]> Drummond Group, LLC

debrap@drummondgroup.com

AS2 EDIINT B2B This document provides an applicability statement (RFC 2026, Section 3.2) describing how to securely exchange structured business data over HTTP. Structured business data may be XML; Electronic Data Interchange (EDI) in either the American National Standards Committee (ANSI) X12 format or the UN Electronic Data Interchange for Administration, Commerce, and Transport (UN/EDIFACT) format; or other structured data formats. The data is packaged using standard MIME structures. Authentication and data confidentiality are obtained by using Cryptographic Message Syntax with S/MIME security body parts (see ). Authenticated acknowledgements make use of multipart/signed Message Disposition Notification (MDN) responses to the original HTTP message. This applicability statement is informally referred to as "AS2" because it is the second applicability statement, produced after "AS1" (RFC 3335). This document obsoletes RFC 4130 and stands on its own without reference to AS1 or SMTP, except where required for IANA registry updates. This document also updates IANA registries originally created by RFC 3335 and RFC 4130. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction This document is a revision ("bis") of RFC 4130, which defined the Applicability Statement 2 (AS2) protocol for secure and reliable transport of business data over HTTP. It obsoletes RFC 4130. The purpose of this revision is to modernize the specification, clarify ambiguities, and incorporate implementation experience gathered since the publication of RFC 4130. Subsequent versions of this draft will refine these updates based on discussion and consensus in the IETF community. This revision also adheres to the principle of backward compatibility. Implementations conformant with RFC 4130 remain valid under this specification, and no breaking changes are introduced. In addition, this document updates existing IANA registrations from RFC 3335 and RFC 4130. The specific IANA actions are described in . Note to readers: Some contributors have suggested that this work could eventually be split into two documents: a minimal RFC4130bis for errata and clarifications, and a separate AS2 v2 specification with a clean modern baseline. This document currently attempts to balance both objectives within a single text, but further discussion may refine the scope.

Applicable RFCs Previous work on Internet EDI focused on specifying MIME content types for EDI data. expands on this to specify a comprehensive set of data security features, specifically data confidentiality, data integrity/authenticity, non-repudiation of origin, and non-repudiation of receipt over HTTP. This document recognizes contemporary RFCs and avoids re-inventing mechanisms wherever possible. Although this document focuses on EDI data, any other data types describable in a MIME format are also supported. Internet MIME-based EDI can be accomplished by using and complying with the following RFCs: This specification references S/MIME Version 4.0 as the baseline for algorithm requirements and security message formats. S/MIME 4.0 introduces AuthEnvelopedData, which provides authenticated encryption for algorithms such as AES-GCM and AES-CCM. For backward compatibility with implementations that have not yet migrated to S/MIME 4.0, this specification also permits the use of EnvelopedData from S/MIME 3.2 when using algorithms such as AES-CBC that require separate integrity protection. The choice between AuthEnvelopedData and EnvelopedData is determined by the content encryption algorithm selected (see for details). Our intent here is to define clearly and precisely how these are used together, and what is required by user agents to be compliant with this document. Implementers should note that HTTP/2 and HTTP/3 MAY be used as transports, but are not required for interoperability. 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 and when, and only when, they appear in all capitals, as shown here.

Backward Compatibility and Interoperability A central design principle of this specification is "backward compatibility" with RFC 4130 and with the underlying RFCs it references. This specification does not redefine or override backward-compatibility rules established in those RFCs. Implementations MUST rely on the mechanisms provided in underlying standards. Consistent with the Robustness Principle ("be conservative in what you send and liberal in what you receive"), this document clarifies requirements and aligns terminology but does not introduce breaking changes. Implementations that conformed to RFC 4130 remain conformant to this specification. Any deviations are limited to clarifications intended to improve interoperability. This specification establishes S/MIME Version 4.0 as the baseline for conformant implementations. Implementations MUST support S/MIME 4.0 message formats (including AuthEnvelopedData) and the algorithm requirements specified in . Implementations SHOULD also support S/MIME Version 3.2 for backward compatibility with legacy trading partners that have not yet migrated to S/MIME 4.0. When both partners support S/MIME 4.0, implementations SHOULD use AuthEnvelopedData with authenticated encryption algorithms (AES-GCM, AES-CCM) for improved security. When interoperating with S/MIME 3.2 systems, implementations SHOULD use EnvelopedData with algorithms such as AES-CBC that require separate integrity protection via digital signatures. This specification defines requirements for modern AS2 deployments using contemporary cryptographic algorithms. It does not redefine or extend the use of weak algorithms such as 3DES or SHA-1. When both partners support this version of AS2, only modern algorithms are in scope. When interoperability with RFC 4130 systems is required, implementers SHOULD apply the clarifications provided in (Legacy Interoperability). is non-normative and does not alter the algorithm requirements defined in , but records expected behavior when communicating with legacy systems.

Legacy Interoperability (Non-Normative) This section provides the conditions for interoperability with legacy AS2 implementations conforming to . These provisions apply only when a modern AS2 implementation communicates with a partner that has not migrated to this specification. In such cases, both parties are effectively operating under , not this document. These notes are provided to reduce ambiguity and ensure consistent behavior across implementations. They do not alter the algorithm requirements specified in , nor do they extend the use of deprecated algorithms. Examples of legacy considerations include: These clarifications are provided for reference and consistency across vendors. They are non-normative and are not intended to redefine or to weaken the algorithm requirements of this specification. Refer to for discussion of backward compatibility principles, and Section 7 for normative algorithm requirements.

Rationale The updates in this specification reflect community consensus to: This approach reduces ambiguity, simplifies certification, and ensures interoperability across implementations.

Terms

Overview

Overall Operation An HTTP POST operation is used to send appropriately packaged EDI, XML, or other business data. The Request-URI (, Section 10.5) identifies a process for unpacking and handling the message data and for generating a reply for the client that contains a message disposition acknowledgement (MDN), either signed or unsigned. The MDN is either returned in the HTTP response message body or by a new HTTP POST operation to a URL for the original sender. This request/reply transactional interchange can provide secure, reliable, and authenticated transport for EDI or other business data using HTTP as a transfer protocol. HTTPS is RECOMMENDED as the default transport for modern implementations (see ). The security protocols and structures used also support auditable records of these document data transmissions, acknowledgements, and authentication. The message formats and processing requirements described below maintain strict backward compatibility (see ).

Purpose of a Security Guideline for MIME EDI The purpose of these specifications is to ensure interoperability between B2B EC user agents, invoking some or all of the commonly expected security features. This document is not limited to strict EDI use; it applies to any electronic commerce application for which business data needs to be exchanged securely over the Internet.

Definitions

The Secure Transmission Loop This document's focus is on the formats and protocols for exchanging EDI/EC content securely in the Internet's HTTP environment. In the "secure transmission loop" for EDI/EC, one organization sends a signed, encrypted and compressed EDI/EC interchange to another organization and requests a signed receipt, and later the receiving organization sends this signed receipt back to the sending organization. In other words, the following transpires: The above describes functionality that, if implemented, will satisfy all security requirements and implement non-repudiation of receipt for the exchange. This specification, however, leaves full flexibility for users to decide the degree to which they want to deploy those security features with their trading partners.

Definition of Receipts The term used for both the functional activity and the message for acknowledging delivery of an EDI/EC interchange is "receipt" or "signed receipt". The first term is used if the acknowledgment is for an interchange resulting in a receipt that is NOT signed. The second term is used if the acknowledgement is for an interchange resulting in a receipt that IS signed. The term non-repudiation of receipt (NRR) is often used in combination with receipts. NRR refers to a legal event that occurs only when the original sender of an interchange has verified the signed receipt coming back from the recipient of the message, and has verified that the returned MIC value inside the MDN matches the previously recorded value for the original message. NRR is best established when both the original message and the receipt make use of digital signatures. See the Security Considerations section for some cautions regarding NRR. For information on how to format and process receipts in AS2, refer to refer to Section 8.

Assumptions

EDI/EC Process Assumptions

Flexibility Assumptions Without Compression (12 permutations)

1. Sender sends un-encrypted data and does NOT request a receipt.
2. Sender sends un-encrypted data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
3. Sender sends un-encrypted data and requests a signed receipt. Receiver sends back the signed receipt.
4. Sender sends encrypted data and does NOT request a receipt.
5. Sender sends encrypted data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
6. Sender sends encrypted data and requests a signed receipt. Receiver sends back the signed receipt.
7. Sender sends signed data and does NOT request a signed or unsigned receipt.
8. Sender sends signed data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
9. Sender sends signed data and requests a signed receipt. Receiver sends back the signed receipt.
10. Sender sends encrypted and signed data and does NOT request a signed or unsigned receipt.
11. Sender sends encrypted and signed data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
12. Sender sends encrypted and signed data and requests a signed receipt. Receiver sends back the signed receipt.

With Compression (12 permutations)

13. Sender sends compressed data (not encrypted or signed) and does NOT request a receipt.
14. Sender sends compressed data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
15. Sender sends compressed data and requests a signed receipt. Receiver sends back the signed receipt.
16. Sender sends compressed and encrypted data and does NOT request a receipt.
17. Sender sends compressed and encrypted data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
18. Sender sends compressed and encrypted data and requests a signed receipt. Receiver sends back the signed receipt.
19. Sender sends compressed and signed data and does NOT request a signed or unsigned receipt.
20. Sender sends compressed and signed data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
21. Sender sends compressed and signed data and requests a signed receipt. Receiver sends back the signed receipt.
22. Sender sends compressed, encrypted, and signed data and does NOT request a signed or unsigned receipt.
23. Sender sends compressed, encrypted, and signed data and requests an unsigned receipt. Receiver sends back the unsigned receipt.
24. Sender sends compressed, encrypted, and signed data and requests a signed receipt. Receiver sends back the signed receipt.

Key Notes

Referenced RFCs and Their Contributions

RFC 2616 HTTP v1.1 specifies how data is transferred using HTTP.

RFC 1847 MIME Security Multiparts defines security multipart for MIME: multipart/encrypted and multipart/signed.

RFC 3462 Multipart/Report defines the use of the multipart/report content type, something that the MDN RFC 3798 builds upon.

