]> Unobtrusive End-to-End Email Signatures PGPKeys.EU
andrewg@andrewg.com
ACLU
dkg@fifthhorseman.net
Thunderbird
kaie@thunderbird.net
int none Internet-Draft This document deals with end-to-end cryptographically signed email. It introduces a novel structure for signed email that is designed to avoid creating any disturbance in legacy email clients. This "unobtrusive" signature structure removes disincentives for signing email. The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the MAILMAINT Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .
Introduction Several different standard structures for end-to-end cryptographically signed email exist (see Sections , and of ). But the existing mechanisms have some undesirable properties which can make such mail difficult for the recipient to handle in some instances, particularly when read by legacy email clients that don't understand the signing structure. This document offers another signed email structure, which is designed to be transparent to legacy email clients. The goal of this mechanism is to help email clients commit to signing every outbound message, which reduces complexity for the user of the mail client. The mechanism is capable of working with any signature mechanism, as well as transporting multiple signatures over a single message. It is specified initially for and , but can be extended to be used with other signature formats. This mechanism is intended only for signed-only messages. A message that is encrypted-and-signed MUST NOT use this mechanism, since any existing MUA that can decrypt an encrypted-and-signed message already handles the signatures on such a message correctly. This document updates by providing an additional mechanism for producing and consuming OpenPGP-signed MIME email. This document also updates by providing an additional mechanism for producing and consuming CMS-signed MIME email.
Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here. "Signed Mail" is used to refer to Internet Mail Messages that are cryptographically signed by the sender of the message, and expected to be validated by the recipient of the message. This document does not consider any cryptographic signature mechanism that is not end-to-end (such as ), and should be agnostic to and non-interfering with any such mechanism. "OpenPGP Signature" refers to an OpenPGP Detached Signature as described by . "CMS Signature" refers to an application/pkcs7-signature object as described by . "MUA" refers to a Mail User Agent, which is also known as an email client. For end-to-end signed mail, the sender's MUA performs message composition and injection into the mail system, and the receiver's MUA performs message ingestion from the mail system and rendering to the user. "Legacy MUA" refers to a MUA that does not know about this specification. "MTA" refers to a Message Transfer Agent, for example an SMTP server that relays Internet mail messages from one point to another. "CRLF" refers to "Carriage Return followed by Line Feed", the standard email line-ending sequence of two octets, CR (0x0D) and LF (0x0A).
Problems With Existing Signature Schemes Existing end-to-end signature schemes for mail can trigger a set of annoyances for a recipient who uses a MUA that doesn't understand these structures. These annoyances can cause the recipient to complain to the sender. The easiest way for the sender to try to accommodate the recipient in this case is to simply not sign mail. The Unobtrusive Signature scheme defined in this document is intended to minimize or eliminate all of these problems.
Unreadable Signed Mail A signed mail message that uses the S/MIME PKCS #7 signed-data Cryptographic Layer described in is unreadable by a receiving MUA that doesn't understand . By contrast, a mail message signed with an Unobtrusive Signature should render normally by any legacy MUA.
Unknown Attachment A signed mail message that uses the S/MIME Multipart Signed Cryptographic Layer described in or the PGP/MIME Signing Cryptographic Layer (multipart/signed) described in has a separate MIME part that contains the message signature. A receiving MUA that doesn't understand these structures will often render the signature as an "attachment". This can cause confusion and anxiety to the user of the MUA, and they will sometimes respond to the sender with the complaint "I can't open your attachment". By contrast, a mail message signed with an Unobtrusive Signature is merely encapsulated in a multipart/mixed outer layer. Legacy MUAs do not render such an encapsulation as an attachment.
Reducing Confusion with name Parameter For existing end-to-end multipart signature schemes, one partial mitigation to this problem is to mark the signature part with an explicit filename that a legacy MUA is likely to display to the recipient. Concretely, some signing MUAs that generate multipart/signed messages using PGP/MIME () will add a name="openpgp-digital-signature.asc" parameter to the Content-Type header of the application/pgp-signature MIME part. For recipients who understand what an OpenPGP digital signature is (even if their MUA can't interpret it), this might reduce the amount of pushback they provide to the sender. The Unobtrusive Signature scheme described in this document intends to offer even less friction to the recipient using a Legacy MUA by hiding the signature entirely.
Broken Signature In some cases, mail is tampered with in transit, whether deliberately or maliciously. In this case, for a MUA that does understand these messages, some MUAs will visibly complain to the recipient that there is a failed signature. If unsigned mail receives no comparable warning, then the act of adding a signature to a message that might traverse a modifiable path is risky. An MUA compliant with will not create such a warning, but many MUAs do not yet comply with that guidance. By contrast, a legacy MUA won't render anything about the cryptographic status of an Unobtrusively Signed message at all. And an MUA compatible with this specification that encounters a message with a broken Unobtrusive Signtature will never render an error that it wouldn't have rendered on an unsigned message anyway, which removes this disincentive to sign.
