefixed with the "*." ANY label, then all subdomains of this domain are to be considered included within the list. When the 'tpa' tag is not present or has no value, it should be assumed to compare with the domain used to generate the TPA-Label.

12.2. TPA-Label Listed Domain Authorization

12.2.1. From (Author) Header Field

Tag	Function
dkim	Outbound Extended Signing Practices (dkim-tag)
scope	Authorization Scope List (scope-tag)
tpa	Authorized Domains List (tpa-tag)

dkim values	Field or Parameter
tpa-sig	Third-Party Signature
tpa-path	Third-Party Path or Signature
all tpa-sig	Third-Party Signature
discardable	Selective discardable

scope values	Field or Parameter	Method
F	From (Author) Header Field	Match Address Domain
L	List-ID Header Field	Match List-ID Identifier
S	Sender Header Field	Match Address Domain
e	SMTP Hostname	Resolve Hostname IP Addr
h	SMTP Hostname	Pass SPF with Hostname
m	MailFrom	Pass SPF with MailFrom
t	SMTP Hostname	Cert of Hostname

The "F" scope asserts that messages carrying the Author Domain within the From header field are authorized to be signed by the TPA-LLD.

12.3. Header Dependent Authorizations

12.3.1. List-ID Header Field

12.3.2. Sender Header Field

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

12.3.3. Combined 'L' or 'S' Scopes

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

12.4. SMTP Host domains

12.5. SMTP Host domains

12.6. MailFrom Parameter

12.7. SMTP Host domains

13. TPA-Label Resource Record Query Transactions

The discovery of TPA-Label resource records need not be subsequent to the discovery of the ADSP record specified by [RFC5617] (Allman, E., Fenton, J., Delany, M., and J. Levine, “DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP),” August 2009.). However, when no ADSP record is discovered or when it does not contain a "dkim" tag value of either "tpa-sig" or "tpa-path", the verifier MAY assume that no TPA-Label Resource Records have been published. Otherwise, when there is a Third Party Signature without any Author Domain Signature, the discovery of TPA-Label Resource Records should be attempted.

14. TPA-Label Resource Record Compliance Assessment

The signing practice compliance assessment of Third Party Signatures is a discretionary operation performed by the verifier. Messages that have valid Author Domain Signatures are already considered to result in a pass. When a verifier decides to assess compliance for Third Party Signatures with an Author Domain ADSP "dkim" tag value "tpa-sig" or "tpa-path", then, checked in succession, one of the following sets of conditions MUST be met for the result to be considered a pass.

For Third Party Signatures, the following represents the set of "tpa-sig" assessment conditions to be checked:

Meeting all the conditions in this set results in a "tpa-sig" pass, where subsequent checks are then skipped.

For Third Party Services where the Author Domain ADSP "dkim" tag value contains "tpa-path", and where the preceding assessment conditions were not met, then the following represents "tpa-path" assessment conditions to be checked:

The TXT record discovery process continues until a TPA-Label TXT Resource Record structure is found where:

Meeting all four conditions in this set, and confirming the domain, results in a "tpa-path" pass.

When the TPA-Label TXT Resource Record retrieval returns a DNS "NOERROR", but not with a single record, the result of the TPA-Label Resource Record compliance assessment is "permerror".

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

15. IANA Considerations

15.1. Author Domain Signing Practices (ADSP) Parameters

15.2. Email Authentication Method Registry

15.3. Email Authentication Result Names Registry

15.4. Third Party Authorizations Labels Registry

15.5. Third Party Authorizations Scope Registry

16. Security Considerations

16.1. Benefits to Recipients

The verifier, after finding either an Author's Domain Acceptable Third-Party Signature or Author's Domain Acceptable Third-Party Service in a message, will have significantly greater confidence in the Third-Party, than when no TPA-Label Resource Record is obtained. This enhanced confidence may, at the recipients' discretion, cause a message to be delivered to the recipient without further source related assessment.