RFC 1767 EDI Content defines the use of content type "application" for ANSI X12 (application/EDI-X12), EDIFACT (application/EDIFACT), and mutually defined EDI (application/EDI-Consent).

RFC 2045, 2046, and 2049 MIME , , and are the basic MIME standards, upon which all MIME related RFCs build, including this one. Key contributions include definitions of "content type", "sub-type", and "multipart", as well as encoding guidelines, which establish 7-bit US-ASCII as the canonical character set to be used in Internet messaging.

RFC 3798 Message Disposition Notification defines how an MDN is requested, and the format and syntax of the MDN. The MDN is the basis upon which receipts and signed receipts are defined in this specification.

RFC 5751 and 5652 S/MIME Version 3.2 Message Specifications and Cryptographic Message Syntax (CMS) and describe how S/MIME carries CMS Objects.

RFC 3023 XML Media Types defines the use of content type "application" for XML (application/xml).

RFC 3274 Compressed Data Content Type for Cryptographic Message Syntax (CMS) defines a mechanism for compressing data within the Cryptographic Message Syntax (CMS), which is the foundation for Secure/Multipurpose Internet Mail Extensions (S/MIME). It specifies a CompressedData content type that allows data to be compressed prior to being signed or encrypted. This reduces the size of transmitted messages and improves efficiency without altering the security services provided by signing or encryption. AS2-Version 1.1 incorporated the compression capability described in RFC 3274, enabling trading partners to optionally apply compression to message payloads before signing and/or encrypting. Most modern AS2 implementations support this feature to reduce bandwidth usage and improve transmission performance, particularly for large payloads.

Structure of an AS2 Message

Introduction The basic structure of an AS2 message consists of MIME format inside an HTTP message with a few additional specific AS2 headers. The structures below are described hierarchically in terms of which RFCs are applied to form the specific structure. For details on how to code in compliance with all RFCs involved, refer to the specific RFCs. Any difference between AS2 implementations and RFCs are mentioned specifically in the sections below.

Structure of an Internet EDI MIME Message Key Notes

HTTP Considerations This specification permits but does not require the use of HTTP/2 or HTTP/3 as transport protocols. Interoperability testing and certification are scoped to HTTP/1.1 unless otherwise agreed upon between trading partners.

Sending EDI in HTTP POST Requests The request line will have the form: "POST Request-URI HTTP/1.1", with spaces and followed by a CRLF. The Request URI is typically exchanged out of band, as part of setting up a bilateral trading partner agreement. Applications SHOULD be prepared to deal with an initial reply containing a status indicating a need for authentication of the usual types used for authorizing access to the Request-URI (, Section 10.4.2 and elsewhere). The request line is followed by entity headers specifying content length (, Section 14.14) and content type (, Section 14.18). The Host request header (, Sections 9 and 14.23) is also included. When using Transport Layer Security (TLS), the request-URI MUST indicate the appropriate scheme value, HTTPS. Implementations MUST support TLS 1.2 or higher and SHOULD follow the guidance in . Encrypted message bodies MAY be used in addition to TLS when required by business policy. The receiving AS2 system MAY disconnect from the sending AS2 system before completing the reception of the entire entity if it determines that the entity being sent is too large to process. For HTTP version 1.1, TCP persistent connections are the default, ( Sections 8.1.2, 8.2, and 19.7.1). A number of other differences exist because HTTP does not conform to MIME as used in SMTP transport. Relevant differences are summarized below.

Unused MIME Headers and Operations

Content-Transfer-Encoding Not Used in HTTP Transport HTTP can handle binary data and so there is no need to use the content transfer encodings of MIME . This difference is discussed in , Section 19.4.5. However, a content transfer encoding value of binary or 8-bit is permissible but not required. The absence of this header MUST NOT result in transaction failure. Content transfer encoding of MIME bodyparts within the AS2 message body is also allowed.

Message Bodies In , Section 3.7.2, it is explicitly noted that multiparts MUST have null epilogues. For HTTP transport, large files SHOULD be handled correctly by the TCP layer. In addition, , Sections 3.5 and 3.6 describe options for compressing or chunking entities to be transferred, and Section 8.1.2.2 describes a pipelining option that is useful for segmenting large amounts of data. These clarifications are consistent with existing AS2 practice and maintain full backward compatibility (see ).

Modification of MIME or Other Headers or Parameters Used

Content-Length The use of the content-length header MUST follow the guidelines of , specifically Sections 4.4 and 14.13.

Final Recipient and Original Recipient The final and original recipient values SHOULD be the same value. These values MUST NOT be aliases or mailing lists.

Message-Id and Original-Message-Id The Message-Id and Original-Message-Id headers identify a message uniquely and are formatted as defined in , Section 3.6.4: " ]]> The length of a Message-Id value MUST NOT exceed 998 characters. For maximum interoperability, the length SHOULD be 255 characters or less. The Message-Id value MUST be globally unique, and the id-right portion SHOULD be something unique to the sending host environment (for example, a fully qualified domain name). Implementations that generate Message-Id values MUST NOT include spaces or control characters. Implementations SHOULD remove spaces rather than substitute another character when constructing identifiers from other message attributes such as AS2-From or AS2-To. Receivers are not required to accept malformed identifiers. If a message is received with a Message-Id that contains spaces or control characters, the implementation SHOULD treat it as syntactically invalid and SHOULD return an MDN with a disposition of processed/error and a human-readable explanation such as "invalid-message-id" (see ). If an implementation chooses to proceed despite the malformed identifier, it MUST NOT propagate or generate a new message using that malformed value. When sending a message, the Message-Id field value MUST be enclosed in angle brackets (“<” and “>”). The brackets are not part of the actual identifier value. For backward compatibility, receiving implementations SHOULD NOT reject a message that omits angle brackets. When creating the Original-Message-Id header in an MDN, always use the exact syntax as received on the original message; do not strip or add angle brackets. See , Section 3.6.4.

Host Header The host request header field MUST be included in the POST request made when sending business data. This field is intended to allow one server IP address to service multiple hostnames, and potentially to conserve IP addresses. See , Sections 14.23 and 19.5.1.

HTTP Response Status Codes Implementations MUST use standard HTTP response codes to signal the outcome of the message transfer. The meaning of the HTTP status code is limited to the success or failure of the transport operation itself, not the semantic processing of the AS2 message content. For example, the status code 401, together with the WWW-Authenticate header, is used to challenge the client to repeat the request with an Authorization header. Other explicit status codes are documented in , Section 6.1.1 and throughout Section 10. Receiving implementations MAY send an interim 102 (Processing) response under HTTP/1.1 to indicate that the inbound message has been fully received and that processing is underway. The 102 response can help prevent sender-side network timeouts for large synchronous transfers by signaling progress while decryption, signature verification, or storage continues. Use of 102 (Processing) is OPTIONAL. It has been deprecated in later HTTP specifications and MUST NOT be used with HTTP/2 or HTTP/3, where interim responses have different semantics. Implementations that do not receive a 102 response MUST NOT assume that a failure has occurred solely because no interim status was returned. They SHOULD continue waiting for the final status response for at least the duration of their configured HTTP read timeout or any timeout agreed upon between trading partners. To minimize the risk of network timeouts during lengthy message processing, receivers SHOULD return an appropriate transfer-layer response as quickly as possible after receiving the full message content. For asynchronous message exchanges, the preferred response is 204 No Content, which indicates that the message has been received successfully and that an asynchronous MDN will follow once processing has completed. This convention is maintained for interoperability with existing AS2 products and certification profiles. Some implementations MAY instead use 202 Accepted to indicate successful receipt and deferred processing; however, 204 No Content remains the recommended and most widely deployed response for asynchronous workflows. Implementations MAY close the connection immediately after sending this response if persistent connections are not required by configuration. After processing completes, the receiver MUST return a final HTTP status code indicating the success or failure of the message transfer. The sender MUST use this final response to determine whether retry is appropriate. Retry MUST NOT be attempted when: Retry MAY be attempted when: Implementations SHOULD refer to Section 5.5 for additional guidance on retry logic, back-off behavior, and use of partial-transfer recovery. The 102 (Processing) status code, if used, MUST NOT be treated as a trigger for retry.

HTTP Error Recovery and Reliability When an AS2 message transfer fails due to a transient transport-layer condition (for example, an HTTP 408 Request Timeout, 425 Too Early, 500 Internal Server Error, 503 Service Unavailable or network interruption before the final response), the sending system SHOULD attempt an automatic retry. Each retry attempt MUST reuse the same Message-ID value so that the receiving system can identify duplicate transmissions and prevent double-processing. A receiving system detecting a duplicate Message-ID MUST NOT treat the message as new and SHOULD return the previously generated MDN, if available. Implementations SHOULD permit configuration of retry behavior rather than enforcing fixed intervals or limits. The following guidelines are RECOMMENDED but not required: Implementations SHOULD NOT retry when: The HTTP 102 (Processing) interim status MAY be used under HTTP/1.1 to indicate progress on long-running synchronous operations. It MUST NOT be used as a signal to initiate or suppress retries. Implementations MUST ignore 102 responses when determining whether a retry is required. The 102 response MUST NOT be used with HTTP/2 or HTTP/3. Implementations MAY also support AS2 Restart, which allows a partially uploaded message to resume from the point of interruption rather than retransmitting the entire payload. This optional feature is defined in . Implementations supporting Restart MUST ensure message integrity through signature or checksum validation of all resumed segments. Additional guidance for retry management, error classification, and duplicate detection is described in . While both of these drafts are expired, they remain widely referenced in AS2 interoperability testing and provide a useful operational baseline for error-recovery behavior. The objective of error recovery is reliability, not speed. Systems SHOULD favor successful delivery over strict timing, provided that duplicate protection and security requirements are preserved.

Additional AS2-Specific HTTP Headers The following headers are to be included in all AS2 messages and all AS2 MDNs. .

AS2 Version Header To promote backward compatibility, AS2 includes a version header. The major version digit indicates wire-level compatibility; minor version digits designate feature sets, clarifications, or extensions that remain compatible within the same major version. Thus, all values in the "1.x" range are compatible with AS2-Version 1.0, while a potential future "2.0" version would indicate a non-backward-compatible revision. Receiving systems MUST NOT fail due to the absence of the AS2-Version header. Its absence MUST be assumed to be equivalent to the default AS2-Version value of 1.0.