Unobtrusively Signed Message An Unobtrusively Signed Message has a specific MIME structure and uses a specific header field.
MIME structure The top-level Content-Type of an unobtrusively signed message is multipart/mixed, and it has a single MIME subpart, which this specification refers to as the "Protected Part". The Protected Part's header sections' first header field is Sig, described in . We hereby specify a third PGP/MIME format in addition to the two listed in :
PGP/MIME Unobtrusive Signing Cryptographic Layer (multipart/mixed)
This MIME layer offers authenticity and integrity if and only if the Protected Part contains one or more valid Sig headers. This format is a Simple Cryptographic Envelope as specified in , and the Protected Part (with all leading Sig Header Fields removed) is the Cryptographic Payload. This MIME structure MUST NOT be used as part of a Multilayer Cryptographic Envelope. If it is found anywhere but the outside of the message it MUST NOT be treated as a Cryptographic Layer.
Sig Header Field This specification defines a new header field, named Sig. Sig is only meaningful if it appears in the Protected Part of an Unobtrusively Signed Message, before any non-Sig header field. It contains parameters, only two of which are currently defined. The t parameter indicates the type of the signature with its value, and the only values currently defined are p (meaning an OpenPGP signature) and c (meaning a CMS signature). See . The b parameter contains a base64-encoded blob that contains the cryptographic signature object of the type described by t. Whitespace is ignored in this value and MUST be ignored when reassembling the original signature. In particular, the signing process can safely insert Folding White Space () in this value in arbitrary places to conform to line-length limits. Note that if multiple Sig header fields appear in a single message, each Sig header field represents a signature over the Protected Part (with all leading Sig Header Fields removed). That is, each Sig signs the same content, and the order of the Sig header fields among themselves doesn't matter as long as every Sig header field precedes all non-Sig header fields in the Header Section of the Protected Part. Sig header fields MUST NOT appear in a non-leading position.
Line Ending Normalization Line endings in the message MUST be converted to CRLF format before signing or verification. This ensures that an OpenPGP signature over the message will be invariant for both binary and text mode signatures.
Sender Guidance
Always Use Header Protection A message signed with an unobtrusive signature MUST always use , signing every header field known to the sending MUA at message composition time.
Message Composition This updates the message composition function found in , using the same parameters. origbody: the traditional unprotected message body as a well-formed MIME tree (possibly just a single MIME leaf part). As a well-formed MIME tree, origbody already has structural header fields present. origheaders: the intended non-structural header fields for the message, represented here as a list of (h,v) pairs, where h is a header field name and v is the associated value. crypto: an indication that the message is to be signed with one or more Unobtrusive Signatures. This contains a list of one or more secret keys. Each key will make one signature. The algorithm returns a MIME object that is ready to be injected into the mail system: Create MIME tree inner as a copy of origbody Ensure Content-Type Header Field of inner has parameter hp set to "clear" For each header name and value (h,v) in origheaders: If h is Sig, skip that header, otherwise Add header h to inner with value v Canonicalize inner (see ) Convert inner to bytestring innerbytes For each signing key key in crypto: The signing system takes innerbytes and the signing key key, yielding a respective signature payload sig. Prepend a Header Field named Sig to inner with two parameters, t (set to the literal string p) and b (set to the base64-encoded value of sig). Create new MIME tree output with Content-Type multipart/mixed, with a single subpart, set to inner For each header name and value (h,v) in origheaders: Add header h to outer with value v Return output
Do Not Use Unobtrusive Signature When Encrypting In accordance with , when sending end-to-end encrypted messages an MUA MUST place end-to-end signatures inside the encrypted data. This mechanism is therefore not applicable to encrypted messages.
Message Canonicalization The Protected Part (with all leading Sig Header Fields removed) SHOULD be canonicalized following the patterns described in : Content-Encoding is used to make the message 7-bit clean End of line trailing whitespace is stripped or encoded to non-whitespace If any line begins with the string "From ", either the Quoted-Printable or Base64 MIME encoding MUST be applied, and if Quoted-Printable is used, at least one of the characters in the string "From " MUST be encoded The canonicalized Protected Part MUST then be line ending normalized as per before creating the signature. A signature over a message is more likely to be verifiable if the message is canonicalized into a format robust against MTA modification in transit.
OpenPGP Signature Details The OpenPGP Signature is made over the canonical bytestring, and binary mode (OpenPGP Signature Type 0x00) SHOULD be used.
CMS Signature Details The CMS signature is a CMS ContentInfo containing a single CMS object of type SignedData. The SignedData encapContentInfo eContent field MUST be absent. The signerInfos field contains the signatures for the canonical bytestring. This specification is directly aligned with the specification of the application/pkcs7-signature media type in .
Recipient Guidance
Detecting an Unobtrusive Signature A receiving MUA detects the presence of an unobtrusive signature on a message by verifying that: the message Content-Type is multipart/mixed, and there is exactly one top-level subpart (though that subpart itself may be multipart), and the Content-Type of that top-level subpart has parameter hp="clear", and the first header field of the top-level subpart is named Sig, and the top-level subpart has a From header field, and its addr-spec matches the addr-spec in the message's From header field. This last requirement (matching From addr-specs) is an anti-spoofing measure, by analogy with .
