DomainKeys Originator Recipient (DKOR)

Abstract

DKIM associates a domain name with a message stream, using cryptographic methods, to permit reliable and accurate reputation-oriented analysis of the stream. It is possible for an authorized user to conspire for additional distribution of a message, leveraging the domain name reputation for promoting spam. This is called DKIM Replay. DKOR defines a means of limiting that ability, by associating original addressing information with the message's DKIM signature, to detect distribution beyond the intended recipient. DKOR uses existing DKIM services and only requires implementation of the additional DKOR features by the signer and any receiving site wishing to participate in DKOR services. Other DKIM receivers can successfully process the same DKIM signature without knowledge of DKOR.¶

1. Introduction

DKIM [RFC6376] associates a domain name with a message stream, using cryptographic methods, to permit reliable and accurate reputation-oriented analysis of the stream. Each message in the stream has a DKIM digital signature attached to it, using the domain name. Receiving sites can then develop a reputation analysis for messages in that stream, without concern that a message was created by an unauthorized actor.¶

However, it is possible for an authorized user of a platform, which is signing messages with a domain name, to conspire for additional distribution of a message, beyond the set of recipients that were included at the time of original posting. The message goes through an additional platform, which then redistributes the message, while preserving the original DKIM signature. Reception and delivery of the message to these additional recipients is aided by the reputation of the domain name used in the DKIM signature. This capability can, therefore, be used for spamming, and is called "DKIM Replay".¶

Ideally, authorized users of a platform are subject to sufficient controls and accountability, so their likelihood -- or even ability -- to effect this abuse is severely limited. Unfortunately, operational realities on some platforms do not achieve such control. There is therefore a need for an additional mechanism, to aid detection of DKIM Replay and to facilitate differential handling of such messages, during email transit.¶

Extensive community discussion converged on the view that a useful line of effort, for enabling this detection, is to include the email envelope address (e.g., SMTP RCPT-TO) [RFC5321] as part of the DKIM hash calculation. There is also interest in similarly encoding the email envelope notification return address (e.g., SMTP MAIL FROM), with the view that this might be helpful in efforts to limit use of the return 'channel' as a spamming path, called backscatter.¶

2. Framework and Terminology

Internet Mail has an extensive architecture and rich associated vocabulary. Unless otherwise indicated, email terminology and reference to functional components are taken from: [RFC5598]¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Reference to SMTP in this document is strictly informative. Nothing in this specification is dependent upon the specific email transport service details, other than being able to obtain recipient and return address information.[RFC5598]¶

The means for obtaining recipient address and return address are outside the scope of this specification.¶

3. Requirements

Assessing the nature and legitimacy of a received message entails a complex and imperfect process. It typically includes a variety of information, derived from a variety of sources, using a variety of methods. This specification seeks to satisfy a narrow and specific part of these broad requirements. Other methods will be separate and complementary to this.¶

This specification assists a receiving system in detecting:¶

• Whether a message has been sent to an address that differs from one originally specified at the time of first posting¶
• What the original recipient and return addresses were¶
• What new recipient and return addresses are (or were, in the case of changes at multiple points along the transit path)¶
• What site(s) made the change(s)¶

4. DKOR Overview

Domain Keys Originator Recipient (DKOR) uses the existing DKIM mechanism, which develops a basic digital signature, over a specified set of message data. DKOR permits basic detection of message re-submission, upon implementation of DKOR by the signers and any receiving sites wishing to participate in DKOR services. DKOR's essential mechanism is inclusion of recipient and return addresses into the message object, by the originating site and by each re-submission point along the transit path.¶

A given message might be subject to multiple, legitimate re-submissions, such as through alias and mail list sites. As each of these adopts DKOR, the message will be able to provide additional information about the addressing changes it has been subject to. This enables receivers to make more accurate and reliable assessments about message handling. However, use of DKOR needs to accomodate incremental, uncoordinated, and piecemeal adoption. So for example, a message that does go through multiple, legitimate resubmissions might produce DKOR-based information from only some of the sites in the sequence.¶

Because DKOR relies on the existing DKIM mechanisms, other DKIM receivers that have not adopted DKOR can, nonetheless, process the same DKIM signature without knowledge of DKOR.¶

Hence, DKOR is an incremental and compatible enhancement to the existing DKIM email infrastructure.¶

5. DKOR Design

A message covered by DKOR is restricted to having a single recipient address.¶

DKOR defines a use of DKIM, with the DKIM signature covering whatever other header fields it is normally used to cover. For DKOR, it also covers one header field that specifies the original recipient address and the envelope notifications return address. These addresses are redundant with the addresses in their corresponding envelope fields/commands.¶

The presence of the DKOR header field self-declares the use of DKOR. Further there is no benefit in having the signer publish that it uses DKOR. Note that having a transit site simply remove a DKOR header field does not accomplish an effective downgrade attack, since it will cause the associated DKIM signature to fail.¶

The method for obtaining envelope information, to be used by DKOR, is a matter of local implementation and is outside the scope of this specification.¶

If the addresses in the DKOR field match those in their corresponding envelope fields/commands, and the DKIM signature validates, then DKOR passes. If either does not match, or the DKIM evaluation fails, then DKOR fails.¶

Policies and actions that apply to the handling of messages that pass or fail DKOR are local matters for the evaluating service and are outside the scope of this specification. ¶

An example of challenges in determining the handling of a DKOR failure is that the failure can be the result of entirely legitimate reasons, such as a legitimate redistribution service that does not make changes to message content and only uses a new recipient address. Failures can also occur due to the considerable vagaries of email handling idiosyncrasies.¶

Note:

The header field used by this specification is independent of any other header fields that might contain the same addresses, in order to avoid any potential confusion about semantics or purpose.¶

6. DKOR Header Field

The DKOR header field specifies a single recipient address and a single return address. It also indicates the order in which the field was created, in the DKOR handling sequence of the message. Since DKOR support cannot be assumed for every MTA and intermediary in the transit path, the DKOR order might not reflect all of the nodes the message transited.¶

At least one of mf or rt attributes MUST be specified.¶

7. Signing Behavior

1. If the recipient address (e.g., RFC5321.rcpt-to) is to be protected, obtain that envelope address and place a copy of it into the rt attribute of the DKOR header field. [RFC5322]¶
2. If the return address (e.g., RFC5321.mail-from) is to be protected, obtain that envelope) address and place a copy of it into the mf attribute of the DKOR header field.¶
3. DKOR permits either or both attributes to be present, and requires at least one.¶
4. Create a DKIM signature that includes the DKOR header field.¶

8. Evaluating Behavior

1. If a DKOR header field is present, then perform a DKOR evaluation.¶
2. Obtain the envelope address values used by DKOR.¶
3. Compare the values to the corresponding DKOR header field attribute values.¶
4. If either value does not match, then DKOR fails.¶
5. Perform a DKIM validation that includes the DKOR header field.¶
6. If DKIM validation fails, then DKOR fails.¶

11. References