AS2 Product header The AS2-Product header value identifies the AS2 product and version used by the sender. This information enables interoperability testing, certification, and troubleshooting by allowing trading partners to detect known product-specific behaviors or version-related quirks. The AS2-Product header value is OPTIONAL for AS2-Version 1.x systems but MUST be included in messages generated by implementations declaring AS2-Version: 1.3 (or later). The header field value MUST follow the format: : lowercase alphanumeric and hyphen characters (a–z, 0–9, "-") without spaces. * : version string consistent with the product’s release version, with one or more numeric components separated by dots. Examples: AS2-Product: biztalk:2025.1 AS2-Product: example.com-connect:4.2.3 ]]> Implementations MUST NOT use arbitrary identifiers or vendor aliases that do not reflect the actual product in use. The value is static and determined at build time. If a product supports multiple AS2 variants, the version portion MAY include an implementation-specific suffix (e.g., "1.2-drummond"). Implementations MAY use the AS2-Product value for automated interoperability tuning or to apply compatibility workarounds for known product versions. However, this field is not intended for feature-negotiation purposes; supported feature tokens belong in the EDIINT-Features header, as defined in RFC 6017.

AS2 System Identifiers To aid the receiving system in identifying the sending system, AS2-From and AS2-To headers are used. AS2-To: < AS2-name > ]]> These AS2 headers contain textual values, as described below, identifying the sender/receiver of a data exchange. Their values may be company specific, such as Data Universal Numbering System (DUNS) numbers, or they may be simply identification strings agreed upon between the trading partners. The AS2-From header value and the AS2-To header value: The string definitions given above are in ABNF format . The AS2-quoted-name SHOULD be used only if the AS2-name does not conform to AS2-atomic-name. This explicitly includes situations where embedded spaces are part of the AS2-name. The AS2-To and AS2-From header fields MUST be present in all AS2 messages and AS2 MDNs whether they are synchronous or asynchronous in nature. The AS2-name for the AS2-To header in a response or MDN MUST match the AS2-name of the AS2-From header in the corresponding request message. Likewise, the AS2-name for the AS2-From header in a response or MDN MUST match the AS2-name of the AS2-To header in the corresponding AS2 request message. The sending system may choose to limit the possible AS2-To/AS2-From textual values but MUST not exceed them. The receiving system MUST make no restrictions on the textual values and SHOULD handle all possible implementations. However, implementers must be aware that older AS2 products may not adhere to this convention. Trading partner agreements should be made to ensure that older products can support the system identifiers that are used. There is no required response to a client request containing invalid or unknown AS2-From or AS2-To header values. The receiving AS2 system MAY return an unsigned MDN with an explanation of the error, such as an MDN error disposition value of "unknown-trading-relationship", if the sending system requested an MDN.

Algorithm Requirements This section defines the normative requirements for cryptographic algorithms used in AS2. These requirements apply to all conformant implementations. Guidance on interoperability with legacy AS2 systems that continue to use older algorithms is provided separately in Section 1.2.1.

Hash Algorithms Implementations MUST support SHA-256 for message integrity check (MIC) calculations and digital signatures. Implementations SHOULD support SHA-384 or stronger algorithms. SHA-1 and MD5 are deprecated due to known weaknesses and SHOULD NOT be generated by conformant implementations. See Section 1.2.1 for clarifications on handling legacy algorithms when interoperating with RFC 4130 systems.

Encryption Algorithms Implementations MUST support AES encryption algorithms as defined in S/MIME Version 4.0 . At a minimum, AES-128-CBC and AES-256-CBC MUST be supported. Implementations are also RECOMMENDED to support AES-128-GCM and AES-256-GCM. Support for AES-CCM is also RECOMMENDED for environments requiring authenticated encryption. Triple DES (3DES) and RC2 are deprecated and SHOULD NOT be generated by conformant implementations.

EnvelopedData vs AuthEnvelopedData The choice between EnvelopedData and AuthEnvelopedData depends on the content encryption algorithm selected: Implementations MUST NOT mix content encryption algorithms for different recipients of the same message. A single content encryption algorithm MUST be selected and used for all recipients. For example, if a message is encrypted with AES-128-GCM, all recipient information MUST use AES-128-GCM; it is not permitted to encrypt the content- encryption key with AES-CBC for some recipients and AES-GCM for others.

Multiple-Recipient Encryption To support recoverable decryption and regulatory requirements, implementations SHOULD support multiple-recipient encryption of the content-encryption key (CEK), consistent with Section 3.3. A copy of the CEK encrypted for the originator SHOULD also be included in the EnvelopedData, and the same principle applies to AuthEnvelopedData when using AES-CCM or AES-GCM. See Section 1.2.1 for guidance on handling 3DES and other weak algorithms when interoperating with legacy AS2 systems.

Structure and Processing of an MDN Message This document aligns MDN behavior with RFC 8098, clarifying semantics for interoperability. It does not redefine the MDN format. Implementations MUST be able to parse historic MDN forms as described in RFC 3798 for backward compatibility.

Introduction In order to support non-repudiation of receipt, a signed receipt, based on digitally signing a message disposition notification, is to be implemented by a receiving trading partner's UA. The message disposition notification, specified by RFC 3798, is digitally signed by a receiving trading partner as part of a multipart/signed MIME message. The requirements in this section update but do not alter the compatibility of MDN formats with existing AS2 implementations (see ). This ensures interoperability with both RFC 3798 and RFC 8098 implementations. The following support for signed receipts is REQUIRED: The signed receipt is used to notify a sending trading partner that requested the signed receipt that: Regardless of whether the EDI/EC Interchange was sent in S/MIME format, the receiving trading partner's UA MUST provide the following basic processing: The EC Interchange and the RFC 1767 MIME EDI content header can actually be part of a multi-part MIME content-type. When the EDI Interchange is part of a multi-part MIME content-type, the MIC MUST be calculated across the entire multi-part content, including the MIME headers contained within the multi-part MIME content. The signed MDN, when received by the sender of the EDI Interchange, can be used by the sender as follows:

Synchronous and Asynchronous MDNs The AS2-MDN exists in two varieties: synchronous and asynchronous. The synchronous AS2-MDN is sent as an HTTP response to an HTTP POST or as an HTTPS response to an HTTPS POST. This form of AS2-MDN is called synchronous because the AS2-MDN is returned to the originator of the POST on the same TCP/IP connection. The synchronous response MUST indicate transfer-layer success or failure, such as 200 OK or 202 Accepted. The format of this response MAY be identical to that used when no AS2-MDN is requested. The asynchronous AS2-MDN is sent on a separate HTTP or HTTPS TCP/IP connection. Logically, the asynchronous AS2-MDN is a response to an AS2 message. However, at the transfer-protocol layer, assuming that no HTTP pipelining is utilized, the asynchronous AS2-MDN is delivered on a unique TCP/IP connection, distinct from that used to deliver the original AS2 message. When handling an asynchronous request, the receiving system SHOULD return a transfer-layer response (typically 202 Accepted or 204 No Content) as soon as the last byte of the inbound message has been received, without waiting for decryption, signature verification, or message persistence. This minimizes the risk of network timeouts and ensures that the sender can begin awaiting the asynchronous MDN promptly. The asynchronous MDN MUST be transmitted as an independent HTTP message, separate from the original connection used to submit the AS2 message. Implementations MAY use persistent (keep-alive) HTTP connections. Closing the TCP connection immediately after sending the response is RECOMMENDED for simplicity, but not required. Some application servers and frameworks manage connection lifecycles automatically and may keep the socket open. The AS2 specification does not mandate that the AS2 layer explicitly close the connection. The following diagram illustrates the synchronous versus asynchronous varieties of AS2-MDN delivery using HTTP: {Peer2} {Peer1} -----( send )------> {Peer2} HTTP Request {AS2-Message} {Peer1} <---( receive )----- {Peer2} HTTP Response {AS2-MDN} Asynchronous AS2-MDN {Peer1} ----( connect )----> {Peer2} {Peer1} -----( send )------> {Peer2} HTTP Request {AS2-Message} {Peer1} <---( receive )----- {Peer2} HTTP Response (e.g., "200 OK" or "204 No Content") {Peer1}*<---( connect )----- {Peer2} {Peer1} <--- ( send )------- {Peer2} HTTP Request {AS2-MDN} {Peer1} ----( receive )----> {Peer2} HTTP Response ]]>

• Note: An AS2-MDN may be directed to a host different from that of the sender of the AS2 message. It may also utilize a transfer protocol different from that used to send the original AS2 message.

The advantage of the synchronous MDN is that it provides the sender of the AS2 message with a verifiable confirmation of delivery within a single synchronous logic flow. However, if the message is large, the time required to process it and return the AS2-MDN on the same connection may exceed the maximum configured time permitted for maintaining an open connection. The advantage of the asynchronous MDN is that it provides for the rapid return of a transfer-layer acknowledgment from the receiver, confirming receipt of data, while allowing full processing to occur later. This reduces connection duration and timeout risk. However, the asynchronous AS2-MDN MUST include sufficient identifying information (for example, Original-Message-ID and Final-Recipient) so that the message originator can correlate the MDN with its original message and update the processing status accordingly. Synchronous and asynchronous HTTP or HTTPS MDNs are both valid under this specification. Implementations MUST support receiving both types and SHOULD support sending both.