Validating an Unobtrusive Signature When validating an unobtrusive signature, the signature data (that is, the value of the b field) is converted from Base64 to binary format. When t=p, this decoding yields a binary-format OpenPGP Detached Signature. When t=c, it yields a CMS signature in PKCS7 format. The signed object is extracted from the multipart/mixed part by selecting every octet that comes after the CRLF that terminates the last leading Sig header, and before the CRLF that immediately precedes the trailing MIME boundary. The signed object is then normalized as described in . The normalized data is then passed to the signature verification routine as a raw bytestream.
Message Rendering and the Cryptographic Summary If the message has at least one Unobtrusive Signature which validates, then the MUA renders the message as though the top-level subpart is the message itself. The Cryptographic Summary of the message SHOULD indicate that the message is signed-only, and that all header fields present in the top-level subpart share that Cryptographic Status.
Example Rendering For example, consider a message with this structure:
If at least one Unobtrusive Signature is present as a leading Sig header field in B, and it validates correctly, the message should be rendered the same way as this message:
And its Cryptographic Status will be signed-only.
Consistency with Summary View for Tampered Messages Many MUAs have two different views of a given message: A summary view, when rendering an overview of the contents of a mailbox, for example showing only From, Subject, Date, and "unread" status information for any given message, and A message view, for displaying the message itself, with its header context, cryptographic summary, and full contents. The user reasonably expects that, for any given message, the information available in the summary view should match the message view. Some MUAs render the summary view after ingesting the full message, but other MUAs might render the summary view without ever accessing anything other than the Outer Header Section. If the latter style of MUA gains access to a full message that has a valid unobtrusive signature, it can construct a candidate summary view using the signed header field information. If the candidate summary view differs from the already displayed summary view, then the Outer Header Section has most likely been tampered with in transit. The MUA MUST NOT render the message view in such a way that its header information does not match the summary view, as this will lead to user confusion about the message itself. In such a situation, the MUA has two reasonable choices: The MUA MAY treat the unobtrusive signature as invalid, and show a message view that aligns with the already displayed summary view by rendering only the Outer Header Section. Such a message would have a cryptographic summary of unprotected. The MUA MAY accept the unobtrusive signature (yielding a cryptographic summary of signed-only), and update the summary view to use the candidate summary view instead. Such an updated summary view may surprise a user who is used to the summary view only sustaining minor changes (e.g., from "unread" to "read") upon rendering the message view. A more complex approach in such a situation would be for the MUA to alert the user that the message may have been tampered with in transit, and allow them to choose to view either form of the message. This is similar to the approach described in . Note that an MUA that renders the summary view only after evaluating the full message will never encounter this problem, as the summary view will be fully aligned with the message view from the start. Note also that this concern applies to all forms of signed-only mail with header protection, not just to mail protected with an unobtrusive signature.
Unprotected Header Fields Added In Transit As noted in , it's possible that a MUA encounters some Header Fields on the outer message (in the Header Section of A in the example above) which could not have been known by the sender. If any such fields would normally be rendered in some fashion by the MUA on an unsigned message, it MAY consider rendering them even on a signed-only Unobtrusively Signed message, but it should take care to indicate that they do not share the signed-only Cryptographic Status with the rest of the message.
Signature Failure Handling Sometimes a receiving MUA encounters an unobtrusively signed message where all unobtrusive signatures fail to validate. The receiving MUA MUST NOT present the user with a cryptographic status that is different from a message with no signature at all. That is, the message's Cryptographic Status SHOULD be unprotected. If a message gets tampered with in such a way that all unobtrusive signatures are broken, the recipient should see the message as though it were a normal unsigned message.
Handling Multiple Signatures If more than one unobtrusive signature is present in a message, the receiving MUA MUST verify each signature against the known certificates associated with the indicated sender. As long as one of the signatures validates, the message should be treated as correctly signed, even if all the other signatures are invalid.
Multiple OpenPGP Signatures Note that when a message is signed by multiple OpenPGP keys, the composer MAY structure the message with each OpenPGP signature packet (see ) in its own Sig: t=p header field, or it MAY pack the OpenPGP signature packets together into a single OpenPGP Detached Signature (see ) and place them in a single Sig: t=p header field. The verifying implementation MUST consider all appropriately placed Sig: t=p header fields, regardless of how many signature packets are included in each header field. It MAY coalesce the decoded b= data from multiple Sig: t=p header fields into a single OpenPGP Detached Signature (by simply concatenating the base64-decoded b values) before attempting verification. Some MIME parsers have a fixed upper bound on the size of any MIME header field. A composer signing the message with more than one key should consider the size of the OpenPGP signatures when generating the Sig: t=p header fields to avoid breaking the message for recipients who use those constrained parsers. If the total size of the cumulative signature packets are very large, the composer MAY split up the OpenPGP Detached Signature at OpenPGP Signature packet boundaries, and place each disaggregated OpenPGP Detached Signature into a separate header field. A good rule of thumb is to ensure each Sig: t=p header field is no larger than 50 KiB.