16.2. Risks to Recipients

The decisions a recipient makes in regard to message filtering based on TPA-Label Resource Records are likely to depend on the system integrity of the Third Party with respect to the Authentication (see Section 5.1 (Third Party Authentication)) and the provided scope labels. When the 'e', 'h', or 'm' scoped domain is not confirmed or the third-party domain does not authenticate the sender, there is a risk of accepting potentially spoofed messages. When there is no out-of-band authentication confirming the sender, Authentication-Results header fields then play an important role. Without proper Authentication-Results handling by the third-party, there is also risk of accepting potentially spoofed messages.

With this specification, third party signatures have verifiable value. When implementing the compliance assessment of third party signatures and TPA-Label Resource Records, implementers need to consider the possibility that a Bad Actor will send the recipient a message with a large number of valid DKIM Signatures. Verifying all of these may consume a large amount of processing resources such that it may be worth checking the existence of a TPA-Label Resource Record first. Section 13 (TPA-Label Resource Record Query Transactions) describes a quick check to see if TPA-Label Resource Records may exist. Additionally validating DKIM signatures and obtaining related resource records might be limited to known trustworthy domains.

Services that depend only upon path authorizations might permit the Author Domain to be spoofed and yet obtain acceptance. During such events, the Author Domain might need to retract its authorization from the service. For this reason, "tpa-path" authorization should only be used as a carefully monitored interim solution.

16.3. Benefits to Author Domains

TPA-Label resource records can replace domain delegations, selector/key record mirroring, or key exchanges. A significant number of details are associated with selector/key records. These details include user limitations, suitable services, key resource record's Time-To-Live, revocation and update procedures, and how the DKIM Signature header field's 'i=' semantics are to be applied. In addition, services that depend upon DKIM keys are better secured by not delegating these DKIM keys, where instead the TPA-LLD scheme allows Author Domains an ability to limit the scope of their authorizations, while also not being mistaken for having authenticated the entity submitting the message.

TPA-Label Resource Records convey which domains are authoritative even when they are not the Author Domain. However, authorized domains are unable to utilize the DKIM signature's 'i=' semantics to directly assert which identifiers on whose behalf a signature was added. As such, no domain should be authorized unless it is trusted to ensure the Alleged Author of an email undergoes authentication that offers acceptable protections for the Author Domain. For example, such authentication might ensure submitting entities have demonstrated receipt of "pingback" messages sent to the Author Address contained within the messages being signed.

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

Broader use of restrictive ADSP policies provides a better likelihood of being able to eliminate a greater range of non-compliant messages, in addition to improving acceptance from authorized sources. With authorization, scope labels allow the Author Domain to control message attributes even from the authorized third parties.

Signing domains having good reputations referenced by a TPA-LLD might therefore provide a means to safely extend limited compliance assessment resources to otherwise unknown domains or SMTP Clients.

16.4. Risks to Author Domains

As indicated in Section 5 (Evaluating the Third-party Signing Domain or Service), there is ultimately a trust of the third party domain to do the right thing and not generate, or allow others to generate, messages that falsely appear to be from the Author Domain. The authentication methods in place for different email elements need to be carefully reflected in the scope of the TPA records.

By authorizing mailing lists with TPA-Label Resource Records, this could cause a loss of confidentiality in mailing list participation by the Author Domain. This might help Bad Actors deduce which subscription related email the Author Domain may receive. Because of the hashing function in generating the TPA-label, anyone wishing to discover which domains are being authorized, has to probe each TPA-label based on the exact signing domain. In addition, service organizations or community groups are able to share comprehensive lists which means, even though the domain has been authorized, that in itself does not mean the Author Domain is exchanging messages with the authorized domain.

16.5. Benefits to Third Party Signers

Third Party Signers benefit by allowing those using their service, the autonomy to authorize their service without needing to exchange DKIM key related details. This is particularly useful for mailing lists.

16.6. Risks caused by Third Party Signers

As mentioned before, Third Party Signers need to authenticate messages from Author Domains. This authentication provides a safety mechanism for the Author Domain and their recipients. The Third Party may not be aware of the authentication value or the message elements involved and make changes without understanding the impact this may have upon targeted Author Domains and their recipients. For example, the Third Party might stop DKIM signing or stop applying Authentication-Results header fields. The unexpected exposure might enable wide spread abuse and prove detrimental for both the Author Domain and their recipients.