Requesting a Signed Receipt Message disposition notifications are requested as per RFC 3798. A request that the receiving user agent issue a message disposition notification is made by placing the following header into the message to be sent: The following example is for requesting an MDN: The "Disposition-notification-to" header field is retained for compatibility with the MDN specification , but its value is not used by AS2 implementations to determine where to return the MDN. Its presence just indicates that an MDN receipt is to be returned to the originator. In AS2, the field value may be an email address, a URL, a fully qualified domain name, an AS2 identifier, or any other implementation-specific string. Implementations MUST NOT reject a message based on the syntax of this field. This document relaxes the original requirement from RFC 4130, which mandated an email address, in order to reflect current AS2 practice while maintaining backward compatibility (see ). When requesting MDN-based receipts, the originator supplies additional extension headers that precede the message body. These header "tags" are as follows: A Message-ID header is added to support message reconciliation, so that an Original-Message-Id value can be returned in the body part of MDN. Other headers, especially "Subject" and "Date", SHOULD be supplied; the values of these headers are often mentioned in the human-readable portion of a MDN to aid in identifying the original message. MDNs will be returned in the HTTP response when requested, unless an asynchronous MDN is requested. To request an asynchronous message disposition notification, the following header is placed into the message that is sent: This is an example requesting that the MDN be asynchronous: Receipt-delivery-option syntax allows the return-url to use some schemes other than HTTP using the POST method. The "receipt-delivery-option: return-url" string indicates the URL to use for an asynchronous MDN. This header is NOT present if the receipt is to be synchronous. The email value in Disposition- notification-to is not used in this specification because it was limited to RFC 2822 addresses (now replaced by ); the extension header "Receipt-delivery-option" has been introduced to provide a URL for the MDN return by several transfer options. The receipt-delivery-option's value MUST be a URL indicating the delivery transport destination for the receipt. An example request for an asynchronous MDN via an HTTP transport: An example request for an asynchronous MDN via an HTTP/S transport: Finally, the header, Disposition-notification-options, identifies characteristics of message disposition notification as in . The most important of these options is for indicating the signing options for the MDN, as in the following example: For signing options, consider the disposition-notification-options syntax: So the Disposition-notification-options string could be: ; signed-receipt-micalg=optional, , ,...; ]]> The currently used value for <protocol symbol> is "pkcs7-signature" for the S/MIME detached signature format. The currently supported values for MIC algorithm <micalg> values are: The semantics of the "signed-receipt-protocol" and the "signed-receipt-micalg" parameters are as follows:

1. The "signed-receipt-protocol" parameter is used to request a signed receipt from the recipient trading partner. The "signed-receipt-protocol" parameter also specifies the format in which the signed receipt SHOULD be returned to the requester. The "signed-receipt-micalg" parameter identifies one or more message integrity check (MIC) algorithms, in order of preference, that the requester supports for signing the returned receipt. Although multiple values MAY be listed to indicate fallback options, only a single MIC algorithm is used in the returned MDN because the "Received-content-MIC" field conveys exactly one digest value. Recipients MUST select the first algorithm in the list that they also support and MUST compute the Received-content-MIC using that algorithm. Senders SHOULD list the strongest algorithm first. Modern implementations SHOULD include only a single value unless multiple values are needed to support phased migration away from weaker algorithms. Implementations MUST accept messages that contain multiple values and MUST ignore unsupported values. When a sender lists multiple algorithms, recipients MUST NOT fall back to an algorithm that is not explicitly listed by the sender. Trading partners typically pre-configure acceptable MIC algorithms through bilateral agreement, and runtime negotiation is not needed. If none of the algorithms listed is supported, the recipient SHOULD reject the message and MAY return an unsigned MDN indicating "unsupported-mic-algorithm" rather than silently selecting a weaker algorithm. When the header is absent (e.g., unsigned messages), an implementation MUST use a locally configured default algorithm; SHA-256 SHOULD be preferred, but SHA-1 MAY be used when required for legacy partners.

The following algorithm requirements apply to all implementations:

1. The "importance" attribute of "Optional" is defined in RFC 3798, Section 2.2, and has the following meaning: Parameters with an importance of "Optional" permit a UA that does not understand the particular options parameter to still generate an MDN in response to a request for a MDN. A UA that does not understand the "signed-receipt-protocol" parameter or the "signed-receipt-micalg" will obviously not return a signed receipt. The importance of "Optional" is used for the signed receipt parameters because it is RECOMMENDED that an MDN be returned to the requesting trading partner even if the recipient could not sign it. The returned MDN will contain information on the disposition of the message and on why the MDN could not be signed. See the Disposition field in .5 for more information. Within an EDI trading relationship, if a signed receipt is expected and is not returned, then the validity of the transaction is up to the trading partners to resolve. In general, if a signed receipt is required in the trading relationship and is not received, the transaction will likely be considered invalid.

Signed Receipt Considerations The method used to request a receipt or a signed receipt is defined in RFC 3798, "An Extensible Message Format for Message Disposition Notifications". The "rules" are as follows:

1. When a receipt is requested, explicitly specifying that the receipt be signed, then the receipt MUST be returned with a signature.
2. When a receipt is requested, explicitly specifying that the receipt be signed, but the recipient cannot support either the requested protocol format or the requested MIC algorithms, then either a signed or unsigned receipt SHOULD be returned.
3. When a signature is not explicitly requested, or if the signed receipt request parameter is not recognized by the UA, then no receipt, an unsigned receipt, or a signed receipt MAY be returned by the recipient.

NOTE: For Internet EDI, it is RECOMMENDED that when a signature is not explicitly requested, or if parameters are not recognized, the UA send back, at a minimum, an unsigned receipt. If, however, a signed receipt was always returned as a policy, whether requested or not, then any false unsigned receipts can be repudiated. When a request for a signed receipt is made, but there is an error in processing the contents of the message, a signed receipt MUST still be returned. The request for a signed receipt SHALL still be honored, though the transaction itself may not be valid. The reason why the contents could not be processed MUST be set in the "disposition-field". When a signed receipt request is made, the "Received-content-MIC" MUST always be returned to the requester (except when corruption prevents computation of the digest in accordance with the following specification). The "Received-content-MIC" MUST be calculated as follows:

MDN Format and Values This section defines the format of the AS2 Message Disposition Notification (AS2-MDN).

AS2-MDN General Formats

AS2-MDN Construction The AS2-MDN-body is formatted as a MIME multipart/report with a report-type of "disposition-notification". When the message is unsigned, the transfer-layer ("outermost") entity-headers of the AS2-MDN contain the content-type header that specifies a content-type of "multipart/report" and parameters indicating the report-type, and the value of the outermost multipart boundary. When the AS2-MDN is signed, the transfer-layer ("outermost") entity- headers of the AS2-MDN contain a content-type header that specifies a content-type of "multipart/signed" and parameters indicating the algorithm used to compute the message digest, the signature- formatting protocol (e.g., pkcs7-signature), and the value of the outermost multipart boundary. The first part of the MIME multipart/signed message is an embedded MIME multipart/report of type "disposition-notification". The second part of the multipart/signed message contains a MIME application/pkcs7-signature message. The first part of the MIME multipart/report is a "human-readable" portion containing a general description of the message disposition. The second part of the MIME multipart/report is a "machine-readable" portion that is defined as:

AS2-MDN Fields The rules for constructing the AS2-disposition-notification content are identical to the disposition-notification-content rules provided in of RFC 3798 , except that the RFC 3798 disposition- field has been replaced with the AS2-disposition-field and that the AS2-received-content-MIC field has been added. The differences between the RFC 3798 disposition-field and the AS2-disposition-field are described below. Where there are differences between this document and RFC 3798, those entity names have been changed by pre-pending "AS2-". Entities that do not differ from RFC 3798 are not necessarily further defined in this document; refer to RFC 3798, Section 7, "Collected Grammar", for the original grammar. To improve error reporting and interoperability, this specification introduces additional standardized disposition modifiers beyond those defined in and . These modifiers are used to indicate specific failure conditions that cannot be adequately represented by the existing error codes and may not be compatible with earlier implementations of AS2. Implementations MUST include a human-readable explanation in the MDN Explanation field when returning these modifiers. Future modifiers may be registered through the IANA registry for AS2 Disposition Values and Modifiers (see ). The "Received-content-MIC" extension field is set when the integrity of the received message is verified. The MIC value is the base64-encoded message-digest computed over the received message using a hash function. This field is required for signed receipts but optional for unsigned receipts. For details defining the specific content over which the message digest is to be computed, see .3.1 of this document. For signed messages, the algorithm used to calculate the MIC MUST be the same as that used on the message that was signed. If the message is not signed, then the SHA-256 algorithm SHOULD be used. SHA-1 MAY be used only for legacy interoperability (see .1). This field is set only when the content of the message is processed successfully. This field is used in conjunction with the recipient's signature on the MDN so that the sender can verify non-repudiation of receipt. AS2-MDN field names (e.g., "Disposition:", "Final-Recipient:") are case insensitive (cf. RFC 3798, Section 3.1.1). AS2-MDN action- modes, sending-modes, AS2-disposition-types, and AS2-disposition- modifier values, which are defined above, and user-supplied *( TEXT ) values are also case-insensitive. AS2 implementations MUST NOT make assumptions regarding the values supplied for AS2-MDN-warning- description or AS2-MDN-failure-description, or for the values of any (optional) error, warning, or failure fields.

AS2-MDN Field Requirements The following fields have clarified requirements for interoperability:

Additional AS2-MDN Programming Notes

Disposition Mode, Type, and Modifier

Disposition Mode Overview This section provides a brief overview of how "processed", "error", "failure", and "warning" are used.

Successful Processing Status Indication When the request for a receipt or signed receipt, and the received message contents are successfully processed by the receiving EDI UA, a receipt or MDN SHOULD be returned with the disposition-type set to "processed". When the MDN is sent automatically by the EDI UA, and there is no explicit way for a user to control the sending of the MDN, then the first part of the "disposition-mode" SHOULD be set to "automatic-action". When the MDN is being sent under user- configurable control, then the first part of the "disposition-mode" SHOULD be set to "manual-action". Since a request for a signed receipt should always be honored, the user MUST not be allowed to configure the UA to disallow sending of a signed receipt when the sender requests one. The second part of the disposition-mode is set to "MDN-sent-manually" if the user gave explicit permission for the MDN to be sent. Again, the user MUST not be allowed to explicitly refuse to send a signed receipt when the sender requests one. The second part of the "disposition-mode" is set to "MDN-sent-automatically" whenever the EDI UA sends the MDN automatically, regardless of whether the sending was under the control of a user, administrator, or the software. Because EDI content is generally handled automatically by the EDI UA, a request for a receipt or signed receipt will generally return the following in the "disposition-field": Note that this specification does not restrict the use of the "disposition-mode" just to automatic actions. Manual actions are valid as long as it is kept in mind that a request for a signed receipt MUST be honored.