Multiple CMS Signatures When a message is signed by multiple CMS keys, each CMS signature SHOULD be placed in its own Sig: t=c header field.
Ignore Out-of-place Unobtrusive Signatures An unobtrusive signature Sig header field MUST NOT be evaluated unless: it is within the MIME headers of the only subpart of a multipart/mixed message, and it appears before any non-Sig header field Evaluating a Sig header outside of this location might mean that a modified message could still appear to be successfully verified. For example, an unobtrusively signed message might be included as a sub-part of another multipart message, or be transformed into a non-MIME message with different message headers than the original email. This could conceivably be used by an attacker to make subtle changes to the meaning of a message without altering the content of the Protected Part.
MTA Guidance An MTA or any other message relay service that observes a message with Content-Type multipart/mixed that is a single part MUST NOT alter the content of this message body in any way, including, but not limited to, changing the content transfer encoding of the body part or any of its encapsulated body parts. This corresponds to the guidance in about the first section of multipart/signed messages.
Security Considerations Based on the principle that "a broken signature is the same as no signature", a receiving MUA MUST NOT display any warnings if an Unobtrusive Signature fails to verify, unless the user has requested debugging output. This is because if an MITM can modify a message in transit, then they can choose whether or not to also remove the (now invalid) signature. If the receiving MUA displayed a more severe warning for a broken signature than for a missing one (or vice versa), the MITM could choose to modify the message in such a way that would result in the less-severe warning. The warning message is thus attacker-controlled. Otherwise, the security properties are equivalent to those of a multipart/signed message.
Performance Considerations
Rationale for Signature in MIME Part An alternate design considered for unobtrusive signatures was to simply place the Sig header in the Outer Header Section of the message itself, without requiring any additional MIME structure. This was rejected in favor of the MIME structure described in for the following reasons: Unobtrusive signatures always offer Header Protection aligned with , so the signature needs to be able to cover those Header Fields generated by the sending MUA. But we know that most received messages contain a mix of Header Fields generated by the sending MUA and Header Fields injected by MTAs that touch the message in transit. Placing the signature as a Header Field in the Outer Header Section raises challenges in identifying which Header Fields are covered by the signature. An MTA is more likely to modify, reorder, or enforce limits on Header Fields in the message's Outer Header Section than it is to corrupt Header Fields in the subpart. Any DKIM signature that includes the body of the message will cover the end-to-end signature if it is part of the message body. If the end-to-end signature was in the message's Outer Header Section it would not normally be signed by DKIM, and would be vulnerable to inadvertent breakage by naive MTAs.
No One-pass Message Generation Because the signature is included first in the message, it is not possible to generate the message in a single pass. A sending MUA that needs to generate a signed outbound message in a single pass should use another end-to-end signing mechanism, like multipart/signed.
IANA Considerations
Register the Sig Header Field IANA is requested to update the Permanent Message Header Field Names registry to add the following entry: Header Field Name Protocol Status Trace Reference Sig MIME informational no This document The registration template called for in is: Template Field Value Header field name Sig Applicable protocol MIME Status informational Author/Change controller IETF Specification document(s) This document Related information RFC9580 describes OpenPGP detached signatures, RFC8551 describes application/pkcs7-signature objects.
Create Registry for Sig Message Header Parameters IANA is requested to create a registry titled "Sig Message Header Parameters" in the "Message Headers" group of registries, with the following initial contents: Name Description Reference t Type of Signature (see ) This document b Base64-encoded Signature Content (whitespace permitted and ignored) This document (( TODO: do we need a registry for this? Are we expecting any new parameters? ))
Create Registry For t Parameter IANA is requested to create a registry titled "Sig Message Header Signature Types" in the "Message Headers" group of registries, with the following initial contents: Value Description Reference p An OpenPGP Detached Signature This document c A CMS Signature This document
Update multipart/mixed to Refer Here IANA is requested to update the "multipart/mixed" entry in the Media Types registry, to add a reference to this document.
Registration Policies IANA is requested to set all registries within this document to use the SPECIFICATION REQUIRED registration policy, see . This policy means that review and approval by a designated expert is required, and that the IDs and their meanings must be documented in a permanent and readily available public specification, in sufficient detail so that interoperability between independent implementations is possible.