16.7. SHA-1 Collisions

The use of the SHA-1 hash algorithm does not represent a security concern. The hash simply ensures a deterministic domain-name size is achieved. Unexpected collisions can be detected and handled by using the extended TPA-Label Resource Record "tpa=" option. The use of TPA-Label Resource Records without the TPA-Label "tpa=" options does present an opportunity for an adversary to attempt to find a hash collision. Message spoofing outside the realm of DKIM protection is likely easier to achieve than finding hash collisions. There is minimal risk of TPA-Labels colliding. Listing 3 x 10^45 domains has less than a 0.1 percent risk of any two domain labels colliding.

16.8. DNS Limits

Use of the TPA-Label Resource Records, rather than simply listing the authorized domain, ensures the DNS record size is independent of the Third Party Domain. The typical domain name size has been steadily increasing. This increase has been caused by domain names that encode international character sets. Perhaps soon there will be a further increase spurred by an expanse of TLDs having larger international labels.

The maximum domain name size allowed, per [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) Section 3, is 255 bytes (or octets). Each label has a byte for its length. Every domain name has a right most label representing the root with a zero length, for another byte. A scheme that concatenates a listed domain with the publishing domain, separated by some conventional label, reduces the maximal domain name in half, where the conventional label reduces this further.

If "_tpa." were used as the conventional label with a simple listing method, the maximum domain name size this supports would be 122 bytes. The suffix for TPA-Labels is "_tpa.domainkey." which consumes 16 bytes. The TPA-Label itself consumes 34 bytes. A domain that publishes the TPA-labels in its domain would then have 205 bytes available for their Author Domain. Since an Author's Domain Acceptable Third-Party Service might not implement DKIM, the TPA-Label is still able to authorize any domain name with a valid length. As a result, the maximum allowable Author Domain is increased by 83 bytes or 68% over simple name concatenation.

Normally, DNS messages should not exceed 512 bytes as per Section 2.3.4 of [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.). Using TPA-Label Resource Records in the DNS, as described by this document, consumes a consistent 50 bytes, in addition to the domain name publishing the TPA-Labels. With this being constant, a limit can be determined as a constraint to resource record size, to ensure a response does not exceed the maximum DNS message size. DNS servers that add additional resource records, for nameservers as an example, will further reduce available resource record capacity. Domains publishing TPA-Labels exceeding the DNS message limit will need to rely on recipients using TCP for DNS retrieval, or EDNS0 [RFC2671] (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) for extended DNS lengths.

17. Acknowledgements

Jeff MacDonald, Michael Deutschmann, Frank Ellermann, Murray Kucherawy, and Wietse Venema.

18. References

18.1. Normative References

18.2. Informative References

Appendix A. DNS Example of TPA-Label Resource Record placement

Appendix B. C code for label generation

/*
 * TPA-Label generation utility
 * Copyright (c) 2010 IETF Trust and the persons identified as the
 * document authors.  All rights reserved.
 *
 * This document is subject to BCP 78 and the IETF Trust's Legal
 * Provisions Relating to IETF Documents
 * (http://trustee.ietf.org/license-info) in effect on the date of
 * publication of this document.  Please review these documents
 * carefully, as they describe your rights and restrictions with respect
 * to this document.  Code Components extracted from this document must
 * include Simplified BSD License text as described in Section 4.e of
 * the Trust Legal Provisions and are provided without warranty as
 * described in the Simplified BSD License.
 *
 * This document and the information contained herein are provided on an
 * "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 * OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
 * THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
 * THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 * WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
 */

#include <stdio.h>
#include <sys/types.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <openssl/sha.h>