Unsuccessful Processed Content The request for a signed receipt requires the use of two "disposition-notification-options", which specify the protocol format of the returned signed receipt, and the MIC algorithm used to calculate the MIC over the message content. The "disposition-field" values that should be used if the message content is being rejected or ignored (for instance, if the EDI UA determines that a signed receipt cannot be returned because it does not support the requested protocol format, the EDI UA chooses not to process the message contents itself) MUST be specified in the MDN "disposition-field" as follows: The "failed" AS2-disposition-type MUST be used when a failure occurs that prevents the proper generation of an MDN. For example, this disposition-type would apply if the sender of the message requested the application of an unsupported message-integrity-check (MIC) algorithm. The "failure:" AS2-disposition-modifier-extension SHOULD be used with an implementation-defined description of the failure. Further information about the failure may be contained in a failure-field. The syntax of the "failed" disposition-type is general, allowing the sending of any textual information along with the "failed" disposition-type. Implementations MUST support any printable textual characters after the Failure disposition-type. For use in Internet EDI, the following "failed" values are pre-defined and MUST be supported:

Unsuccessful Non-Content Processing When errors occur in processing the received message (other than content), the "disposition-field" MUST be set to the "processed" value for disposition-type and the "error" value for disposition- modifier. The "error" AS2-disposition-modifier with the "processed" disposition-type MUST be used to indicate that an error of some sort occurred that prevented successful processing of the message. Further information may be contained in an error-field. An "error:" AS2-disposition-modifier-extension SHOULD be used to combine the indication of an error with a predefined description of a specific, well-known error. Further information about the error may be contained in an error field. For Internet EDI use, the following "error" AS2-disposition-modifier values are defined: An example of how the "disposition-field" would look when errors other than those in content processing are detected is as follows:

Processing Warnings Situations arise in EDI when, even if a trading partner cannot be authenticated correctly, the trading partners still agree to continue processing the EDI transactions. Transaction reconciliation is done between the trading partners at a later time. In the content processing warning situations as described above, the "disposition- field" MUST be set to the "processed" disposition-type value, and the "warning" to the "disposition-modifier" value. The "warning" AS2-disposition-modifier MUST be used with the "processed" disposition-type to indicate that the message was successfully processed but that an exceptional condition occurred. Further information may be contained in a warning-field. A "warning:" AS2-disposition-modifier-extension SHOULD be used to combine the indication of a warning with an implementation-defined description of the warning. Further information about the warning may be contained in a warning-field. For use in Internet EDI, the following "warning" disposition-modifier-extension value is defined: An example of how the "disposition-field" would look when warning other than those for content processing are detected is as follows: Example:

Backward Compatibility with Disposition Type, Modifier, and Extension The following set of examples represents typical constructions of the Disposition field that have been in use by AS2 implementations. This is NOT an exhaustive list of possible constructions. However, AS2 implementations MUST accept constructions of this type to be backward compatible with earlier AS2 versions. The following set of examples represents allowable constructions of the Disposition field that combine the historic constructions above with optional RFC 3798 error, warning, and failure fields. AS2 implementations MAY produce these constructions. However, AS2 servers are not required to recognize or process optional error, warning, or failure fields at this time. Note that the use of the multiple error fields in the second example below provides for the indication of multiple error conditions. The following set of examples represents allowable constructions of the Disposition field that employ pure RFC 3798 Disposition-modifiers with optional error, warning, and failure fields. These examples are provided as informational only. These constructions are not guaranteed to be backward compatible with AS2 implementations prior to version 1.1.

Receipt Reply Considerations in an HTTP POST The details of the response to the POST command vary depending upon whether a receipt has been requested. With no extended header requesting a receipt, and with no errors accessing the request-URI specified processing, the status line in the Response to the POST request SHOULD be in the 200 range. Status codes in the 200 range SHOULD also be used when an entity is returned (a signed receipt in a multipart/signed content type or an unsigned receipt in a multipart/report). Even when the disposition of the data was an error condition at the authentication, decryption or other higher level, the HTTP status code SHOULD indicate success at the HTTP level. The HTTP server application may respond with an unsolicited multipart/report as a message body that the HTTP client might not have solicited, but the client may discard this. Applications SHOULD avoid emitting unsolicited receipt replies because bandwidth or processing limitations might have led administrators to suspend asking for acknowledgements. Message Disposition Notifications, when used in the HTTP reply context, follow the same semantics as those defined in . For example, the disposition field is a required element in the machine-readable second part of a multipart/report for a MDN. The final-recipient- field (, Section 3.1) value SHOULD be derived from the entity headers of the request. In an MDN, the first part of the multipart/report (the human-readable portion) SHOULD include items such as the subject, the date, and other information when those fields are present in entity header fields following the POST request. An application MUST report the Message- ID of the request in the second part of the multipart/report (the machine-readable portion). Also, an MDN SHOULD have its own unique Message-ID HTTP header. The HTTP reply SHOULD normally omit the third optional part of the multipart/report (this was historically used to return the original message or its headers within the SMTP context).

Public Key Certificate Handling The initial exchange and certification of public keys are essential steps in establishing a secure trading partnership. This process MAY occur manually during partner onboarding or automatically through supported mechanisms such as the AS2 GET method (see ). Implementations MUST maintain a database of public keys used for encryption and signature verification, together with the mapping between the EDI trading partner identifier and its associated RFC 5322 email address and HTTP URL/URI. The exact procedures for establishing and configuring secure AS2 messaging can vary among trading partners and software implementations. X.509 certificates are REQUIRED. It is RECOMMENDED that trading partners self-certify each other if an agreed-upon certification authority is not used. This applicability statement does NOT require the use of a certification authority (CA) and the use of a CA is therefore OPTIONAL. Certificates MAY be self-signed. It is RECOMMENDED that when trading partners are using S/MIME they also exchange public key certificates, considering the advice provided in . The message formats useful for certificate exchange are found in and .

Certificate Roles and Requirements While TLS certificates and AS2 message-signing certificates both use the X.509 standard, they serve distinct purposes and MUST be managed separately: Although short certificate lifetimes are now common in the TLS ecosystem due to CA/Browser Forum requirements and industry regulations, AS2 certificates generally do not require the same frequency of renewal. AS2 systems handle far fewer encrypted transactions than high-volume web servers, and certificate rollover can be operationally complex. Implementations SHOULD allow independent lifetime policies for AS2 and TLS certificates.

Certificate Exchange and Renewal Automated certificate management significantly reduces operational risk. Implementations SHOULD support Certificate Exchange Messaging (CEM) to enable secure, automated exchange of AS2 certificates between trading partners. When CEM is not available, manual exchange processes MUST ensure integrity and authentication of keys prior to activation. The AS2 GET method MAY be used for initial retrieval of partner certificates. Implementations using AS2 GET MUST ensure that certificate retrieval is authenticated (to verify the requester's identity) and authorized (to ensure only legitimate trading partners can access certificates). While certificates themselves are digitally signed by their issuer and thus tamper-evident, authentication and authorization are required to prevent unauthorized parties from obtaining certificates and using them to identify legitimate trading partners or map relationships. For self-signed certificates, additional out-of-band verification (such as fingerprint confirmation via secure channel) is REQUIRED to establish initial trust before use in production.

Operational Guidance For security and algorithm lifecycle considerations, see and Section 10.

Security Considerations This entire document is concerned with secure transport of business data, covering confidentiality, authentication, and non-repudiation. Cryptographic algorithms used for signatures, MIC calculations, and encryption are subject to modernization and deprecation guidance. The definitive algorithm requirements (for hash functions and encryption) are specified in . This section provides security rationale and additional guidance. Legacy algorithms such as SHA-1, MD5, 3DES, and RC2 are deprecated due to known weaknesses. Implementations SHOULD NOT generate these algorithms in modern implementations. However, legacy use cases may still be encountered when interoperating with older systems conforming to . See Section 1.2.1 for clarifications on legacy interoperability. Modern implementations are expected to support strong algorithms such as SHA-256 or stronger for MIC calculations, and AES (128-bit or greater) for encryption. Implementations SHOULD also support advanced AES modes such as AES-GCM and AES-CCM for improved efficiency and authenticated encryption. See for details. Implementations must ensure robust handling of all cryptographic failures. Administrators are encouraged to monitor IETF and NIST publications for algorithm lifecycle updates and to update deployed systems accordingly. These compatibility allowances are described in more detail in Section 1.2 and Section 1.2.1. When processing certificates, failures such as expired, revoked, or untrusted certificates MUST result in immediate and noticeable error reporting. See and for guidance on certificate path validation. For guidance on certificate management, key exchange, and renewal, including use of Certificate Exchange Messaging (CEM) and AS2 GET, see and .

HTTPS and TLS Requirements Consensus Update: Implementations MUST support TLS 1.2 or higher and SHOULD support TLS 1.3. TLS 1.3 is strongly encouraged as the default where platform support allows, but TLS 1.2 remains the mandatory baseline to ensure interoperability with older environments (e.g., legacy JVMs). Products SHOULD allow administrators to configure which TLS versions are enabled, so that TLS 1.3 can be mandated by policy where required. Future revisions of this specification are expected to mandate TLS 1.3 as the minimum required version once deployment is widespread. Administrators SHOULD use only cipher suites listed as “Recommended (Y)” in the IANA TLS Parameters registry. Implementations SHOULD provide configurable cipher selection rather than hardcoding cipher lists. New implementations of AS2 SHOULD use HTTPS as the default transport protocol to provide confidentiality and integrity in transit. Plain HTTP remains permitted to support message-level encryption and backward compatibility with existing deployments. This guidance promotes strong encryption, aligns with current best practices, and ensures that AS2 remains interoperable with existing deployments while allowing administrators to phase out weaker protocols and cipher suites over time. Future Considerations Per BCP 195/RFC 7525bis, new IETF protocols and major revisions are expected to mandate TLS 1.3. AS2-bis is a backward-compatible revision, and therefore specifies MUST support of TLS 1.2 and SHOULD support of TLS 1.3 to preserve interoperability with existing implementations. At the same time, implementers are strongly encouraged to deploy TLS 1.3 as the default for all new products and configurations. Future versions of AS2 (for example, a potential AS2-Version 1.3 or subsequent update) are expected to raise the baseline to MUST support TLS 1.3 or higher, and may deprecate TLS 1.2 once widespread deployment makes this feasible. Implementers are encouraged to prepare for a future revision of this specification that will mandate TLS 1.3 as the minimum required version.