Guidance on End-to-End Email Security End-to-end cryptographic protections for email messages can provide useful security. However, the standards for providing cryptographic protection are extremely flexible. That flexibility can trap users and cause surprising failures. This document offers guidance for Mail User Agent (MUA) implementers to help mitigate those risks and to make end-to-end email simple and secure for the end user. It provides a useful set of vocabulary as well as recommendations to avoid common failures. It also identifies a number of currently unsolved usability and interoperability problems. Header Protection for Cryptographically Protected Email S/MIME version 3.1 introduced a mechanism to provide end-to-end cryptographic protection of email message headers. However, few implementations generate messages using this mechanism, and several legacy implementations have revealed rendering or security issues when handling such a message. This document updates the S/MIME specification (RFC 8551) to offer a different mechanism that provides the same cryptographic protections but with fewer downsides when handled by legacy clients. Furthermore, it offers more explicit usability, privacy, and security guidance for clients when generating or handling email messages with cryptographic protection of message headers. The Header Protection scheme defined here is also applicable to messages with PGP/MIME (Pretty Good Privacy with MIME) cryptographic protections. Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. OpenPGP This document specifies the message formats used in OpenPGP. OpenPGP provides encryption with public key or symmetric cryptographic algorithms, digital signatures, compression, and key management. This document is maintained in order to publish all necessary information needed to develop interoperable applications based on the OpenPGP format. It is not a step-by-step cookbook for writing an application. It describes only the format and methods needed to read, check, generate, and write conforming packets crossing any network. It does not deal with storage and implementation questions. It does, however, discuss implementation issues necessary to avoid security flaws. This document obsoletes RFCs 4880 ("OpenPGP Message Format"), 5581 ("The Camellia Cipher in OpenPGP"), and 6637 ("Elliptic Curve Cryptography (ECC) in OpenPGP"). Cryptographic Message Syntax (CMS) This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] MIME Security with OpenPGP This document describes how the OpenPGP Message Format can be used to provide privacy and authentication using the Multipurpose Internet Mail Extensions (MIME) security content types described in RFC 1847. [STANDARDS-TRACK] Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0 Message Specification This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Internet Message Format This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK] Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted This document defines a framework within which security services may be applied to MIME body parts. [STANDARDS-TRACK] This memo defines a new Simple Mail Transfer Protocol (SMTP) [1] reply code, 521, which one may use to indicate that an Internet host does not accept incoming mail. This memo defines an Experimental Protocol for the Internet community. This memo defines an extension to the SMTP service whereby an interrupted SMTP transaction can be restarted at a later time without having to repeat all of the commands and message content sent prior to the interruption. This memo defines an Experimental Protocol for the Internet community. OpenPGP Example Keys and Certificates Mailpile ehf Independent American Civil Liberties Union The OpenPGP development community benefits from sharing samples of signed or encrypted data. This document facilitates such collaboration by defining a small set of OpenPGP certificates and keys for use when generating such samples. DomainKeys Identified Mail (DKIM) Signatures DomainKeys Identified Mail (DKIM) permits a person, role, or organization that owns the signing domain to claim some responsibility for a message by associating the domain with the message. This can be an author's organization, an operational relay, or one of their agents. DKIM separates the question of the identity of the Signer of the message from the purported author of the message. Assertion of responsibility is validated through a cryptographic signature and by querying the Signer's domain directly to retrieve the appropriate public key. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the DKIM signature. This memo obsoletes RFC 4871 and RFC 5672. [STANDARDS-TRACK] Registration Procedures for Message Header Fields This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. S/MIME Example Keys and Certificates The S/MIME development community benefits from sharing samples of signed or encrypted data. This document facilitates such collaboration by defining a small set of X.509v3 certificates and keys for use when generating such samples. Improving Awareness of Running Code: The Implementation Status Section This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. This will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice.
Test Vectors These test vectors show different examples of unobtrusive signed messages.
From Alice to Bob The message below is a common multipart/alternative email, signed with an unobtrusive signature. The signature should be verifiable using the "Alice" v4 certificate found in .
To: Bob Babbage Subject: This is a Test Date: Thu, 01 May 2025 22:16:15 -0400 Message-ID: --5d6 Sig: t=p; b=wnUEABYKAB0WIQTrhbtfozp14V6UTmPyMVUMT0fjjgUCaBQq 7wAKCRDyMVUMT0fjjr+3AP4nGDsaptk9I6EePoXftyevyH6luB2aSAzrD8o xQVNWDQD/VQ/s85C3v6SAxtFDcBsn2H32Hd/yW5BsDx62gmpL7Aw= MIME-Version: 1.0 From: Alice Lovelace To: Bob Babbage Subject: This is a Test Date: Thu, 01 May 2025 22:16:15 -0400 Message-ID: Content-Type: multipart/alternative; boundary="913"; hp="clear" --913 Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Hi Bob, This is Alice. I need you to: - read this message - reply to it - delete it promptly. Thanks, Alice --913 Content-Type: text/html; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit

Hi Bob,

This is Alice. I need you to:

  • read this message
  • reply to it
  • delete it promptly.

Thanks, Alice

--913-- --5d6-- ]]>
From David to Alice The message below is a simple text/plain email, signed with an unobtrusive signature. The signature should be verifiable using the "David" certificate found in .
To: Alice Lovelace Subject: Checking in Date: Fri, 02 May 2025 13:01:07 -0400 Message-ID: --a21 Sig: t=p; b=wpIGABsIAAAAKSIhBkGZ2eqmaCp41aU09iv3YiKlTk3rx4Xb 5qbFs0WGAm/iBQJoFPpTAAAACgkQQZnZ6qZoKnhDIRA9tgkt50eA1ckzilm 9KndQt3t4iYlab66bvtP+kP9D7zaNzvC1vE+B6jPY1gUBOQMyF5CK3yC/xZ Ol2ww+x8Y3PZ7OpZ1dPUlshDL5gA7ZAw== MIME-Version: 1.0 Content-Transfer-Encoding: 7bit From: David Deluxe To: Alice Lovelace Subject: Checking in Date: Fri, 02 May 2025 13:01:07 -0400 Message-ID: Content-Type: text/plain; charset="us-ascii"; hp="clear" Alice! So good to see you earlier. I hope you will have a chance to check out our new website: https://openpgp.example/ and tell me what you think. All the best, David --a21-- ]]>
From Alice to David The message below is a multipart/alternative email with an image attached, signed with an unobtrusive signature. The signature should be verifiable using the "Alice" v4 certificate found in .
To: David Deluxe Subject: Re: Checking in Date: Fri, 02 May 2025 17:03:35 -0400 Message-ID: In-Reply-To: References: --3e4 Sig: t=p; b=wnUEABYKAB0WIQTrhbtfozp14V6UTmPyMVUMT0fjjgUCaBUz JwAKCRDyMVUMT0fjjmnRAQDKnIfyPyvE2lVlVOQl+H99TK+VFCvBaTZyTAV xnKgJ1gEAjVDQ3idx4Z4wSN+pLhWS1LdpVbWdH7mW58gS0GBz5AM= MIME-Version: 1.0 From: Alice Lovelace To: David Deluxe Subject: Re: Checking in Date: Fri, 02 May 2025 17:03:35 -0400 Message-ID: In-Reply-To: References: Content-Type: multipart/mixed; boundary="d64"; hp="clear" --d64 Content-Type: multipart/alternative; boundary="f4f" MIME-Version: 1.0 --f4f Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Hi David, I think the attached logo might look good on the website. Thanks, Alice --f4f Content-Type: text/html; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit

Hi David,

I think the attached logo might look good on the website.