#define TPA_LABEL_VERSION   102
#define MAX_DOMAIN_NAME     256
#define MAX_FILE_NAME       1024

static char base32[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
static char sign_on[] =
{"%s v%d.%02d Copyright (C) (2010)  The IETF Trust, Otis & Black\n"};
char err_cmd[] =\
 "ERR: Command error with [%s]\n";
char use_txt[]=\
 "Usage: tpa-label [-i domain_input_file] [-o label_output_file][-v]\n";
char help_txt[]=\
 "The options are as follows:\n"\
 "-i  domain name input. Defaults to stdin. Removes trailing '.'\n"\
 "-o  TPA-Label output.  Defaults to stdout.\n"\
 "-v  Specifies Verbose Mode.\n\n";

static void usage(void);
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

static void
usage(void)
{
    (void) fprintf(stderr, "\n%s%s", use_txt, help_txt);
    exit(1);
}
/*- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

int
main (int argc, char * argv[])
{
    int  ret_val, in_mode, out_mode, verbose, done, i, j, k;
    char ch;
    unsigned int len;
    unsigned long long b_5;
    char in_fn[MAX_FILE_NAME], out_fn[MAX_FILE_NAME];
    unsigned char in_buf[MAX_DOMAIN_NAME + 2];
    unsigned char sha_res[20], tpa_label[33];
    FILE *in_file, *out_file;

    ret_val = in_mode = out_mode = verbose = done = 0;
    len = 0;

    while ((ch = getopt(argc, argv, "i:o:v")) != -1)
    {
        switch (ch)
        {
            case 'i':
                in_mode = 1;          /* input from file */
                (void) strncpy(in_fn, optarg, sizeof(in_fn));
                in_fn[sizeof(in_fn) - 1] = '\0';
                break;
            case 'o':
                out_mode = 1;         /* out to file */
                (void) strncpy(out_fn, optarg, sizeof(out_fn));
                out_fn[sizeof(out_fn) - 1] = '\0';
                break;
            case 'v':
                verbose = 1;
                break;
            case '?':
            default:
                (void) usage();
                break;
        }
    };

    if (in_mode)
    {
        if ((in_file = fopen(in_fn, "r")) == NULL)
        {
            (void) fprintf(stderr,
                           "ERR: Error opening [%s] input file.\n",
                           in_fn);
            exit(2);
        }
    }
    else
    {
        in_file = stdin;
    }

    if (out_mode)
    {
        if ((out_file = fopen(out_fn, "w")) == NULL)
        {
            (void) fprintf(stderr,
                           "ERR: Error opening [%s] output file.\n",
                           out_fn);
            exit(3);
        }
    }
    else
    {
        out_file = stdout;
    }

    if (out_mode && verbose)
    {
        (void) printf(sign_on, "tpa-label utility",
                      TPA_LABEL_VERSION / 100,
                      TPA_LABEL_VERSION % 100);
    }

    for (i = 0; i < MAX_DOMAIN_NAME && !done; i++)
    {
        if ((ch = fgetc(in_file)) == EOF)
        {
            ch = 0;
        }
        else  if (ch == '\n' || ch == '\r')
        {
            ch = 0;
        }

        in_buf[i] = tolower(ch);

        if (ch == 0)
        {
            len = i;         /* string length */
            done = 1;
        }
    }

    if (!done)
    {
        (void) fprintf(stderr, "ERR: Domain name too long.\n");
        exit (4);
    }

    if (len && in_buf[len - 1] == '.')    /* remove any trailing "." */
    {
        len--;
        in_buf[len] = 0;     /* replace trailing "." with 0 */
    }

    in_buf[len] = 0;         /* terminate string */

    if (len < 2)
    {
        (void)
        fprintf(stderr,
                "ERR: Domain name [%s] too short with %d length.\n",
                in_buf,
                len);
        exit (5);
    }

    SHA1(in_buf, len, sha_res);

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

        for (i = 0; i < 20; i++)
        {
            printf("%02x", sha_res[i]);
        }
        printf("\nTPA-Label: 5 bit intervals left to right.\n");
    }