TLS Server Certificates The following certificate types MUST be supported for TLS server certificates: The URL, which matches the source server identity, SHOULD be carried in the certificate. However, it is not required that DNS checks or reverse lookups to vouch for the accuracy of the URL or server value. The complete certification chain MUST be included in all certificates. All certificate verifications MUST "chain to root" or to an accepted trust anchor. Additionally, the certificate hash SHOULD match the hash recomputed by the receiver. Because server certificates are exchanged, and also trust is established during the configuration of the trading partner relationship, runtime validation (including hostname matching and certificate path validation) SHOULD be performed unless an out-of-band trust model has been explicitly agreed upon by trading partners. If a self-signed TLS certificate is used, it SHOULD contain a Subject Alternative Name (SAN) extension that includes the DNS name and/or IP address of the sender. If included, this certificate extension MUST be marked as non-critical. Note: Although not restricted by this specification, self-signed TLS certificates should be used with great care, especially in production environments.

NRR Cautions This specification seeks to provide multiple mechanisms that can be combined in accordance with local policies to achieve a wide range of security needs as determined by threat and risk analyses of the business peers. It is required that all these mechanisms be implemented by AS2 software so that the software has capabilities that promote strong interoperability, no matter what policies are adopted. One strong cluster of mechanisms (the secure transmission loop) can provide good support for meeting the evidentiary needs of non- repudiation of receipt by the original sender and by a third party supplied with all stated evidence. However, this specification does not itself define non-repudiation of receipt nor enumerate its essential properties because NRR is a business analysis and/or legal requirement, and not relevantly defined by a technical applicability statement. Some analyses observe that non-repudiation of receipt presupposes that non-repudiation of the sender of the original message is obtained, and further that non-repudiation should be implemented by means of digital signature on the original message. To satisfy strict NRR evidence, authentication and integrity MUST be provided by some mechanism, and the RECOMMENDED mechanism is digital signatures on both the original message and the receipt message. Given that this specification has selected several mechanisms that can be combined in several ways, it is important to realize that if a digital signature is omitted from the original message, in order to satisfy the preceding analysis of NRR requirements, some authentication mechanism MUST accompany the request for a signed receipt and its included Received-content-MIC value. This authentication might come from using client-side SSL, authentication via IPsec, or HTTP authentication (while using SSL). In any case, records of the message content, its security basis, and the digest value need to be retained for the NRR process. Therefore, if NRR is one of the goals of the policy that is adopted, by using the mechanisms of the secure transmission loop mentioned above and by retaining appropriate records of authentication at the original message sender site, strong evidentiary requirements proposed for NRR can be fulfilled. Other ways of proceeding may fall short of fulfilling the most stringent sets of evidence required for NRR to obtain, but may nevertheless be part of a commercial trading agreement and, as such, are good enough for the parties involved. However, if MDNs are returned unsigned, evidentiary requirements for NRR are weak; some authentication of the identity of the receiver is needed. If TLS is used for transport, the guidance in applies.

Replay Remark Because business data documents normally contain transaction ids, replays (such as resends of not-yet-acknowledged messages) are discarded as part of the normal process of duplicate detection. Detection of duplicates by Message-Id or by business transaction identifiers is recommended.

IANA Considerations IANA is requested to update the following registries:

Registration RFC 4130 originally defined an extension to the Message Disposition Notification (MDN) protocol for a disposition-modifier in the Disposition field of a body of content-type "message/disposition-notification". This document updates that definition, and IANA is requested to replace RFC 4130 with this document as the reference for the MDN Disposition Modifier Names registry.

Disposition Modifier 'warning'

Acknowledgments Carl Hage, Karen Rosenfeld, Chuck Fenton, Russ Housley, Marc Blanchet, Erik Wrammer, and many others provided valuable suggestions during both the initial review of RFC 4130 that improved that applicability statement and this bis specification. The authors would also like to thank the past and current vendors who have participated in the Drummond AS2 interoperability testing. Their contributions have ultimately led to great improvement in the clarity of this document.

References Normative References This document provides an applicability statement (RFC 2026, Section 3.2) that describes how to exchange structured business data securely using the HTTP transfer protocol, instead of SMTP; the applicability statement for SMTP is found in RFC 3335. Structured business data may be XML; Electronic Data Interchange (EDI) in either the American National Standards Committee (ANSI) X12 format or the UN Electronic Data Interchange for Administration, Commerce, and Transport (UN/EDIFACT) format; or other structured data formats. The data is packaged using standard MIME structures. Authentication and data confidentiality are obtained by using Cryptographic Message Syntax with S/MIME security body parts. Authenticated acknowledgements make use of multipart/signed Message Disposition Notification (MDN) responses to the original HTTP message. This applicability statement is informally referred to as "AS2" because it is the second applicability statement, produced after "AS1", RFC 3335. [STANDARDS-TRACK] This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] This second document defines the general structure of the MIME media typing system and defines an initial set of media types. [STANDARDS-TRACK] This set of documents, collectively called the Multipurpose Internet Mail Extensions, or MIME, redefines the format of messages. This fifth and final document describes MIME conformance criteria as well as providing some illustrative examples of MIME message formats, acknowledgements, and the bibliography. [STANDARDS-TRACK] Since there are many different EDI specifications, the current document defines three distinct categories as three different MIME content-types. [STANDARDS-TRACK] HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as "HTTP/1.1", and is an update to RFC 2068. [STANDARDS-TRACK] This memo defines a MIME content-type that may be used by a mail user agent (MUA) or electronic mail gateway to report the disposition of a message after it has been successfully delivered to a recipient. This content-type is intended to be machine-processable. Additional message headers are also defined to permit Message Disposition Notifications (MDNs) to be requested by the sender of a message. The purpose is to extend Internet Mail to support functionality often found in other messaging systems, such as X.400 and the proprietary "LAN-based" systems, and often referred to as "read receipts," "acknowledgements", or "receipt notifications." The intention is to do this while respecting privacy concerns, which have often been expressed when such functions have been discussed in the past. Because many messages are sent between the Internet and other messaging systems (such as X.400 or the proprietary "LAN-based" systems), the MDN protocol is designed to be useful in a multi-protocol messaging environment. To this end, the protocol described in this memo provides for the carriage of "foreign" addresses, in addition to those normally used in Internet Mail. Additional attributes may also be defined to support "tunneling" of foreign notifications through Internet Mail. [STANDARDS-TRACK] This memo defines a MIME content type that may be used by a Mail User Agent (MUA) or electronic mail gateway to report the disposition of a message after it has been successfully delivered to a recipient. This content type is intended to be machine processable. Additional message header fields are also defined to permit Message Disposition Notifications (MDNs) to be requested by the sender of a message. The purpose is to extend Internet Mail to support functionality often found in other messaging systems, such as X.400 and the proprietary "LAN-based" systems, and are often referred to as "read receipts," "acknowledgements," or "receipt notifications." The intention is to do this while respecting privacy concerns, which have often been expressed when such functions have been discussed in the past. Because many messages are sent between the Internet and other messaging systems (such as X.400 or the proprietary "LAN-based" systems), the MDN protocol is designed to be useful in a multiprotocol messaging environment. To this end, the protocol described in this memo provides for the carriage of "foreign" addresses, in addition to those normally used in Internet Mail. Additional attributes may also be defined to support "tunneling" of foreign notifications through Internet Mail. This document is an Internet Standard. It obsoletes RFC 3798 and updates RFC 2046 (message/partial media type handling) and RFC 3461 (Original-Recipient header field generation requirement). 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. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.2. 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 3851. [STANDARDS-TRACK] This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] The Multipart/Report Multipurpose Internet Mail Extensions (MIME) content-type is a general "family" or "container" type for electronic mail reports of any kind. Although this memo defines only the use of the Multipart/Report content-type with respect to delivery status reports, mail processing programs will benefit if a single content-type is used to for all kinds of reports. This document is part of a four document set describing the delivery status report service. This collection includes the Simple Mail Transfer Protocol (SMTP) extensions to request delivery status reports, a MIME content for the reporting of delivery reports, an enumeration of extended status codes, and a multipart container for the delivery report, the original message, and a human-friendly summary of the failure. [STANDARDS-TRACK] This document standardizes five new media types -- text/xml, application/xml, text/xml-external-parsed-entity, application/xml- external-parsed-entity, and application/xml-dtd -- for use in exchanging network entities that are related to the Extensible Markup Language (XML). This document also standardizes a convention (using the suffix '+xml') for naming media types outside of these five types when those media types represent XML MIME (Multipurpose Internet Mail Extensions) entities. [STANDARDS-TRACK] This memo documents the process used by the Internet community for the standardization of protocols and procedures. It defines the stages in the standardization process, the requirements for moving a document between stages and the types of documents used during this process. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies conventions for X.509 certificate usage by Secure/Multipurpose Internet Mail Extensions (S/MIME) agents. S/MIME provides a method to send and receive secure MIME messages, and certificates are an integral part of S/MIME agent processing. S/MIME agents validate certificates as described in RFC 3280, the Internet X.509 Public Key Infrastructure Certificate and CRL Profile. S/MIME agents must meet the certificate processing requirements in this document as well as those in RFC 3280. [STANDARDS-TRACK] In the early days of the Arpanet, each specification contained its own definition of ABNF. This included the email specifications, RFC733 and then RFC822 which have come to be the common citations for defining ABNF. The current document separates out that definition, to permit selective reference. Predictably, it also provides some modifications and enhancements. [STANDARDS-TRACK] 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. 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. This document defines a format for using compressed data as a Cryptographic Message Syntax (CMS) content type. Compressing data before transmission provides a number of advantages, including the elimination of data redundancy which could help an attacker, speeding up processing by reducing the amount of data to be processed by later steps (such as signing or encryption), and reducing overall message size. Although there have been proposals for adding compression at other levels (for example at the MIME or SSL level), these don't address the problem of compression of CMS content unless the compression is supplied by an external means (for example by intermixing MIME and CMS). [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 Certificate Revocation List (CRL) for use in the Internet. [STANDARDS-TRACK] 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. 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] With the maturity of the Electronic Data Interchange - Internet Integration (EDIINT) standards of AS1, AS2, and AS3, applications and additional features are being built upon the basic secure transport functionality. These features are not necessarily supported by all EDIINT applications and could cause potential problems with implementations. The EDIINT-Features header field provides a means to resolve these problems and support new functionality. This document is not an Internet Standards Track specification; it is published for informational purposes. Informative References This document specifies Version 1.0 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications privacy over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. Web Distributed Authoring and Versioning (WebDAV) consists of a set of methods, headers, and content-types ancillary to HTTP/1.1 for the management of resource properties, creation and management of resource collections, URL namespace manipulation, and resource locking (collision avoidance). RFC 2518 was published in February 1999, and this specification obsoletes RFC 2518 with minor revisions mostly due to interoperability experience. [STANDARDS-TRACK] This document describes how to use Elliptic Curve Cryptography (ECC) public key algorithms in the Cryptographic Message Syntax (CMS). The ECC algorithms support the creation of digital signatures and the exchange of keys to encrypt or authenticate content. The definition of the algorithm processing is based on the NIST FIPS 186-3 for digital signature, NIST SP800-56A and SEC1 for key agreement, RFC 3370 and RFC 3565 for key wrap and content encryption, NIST FIPS 180-3 for message digest, SEC1 for key derivation, and RFC 2104 and RFC 4231 for message authentication code standards. This document obsoletes RFC 3278. This document is not an Internet Standards Track specification; it is published for informational purposes. Procter & Gamble Orion Health One goal of this document is to define approaches to achieve a "once and only once" delivery of messages. The EDIINT AS2 protocol is implemented by a number of software tools on a variety of platforms with varying capabilities and with varying network service quality. Although the AS2 protocol defines a unique "Message-ID", current implementations of AS2 do not provide a standard method to prevent the same message (re-transmitted by the initial sender) from reaching back-end business applications at the initial receiver. A second goal is to reduce retransmissions and failures when AS2 is used in a synchronous mode for transmitting MDNs. There can be a large latency between receipt of the POSTed entity body and the MDN response caused by the operations of decompressing, decrypting, and signature checks. Uncoordinated timeout policies and intermediate devices dropping connections have interfered with reliable data exchange. The use of an HTTP 102(Processing) status code is described to mitigate these difficulties. Use of these reliability features is indicated by presence of the "AS2-Reliability" value in the EDIINT-Features header. Intended Status The intent of this document is to be placed on the RFC track as an Informational RFC. Feedback Instructions: NOTE TO RFC EDITOR: This section should be removed by the RFC editor prior to publication. If you want to provide feedback on this draft, follow these guidelines: -Send feedback via e-mail to the ietf-ediint list for discussion, with "AS2 Reliability" in the Subject field. To enter or follow the discussion, you need to subscribe to ietf-ediint@imc.org. -Be specific as to what section you are referring to, preferably quoting the portion that needs modification, after which you state your comments. -If you are recommending some text to be replaced with your suggested text, again, quote the section to be replaced, and be clear on the section in question. AS2 Restart provides a method for AS2 clients and servers to restart payload transfers from the point of failure without requiring the entire document to be resent. Keywords The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119. Drummond Group Inc. Axway, Inc. The EDIINT AS1, AS2 and AS3 message formats do not currently contain any neutral provisions for transporting and exchanging trading partner profiles or digital certificates. EDIINT Certificate Exchange Messaging provides the format and means to effectively exchange certificates for use within trading partner relationships. The messaging consists of two types of messages, Request and Response, which allow trading partners to communicate certificates, their intended usage and their acceptance through XML. Certificates can be specified for use in digital signatures, data encryption or SSL/TLS over HTTP (HTTPS).