Thanks, Alice

--f4f-- --d64 Content-Type: image/png Content-Transfer-Encoding: base64 Content-Disposition: inline; filename="logo.png" iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg== --d64-- --3e4-- ]]>
Alice to David Followup The message below is a multipart/alternative email that is a self-reply from about a week later. In the meantime, Alice has gotten a new OpenPGP certificate, so the message is signed with both her old key and her new key. This message's signatures should be verifiable respectively using either the "Alice" v4 certificate found in or the "Alice" v6 certificate found in .
To: David Deluxe Subject: Re: Checking in Date: Thu, 08 May 2025 18:41:05 -0400 Message-ID: In-Reply-To: References: --0cd Sig: t=p; b=wnUEABYKAB0WIQTrhbtfozp14V6UTmPyMVUMT0fjjgUCaB0z AQAKCRDyMVUMT0fjjtCJAQCLvEeDH/grJ9szJTEPumRz0lvQm1f3GHNuTnS W9+SV/wD/YpPK4oMy2Cbrzo9JagpO4uxXkbCWQIHI9HFlwkz8Hg0= Sig: t=p; b=wpIGABsIAAAAKSIhBuRqR5oGQqpTb/U1uxxDl7NeiBI/TgFl Z9LvdROjBBHyBQJoHTMBAAAACgkQ5GpHmgZCqlOwFhA99dXXagDzmkcTALm p1ZYQ0IyBvaqxRgRwNHw7VdYBSZ66F1vAjY44pdc8ynahPGHexB4AdfXFeb K1GcqiREQgwF74RoCegJ6ZZkRfWCr3Cw== MIME-Version: 1.0 From: Alice Lovelace To: David Deluxe Subject: Re: Checking in Date: Thu, 08 May 2025 18:41:05 -0400 Message-ID: In-Reply-To: References: Content-Type: multipart/alternative; boundary="97a"; hp="clear" --97a Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Hey David, Also, please spell my name correctly in the website's acknowledgements section. Kind regards, Alice --97a Content-Type: text/html; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit

Hey David,

Also, please spell my name correctly in the website's acknowledgements section.