    /* process sha1 results 4 times by 40 bits (160 bits) */
    for (i = 0, j = 0; i < 4 ; i++)
    {
        b_5 =  (unsigned long long) sha_res[(i * 5)] << 32;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 1] << 24;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 2] << 16;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 3] << 8;
        b_5 |= (unsigned long long) sha_res[(i * 5) + 4];

        if (verbose)
        {
            printf(" {%010llX}->", b_5);
        }

        for (k = 35; k >= 0; k-= 5, j++)    /* convert 40 bits (5x8) */
        {
            tpa_label[j] = base32[(b_5 >> k) & 0x1F];

            if (verbose)
            {
                printf(" %02X:%c",
                       (unsigned int)(b_5 >> k) & 0x1F,
                       tpa_label[j]);
            }
        }
        if (verbose)
        {
            printf ("\n");
        }
    }
    if (verbose)
    {
        printf("\n");
    }
    tpa_label[j] = 0;   /* terminate label string */
    fprintf(out_file, "_%s", tpa_label);
    printf("\n");

    /* close */
    if (out_mode)
    {
        if (fclose (out_file) != 0)
        {
            (void) fprintf(stderr,
                           "ERR: Unable to close %s output file.\n",
                           out_fn);
            ret_val = 6;
        }
    }
    if (in_mode)
    {
        if (fclose (in_file) != 0)
        {
            (void) fprintf(stderr,
                           "ERR: Unable to close %s input file.\n",
                           in_fn);
            ret_val = 7;
        }
    }
    return (ret_val);
 }

Method	Defined	ptype	property	value
tpa-lld	(this document)	header field	d	value of signature "d" tag. The dkim method results from [RFC5451] (Kucherawy, M., “Message Header Field for Indicating Message Authentication Status,” April 2009.) should also be included in a Authenticated Results header field
			scope	value of scope (Third Party Authorizations Scope Registry) tag. (When 'scope' contains 'e', 'h' or 'm', the iprev (Kucherawy, M., “Message Header Field for Indicating Message Authentication Status,” April 2009.) [RFC5451] (Section 3) method results should also be included in the Authenticated-Results header field to capture the SMTP client IP address.
			ca-scope	The scopes (Third Party Authorizations Scope Registry) with a compliance assessment as pass
			tpa	Value of tpa (Authorized Signing Domain) tag at time of compliance assessment

code	method	meaning
none	tpa-lld	No TPA-Label was published
pass	tpa-lld	Section 14 (TPA-Label Resource Record Compliance Assessment)
tempfail	tpa-lld	Section 14 (TPA-Label Resource Record Compliance Assessment)
permfail	tpa-lld	Section 14 (TPA-Label Resource Record Compliance Assessment)
unknown	tpa-lld	The TPA-Label Resource Record had a tag/value of "dkim=unknown" and the Third Party Signature failed its compliance assessment.
discard	tpa-lld	The TPA-Label Resource Record had a tag/value of dkim=discard and the Third Party Signature failed its compliance assessment.
fail	tpa-lld	The TPA-Label Resource Record had a tag/value of dkim=all and the Third Party Signature failed to its compliance assessment.
nxdomain	tpa-lld	When obtaining the TPA-Label Resource Record, DNS indicated this domain does not exist.
Other value defined in the IANA ADSP Outbound Signing Practices Registry	tpa-lld	The TPA-Label Resource Record had a tag/value of dkim={other value} and the Third Party Signature failed its compliance assessment.

tag	defined	definition
dkim	Section 9 (TPA-Label TXT Resource Record Structure)	As per IANA Registry ADSP Outbound Signing Practices
tpa-sig	Section 11 (Outbound Extended Signing Practice)	DKIM auth
tpa-path	Section 11 (Outbound Extended Signing Practice)	DKIM or path auth
all tpa-sig	Section 11 (Outbound Extended Signing Practice)	Same as tpa-sig
scope	Section 12 (TPA-Label Resource Record Scope Syntax)	Section 15.5 (Third Party Authorizations Scope Registry)
tpa	Section 12.1 (Authorized Signing Domain)	List of Authorized Domains or Identifiers

value	defined
F	Section 12.2 (TPA-Label Listed Domain Authorization)
L	Section 12.3.1 (List-ID Header Field)
S	Section 12.3.2 (Sender Header Field)
h	Section 12.5 (SMTP Host domains)
e	Section 12.4 (SMTP Host domains)
m	Section 12.6 (MailFrom Parameter)
t	Section 12.7 (SMTP Host domains)