Message Examples Note to Readers: All examples are provided for illustration only, and are not part of the protocol specification. If an example conflicts with the protocol definitions, the example is wrong. Email addresses in the examples (e.g., in the Disposition-Notification-To field) reflect one valid option but are not required. In AS2, this field may also contain a URL, a fully qualified host name, an AS2 identifier, or another implementation-specific string, as described in .3.

Signed Message Requesting a Signed, Synchronous Receipt Disposition-Notification-To: mrAS2@example.com Disposition-Notification-Options: signed-receipt-protocol=optional, pkcs7-signature; signed-receipt-micalg=optional,sha-256 Content-Type: multipart/signed; boundary="as2BouNdary1as2"; protocol="application/pkcs7-signature"; micalg=sha-256 Content-Length: 2464 --as2BouNdary1as2 Content-Type: application/edi-x12 Content-Disposition: attachment; filename=rfc1767.dat {ISA ...EDI transaction data...IEA...} --as2BouNdary1as2 Content-Type: application/pkcs7-signature {omitted binary pkcs7 signature data} --as2BouNdary1as2-- ]]>

MDN for Message in A.1, Above Content-Type: multipart/signed; micalg=sha-256; protocol="application/pkcs7-signature"; boundary="----=_Part_57_648441049.1028122454671" Connection: Close Content-Length: 1980 ------=_Part_57_648441049.1028122454671 & Content-Type: multipart/report; & report-type=disposition-notification; & boundary="----=_Part_56_1672293592.1028122454656" & &------=_Part_56_1672293592.1028122454656 &Content-Type: text/plain &Content-Transfer-Encoding: 7bit & &MDN for - & Message ID: <200207310834482A70BF63@\"~~foo~~\"> & From: "as2 Name" & To: 0123456780000 & Received on: 2025-07-31 at 09:34:14 (EDT) & Status: processed & Comment: This is not a guarantee that the message has & been completely processed or &understood by the receiving & translator & &------=_Part_56_1672293592.1028122454656 &Content-Type: message/disposition-notification &Content-Transfer-Encoding: 7bit & &Reporting-UA: AS2 Server &Original-Recipient: rfc822; 0123456780000 &Final-Recipient: rfc822; 0123456780000 &Original-Message-ID: <200207310834482A70BF63@\"~~foo~~\"> &Received-content-MIC: 43d9tGY3gNSGuFaut4PAGvuc+48VgW6USgXLDPTxsBU=, sha-256 &Disposition: automatic-action/MDN-sent-automatically; processed & &------=_Part_56_1672293592.1028122454656-- ------=_Part_57_648441049.1028122454671 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQ cp24hMJNbxDKHnlB9jTiQzLwSwo+/90Pc87x+Sc6EpFSUYWGAAAAAAAA ------=_Part_57_648441049.1028122454671-- Notes: 1. The lines proceeded with "&" are what the signature is calculated over. 2. For details on how to prepare the multipart/signed with protocol = "application/pkcs7-signature", see the "S/MIME Message Specification, PKCS Security Services for MIME". 3. Note that the textual first body part of the multipart/report can be used to include a more detailed explanation of the error conditions reported by the disposition headers. The first body part of the multipart/report, when used in this way, allows a person to better diagnose a problem in detail. 4. As specified by RFC 3462 [RFC3462], returning the original or portions of the original message in the third body part of the multipart/report is not required. This is an optional body part. However, it is RECOMMENDED that this body part be omitted or left blank. ]]>

Signed, Encrypted Message Requesting a Signed, Asynchronous Receipt Date: Thu, 19 Dec 2024 15:04:18 GMT Subject: Signed and encrypted message with async MDN request Mime-Version: 1.0 Content-Type: application/pkcs7-mime; smime-type=enveloped-data; name=smime.p7m Content-Transfer-Encoding: binary Content-Disposition: attachment; filename=smime.p7m Recipient-Address: 10.240.1.2// Disposition-Notification-To: http://10.240.1.2:58201/exchange/as2_company Disposition-Notification-Options: signed-receipt-protocol=optional, pkcs7-signature; signed-receipt-micalg=optional,sha-256 Receipt-Delivery-Option: http://10.240.1.2:58201/exchange/as2_company AS2-From: as2_company AS2-To: "AS2 Test" AS2-Version: 1.3 Connection: close Content-Length: 3428 {omitted binary encrypted data} ]]>

Asynchronous MDN for Message in A.3, Above AS2-Version: 1.3 Mime-Version: 1.0 Recipient-Address: http://10.240.1.2:58201/exchange/as2_company AS2-To: as2_company AS2-From: "AS2 Test" Subject: Your Requested MDN Response From: as2debug@example.com Accept-Encoding: deflate, gzip, x-gzip, compress, x-compress Content-Type: multipart/signed; micalg=sha-256; protocol="application/pkcs7-signature"; boundary="----=_Part_337_6452266.1040310218750" Content-Length: 3103 ------=_Part_337_6452266.1040310218750 Content-Type: multipart/report; report-type=disposition-notification; boundary="----=_Part_336_6069110.1040310218718" ------=_Part_336_6069110.1040310218718 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit The message sent to Recipient on Thu, 19 Dec 2024 15:04:18 GMT with Subject has been received. The EDI Interchange was successfully decrypted, and its integrity was verified. In addition, the sender of the message, Sender at Location http://10.240.1.2:58201/exchange/as2_company was authenticated as the originator of the message. There is no guarantee, however, that the EDI interchange was syntactically correct, or that it was received by the EDI application/translator. ------=_Part_336_6069110.1040310218718 Content-Type: message/disposition-notification Content-Transfer-Encoding: 7bit Reporting-UA: AS2@test:8101 Original-Recipient: rfc822; "AS2 Test" Final-Recipient: rfc822; "AS2 Test" Original-Message-ID: <#as2_company#01#a4260as2_companyout#> Disposition: automatic-action/MDN-sent-automatically; processed Received-Content-MIC: Hes6my+vIxIYxmvsA+MNpEOTPAc=, sha1 ------=_Part_336_6069110.1040310218718-- ------=_Part_337_6452266.1040310218750 Content-Type: application/pkcs7-signature; name=smime.p7s Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=smime.p7s BhbWjEfbyXoTAS/H0zpnEqLqbaBh29y2v82b8bdeGw8pipBQWmf53hIcqHGM 4ZBF3CHw5Wrf1JIE+8TwOzdbal30zeChw88WfRfD7c/j1fIA8sxsujvf2d9j UxCUga8BVdVB9kH0Geexytyt0KvWQXfaEEcgZGUAAAAAAAA= ------=_Part_337_6452266.1040310218750- ]]>

Change Log (Non-Normative) This appendix records the substantive changes made during the revision from the original RFC 4130 draft toward the current version of this document.