Kind regards, Alice

--97a-- --0cd-- ]]>
From Carlos to Dana The message below is a multipart/alternative email, signed with an unobtrusive CMS signature. The signature should be verifiable using the "Carlos" X.509 certificate from .
To: Dana Hopper Subject: Touching base on Project Scoop Date: Mon, 01 Dec 2025 20:41:05 -0400 Message-ID: --b8f Sig: t=c; b=MIIDoAYJKoZIhvcNAQcCoIIDkTCCA40CAQExDTALBglghkgB ZQMEAgMwCwYJKoZIhvcNAQcBoIICCzCCAgcwggG5oAMCAQICEz9eH1Qk0bQ BQ3gPc8GKF4UedpYwBQYDK2VwMFkxDTALBgNVBAoTBElFVEYxETAPBgNVBA sTCExBTVBTIFdHMTUwMwYDVQQDEyxTYW1wbGUgTEFNUFMgRWQyNTUxOSBDZ XJ0aWZpY2F0aW9uIEF1dGhvcml0eTAgFw0yMDEyMTUyMTM1NDRaGA8yMDUy MTIxNTIxMzU0NFowOjENMAsGA1UEChMESUVURjERMA8GA1UECxMITEFNUFM gV0cxFjAUBgNVBAMTDUNhcmxvcyBUdXJpbmcwKjAFBgMrZXADIQDCzoAyLN 5hyE2ETWDvkZznna6ufx7kB10j8gCmkvfIraOBsDCBrTAMBgNVHRMBAf8EA jAAMBcGA1UdIAQQMA4wDAYKYIZIAWUDAgEwATAfBgNVHREEGDAWgRRjYXJs b3NAc21pbWUuZXhhbXBsZTATBgNVHSUEDDAKBggrBgEFBQcDBDAOBgNVHQ8 BAf8EBAMCBsAwHQYDVR0OBBYEFGSF4zucHVrN5gu6Gn8IvsSczIQ/MB8GA1 UdIwQYMBaAFGuilX26FJvkLQTRB6TRguQua4y1MAUGAytlcANBAMFRkFm3c uhUCKUxbGlrxtv1Etn50ugfgc4Aq5BkCll9VoJE4+DBDqi/tHBVy6+2UNg0 FKNol8kwLufAUem7XAkxggFbMIIBVwIBATBwMFkxDTALBgNVBAoTBElFVEY xETAPBgNVBAsTCExBTVBTIFdHMTUwMwYDVQQDEyxTYW1wbGUgTEFNUFMgRW QyNTUxOSBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eQITP14fVCTRtAFDeA9zw YoXhR52ljALBglghkgBZQMEAgOggYkwGAYJKoZIhvcNAQkDMQsGCSqGSIb3 DQEHATAcBgkqhkiG9w0BCQUxDxcNMjUxMjAyMDA0MTA1WjBPBgkqhkiG9w0 BCQQxQgRAKL1rWpLN627fT1QCjZRf+3Jhvr6SpbQREptLf0wBzIPktKLT4W nDO68KCYuZMD07gPaw9o47QxhC9C4CnXT3DDAFBgMrZXAEQOQGMe6IlrFn1 owFikFVQ18/x9nqJ3FI2nJDOKv87VStnqsDOnMcjTLxSD5uGnKgUY15yD/Z p+oXksYt791AmQM= MIME-Version: 1.0 From: Carlos Turing To: Dana Hopper Subject: Touching base on Project Scoop Date: Mon, 01 Dec 2025 20:41:05 -0400 Message-ID: Content-Type: multipart/alternative; boundary="12f"; hp="clear" --12f Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Ahoy Dana-- Can you take a look at the most recent document on the W: drive related to Project Scoop? I am happy to talk with you about it Thursday. --Carlos --12f Content-Type: text/html; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit

Ahoy Dana--

Can you take a look at the most recent document on the W: drive related to Project Scoop?

I am happy to talk with you about it Thursday.

--Carlos

--12f-- --b8f-- ]]>
Document History
Changes Between draft-ietf-mailmaint-unobtrusive-signatures-00 and draft-ietf-mailmaint-unobtrusive-signatures-01 Specified CMS signatures Added implementation status section
Changes Between draft-gallagher-email-unobtrusive-signatures-02 and draft-ietf-mailmaint-unobtrusive-signatures-00 Working group adoption.
Changes Between draft-gallagher-email-unobtrusive-signatures-01 and draft-gallagher-email-unobtrusive-signatures-02 Align IANA registration section with requests from IANA. Update references for drafts which are now RFCs. Permit multiple OpenPGP signature packets in each Sig header, aligning with OpenPGP "detached signature". Clarify signing process. Guidance for possible discrepancy between "summary" and "message" views if headers are not aligned.
Changes Between draft-gallagher-email-unobtrusive-signatures-00 and draft-gallagher-email-unobtrusive-signatures-01 Made explicit that Sig MUST NOT appear in a non-leading position. Expanded design rationale section. Clarified use of Quoted-Printable encoding. Terminology and reference cleanup.
Changes Between draft-gallagher-email-invisible-signatures-00 and draft-gallagher-email-unobtrusive-signatures-00 Updated sender canonicalization guidance from MUST to SHOULD. Registries changed to SPECIFICATION REQUIRED. Improved test vectors. Renamed to "Unobtrusive Signatures". Explicitly allow folding whitespace. Document existing convention re attachment filenames. Fixed references. Various clarifications to wording.
Implementation Status RFC Editor: When this section is removed before publishing, please also remove the reference to . As recommended in , this section describes the status of implementations known to the editors at the time of draft publication. 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.
emacs mml-mode The mml MUA within emacs has a patch set for generating unobtrusive signatures. This code is distributed under the General Public License, version 3 or later.
Thunderbird The Thunderbird MUA has a work-in-progress patch set for generating and processing unobtrusive signatures. This code is distributed under the Mozilla Public License, version 2.0 or the General Public License, version 2.
Acknowledgments The authors would like to thank the attendees of the 9th OpenPGP Email Summit for feedback and suggestions. Additionally, Anarcat and Barry Leiba offered useful feedback on the draft.