[FIPS.180-2.2002]	National Institute of Standards and Technology, “Secure Hash Standard,” FIPS PUB 180-2, August 2002.
[I-D.saintandre-tls-server-id-check]	Saint-Andre, P. and J. Hodges, “Representation and Verification of Domain-Based Application Service Identity in Certificates Used with Transport Layer Security,” draft-saintandre-tls-server-id-check-09 (work in progress), August 2010 (TXT).
[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC2919]	Chandhok, R. and G. Wenger, “List-Id: A Structured Field and Namespace for the Identification of Mailing Lists,” RFC 2919, March 2001 (TXT).
[RFC3207]	Hoffman, P., “SMTP Service Extension for Secure SMTP over Transport Layer Security,” RFC 3207, February 2002 (TXT).
[RFC4408]	Wong, M. and W. Schlitt, “Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1,” RFC 4408, April 2006 (TXT).
[RFC4648]	Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” RFC 4648, October 2006 (TXT).
[RFC4871]	Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” RFC 4871, May 2007 (TXT).
[RFC5234]	Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008 (TXT).
[RFC5321]	Klensin, J., “Simple Mail Transfer Protocol,” RFC 5321, October 2008 (TXT).
[RFC5322]	Resnick, P., Ed., “Internet Message Format,” RFC 5322, October 2008 (TXT, HTML, XML).
[RFC5451]	Kucherawy, M., “Message Header Field for Indicating Message Authentication Status,” RFC 5451, April 2009 (TXT).
[RFC5617]	Allman, E., Fenton, J., Delany, M., and J. Levine, “DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP),” RFC 5617, August 2009 (TXT).

[I-D.ietf-dkim-mailinglists]	Kucherawy, M., “DKIM And Mailing Lists,” draft-ietf-dkim-mailinglists-01 (work in progress), July 2010 (TXT).
[RFC1034]	Mockapetris, P., “Domain names - concepts and facilities,” STD 13, RFC 1034, November 1987 (TXT).
[RFC1035]	Mockapetris, P., “Domain names - implementation and specification,” STD 13, RFC 1035, November 1987 (TXT).
[RFC1930]	Hawkinson, J. and T. Bates, “Guidelines for creation, selection, and registration of an Autonomous System (AS),” BCP 6, RFC 1930, March 1996 (TXT).
[RFC2671]	Vixie, P., “Extension Mechanisms for DNS (EDNS0),” RFC 2671, August 1999 (TXT).
[RFC2672]	Crawford, M., “Non-Terminal DNS Name Redirection,” RFC 2672, August 1999 (TXT).
[RFC4686]	Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” RFC 4686, September 2006 (TXT).
[RFC4954]	Siemborski, R. and A. Melnikov, “SMTP Service Extension for Authentication,” RFC 4954, July 2007 (TXT).
[RFC5016]	Thomas, M., “Requirements for a DomainKeys Identified Mail (DKIM) Signing Practices Protocol,” RFC 5016, October 2007 (TXT).
[RFC5226]	Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” BCP 26, RFC 5226, May 2008 (TXT).
[RFC5672]	Crocker, D., “RFC 4871 DomainKeys Identified Mail (DKIM) Signatures -- Update,” RFC 5672, August 2009 (TXT).
[RFC5863]	Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, “DomainKeys Identified Mail (DKIM) Development, Deployment, and Operations,” RFC 5863, May 2010 (TXT).
[apwg-globalphishingsurvey-2H2009]	Anti-Phishing Working Group, “Global Phishing Survey: Trends and Domain Name Use 2H2009,” May 2009.

	Douglas Otis
	Trend Micro
	10101 N. De Anza Blvd
	Cupertino, CA 95014
	USA
Phone:	+1.408.257-1500
Email:	doug_otis@trendmicro.com

	Daniel Black
	Canberra ACT
	Australia
Email:	daniel.subs@internode.on.net

DKIM Third-Party Authorization Labeldraft-otis-dkim-tpa-label-06

Abstract