General

• Removed all references to AS1/SMTP throughout the draft.
• Included descriptions and references for compressed content, which was previously supported since AS2 version 1.1.
• This draft explicitly allows negotiation of newer RFCs while retaining legacy support, where necessary.
• Updated formatting and cross-references so that all internal "Section X.Y" references are properly linked in HTML and XML output.
• Moved legacy interoperability clarifications to Section 1.2.1 to avoid confusion with normative text.
• Clarified that appendices are non-normative unless otherwise indicated.
• Replaced all references to RFC 3851 with RFC 5751.
• Replaced all references to RFC 3852 with RFC 5652.

Changes affecting Section 1.2 - Backward Compatibility and Interoperability

• Expanded to explicitly state the dual-reference policy:
  • Implementations MAY interoperate with S/MIME v3.2 (RFC 5751, which obsoletes RFC 3851/3852).
  • Conformant implementations MUST also support S/MIME v4.0 (RFC 8551, which obsoletes RFC 5751).
• UPDATED (Technical Review): Clarified that S/MIME 4.0 is the baseline for conformant implementations, with explicit guidance on when to use AuthEnvelopedData (S/MIME 4.0 with AES-GCM/AES-CCM) versus EnvelopedData (S/MIME 3.2 with AES-CBC or for backward compatibility).
• Added explicit pointer to Section 1.2.1 for legacy interoperability clarifications.
• Clarified that RFC 8551 forms the baseline for new implementations.
• Added new Section 1.2.1: Legacy Interoperability (non-normative clarifications):
  • Captures behavior when interoperating with RFC 4130 systems.
  • Explicit note: 3DES withdrawn by NIST in 2024; MAY only be accepted for backward compatibility, SHOULD NOT be generated.

Changes affecting Section 5.1 - AS2-Version Header

• Retained definitions for AS2-Version 1.0, 1.1, and included the previously supported version 1.2.
• Added AS2-Version: 1.3:
  • Defines modernization requirements (SHA-256, AES baseline per RFC 8551).
  • Aligns MDN behavior with RFC 8098.
  • Requires support for multiple-recipient encryption (Section 7.2).
  • States weak algorithms (SHA-1, MD5, 3DES, RC2) SHOULD NOT be generated by conformant 1.3 implementations.
  • Points to Section 1.2.1 for legacy interoperability when communicating with 1.2 or earlier partners.
• Added note about future versioning: minor versions (1.x) remain backward compatible; major version (2.0+) would indicate non-backward-compatible revisions.

Changes affecting Section 5.3.3 — Message-Id and Original-Message-Id

• Prohibit spaces and control characters in newly generated Message-Id values; recommend removal (not substitution) when constructing from other attributes.
• Clarify receiver behavior: implementations are not required to accept malformed Message-Id values; they SHOULD return an MDN with processed/error and a human-readable explanation (per ).
• Keep angle-bracket guidance; brackets are required on send and are not part of the identifier value. Receivers SHOULD NOT reject solely for missing brackets.
• Update normative reference to .

Changes affecting Sections 5.4 and 5.5 — Reliability and Restart

• Expanded Section 5.4 to clarify that HTTP 102 (Processing) MAY be used under HTTP/1.1 for progress indication but MUST NOT be used under HTTP/2 or HTTP/3.
• Changed previous requirement to close connections before retry to optional behavior; clarified that 102 is deprecated but still permitted for backward compatibility.
• Expanded Section 5.5 to reference the AS2 Reliability () and AS2 Restart () drafts.
• Clarified that implementations SHOULD support configurable retry logic and MAY implement restart for large or interrupted transfers.
• Added explicit normative language about duplicate detection using Message-ID.

Changes affecting Section 6 — Additional AS2-Specific HTTP Headers

• Added new AS2-Product header to identify the sending product and version.
• Defined header format as <product-name>:<major.minor[.patch]>.
• Required inclusion for AS2-Version 1.3 and possibly 2.0 later.
• Clarified that the header enables interoperability diagnostics and implementation-specific workarounds, not capability negotiation.
• Explicitly stated that arbitrary or misleading product names MUST NOT be used.

Changes affecting Section 7 - Algorithm Requirements

• Introduced a new dedicated section consolidating algorithm requirements.
• Hash Algorithms:
  • MUST support SHA-256.
  • SHOULD support SHA-384 or stronger.
  • SHA-1 and MD5 deprecated; SHOULD NOT be generated by conformant implementations.
• Encryption Algorithms:
  • MUST support AES-128-CBC; SHOULD support AES-256-CBC.
  • RECOMMENDED to support AES-GCM and AES-CCM modes.
  • 3DES and RC2 deprecated; SHOULD NOT be generated.
  • SHOULD support multiple-recipient encryption (per RFC 8551 §3.3).
• UPDATED (Technical Review): Added comprehensive key management algorithm requirements:
  • MUST support RSA with minimum 2048-bit key length
  • MAY support ECDH and Diffie-Hellman (RFC 5753)
  • For elliptic curves, SHOULD support NIST P-256 or stronger
• UPDATED (Technical Review): Added new Section 7.2 subsections:
  • "EnvelopedData vs AuthEnvelopedData" - Explicit rules for when to use each:
    • AuthEnvelopedData MUST be used with AES-GCM and AES-CCM
    • EnvelopedData MUST be used with AES-CBC and for S/MIME 3.2 compatibility
    • Single content encryption algorithm MUST be used for all recipients
  • "Multiple-Recipient Encryption" - Explains support for multiple recipients of the same content-encryption key
• Added explicit cross-references to Section 1.2.1 for legacy interoperability.

Changes affecting Section 8 - MDN Processing

• Updated to align MDN behavior with RFC 8098 (superseding RFC 3798).
• Clarified semantics for signed-receipt-micalg:
  • Allow multiple algorithms in header for backward compatibility.
  • Conformance requires selecting one algorithm in Received-content-MIC.
  • SHA-256 set as the default minimum; SHA-1 only permitted for legacy use.
• Relaxed constraints on the content of the Disposition-notification-to header.
• Definitions have been included for additional supported error dispositions.0
• Clarified required MDN fields:
  • Final-Recipient — MUST always be present.
  • Original-Message-ID — REQUIRED and must match the original message exactly.
  • Message-ID in the MDN — optional.
  • Disposition-Notification-To — MAY use any format (email, URL, hostname); receiving systems MUST ignore syntax issues per RFC 4130.
• Updated asynchronous MDN handling to reflect practical implementation realities:
  • HTTP 200-level responses SHOULD be sent immediately after receiving the last byte, before full decryption or validation, to minimize timeout risk.
  • Persistent (keep-alive) connections MAY be used; closing is optional and implementation-dependent.
  • Receipt of a 200-level response only acknowledges receipt, not successful processing.
• Emphasized that asynchronous MDNs are sent as independent HTTP messages per Section 7.3, and clarified that connection handling should not be mandated at the application layer.
• Added standardized disposition-modifier extensions to improve error reporting.
• New recommended modifiers:
  • error: decompression-failed
  • error/duplicate-filename
  • error/illegal-filename
  • error: insufficient-message-security
  • error/invalid-message-id
  • error/unknown-trading-relationship
• Clarified that implementations returning these modifiers MUST include a human-readable explanation in the MDN Explanation field.

Changes affecting Section 9 — Public Key Certificate Handling

• Revised the opening paragraph to reference the optional AS2 GET method, in addition to manual partner onboarding.
• Expanded the section to clarify separate roles and lifecycle management for TLS certificates (transport security) and AS2 certificates (message signing and encryption).
• UPDATED (Technical Review): Strengthened requirement that TLS and AS2 certificates MUST NOT be the same certificate (changed from SHOULD NOT). Using the same certificate for both purposes creates security dependencies and operational risks that must be avoided.
• Required a minimum RSA key length of 2048 bits (or equivalent elliptic-curve strength such as P-256).
• Clarified that:
  • TLS certificates SHOULD be CA-signed in production; self-signed certificates MAY be used in test environments or by partner agreement, provided they include a Subject Alternative Name (SAN) extension with hostname and/or IP address.
  • UPDATED (Technical Review): Added reference to CA/Browser Forum Baseline Requirements (https://cabforum.org/baseline-requirements-documents/) for public-facing TLS certificates.
  • AS2 certificates MAY be CA-issued or self-signed per partner policy.
• Added guidance that AS2 certificate lifetimes need not mirror the short renewal cycles of TLS certificates; renewal policies SHOULD be independent.
• Recommended CEM for automated certificate exchange between partners to reduce manual errors and downtime.
• UPDATED (Technical Review): Clarified AS2 GET certificate retrieval requirements to focus on authentication (verify requester identity) and authorization (ensure only legitimate partners can access certificates) rather than just integrity protection. While certificates are digitally signed and thus tamper-evident, authentication and authorization prevent unauthorized parties from obtaining certificates and mapping trading partner relationships. For self-signed certificates, added requirement for out-of-band fingerprint verification before production use.
• Added operational recommendations:
  • Maintain separate TLS / AS2 certificates.
  • Include SAN extensions in all self-signed certificates.
  • Support configurable expiry-notification mechanisms.
  • UPDATED (Technical Review): Administrators MUST NOT (strengthened from SHOULD NOT) reuse TLS certificates for AS2 message security.

Changes affecting Section 10 - Security Considerations

• Provided guidance for the usgae of HTTPS and the minimum and recommended usage of TLS versions.
• Expanded discussion of algorithm lifecycle:
  • SHA-1 and MD5 deprecated; 3DES formally withdrawn by NIST (2024).
  • Implementations SHOULD NOT generate deprecated algorithms.
  • Migration guidance provided for interoperability.
• Added explicit references back to Section 7 (Algorithm Requirements) and Section 1.2.1 (Legacy Interoperability).

Changes affecting Section 11 - IANA Considerations

• Clarified that IANA must update existing MDN registries to reference this specification (replacing RFC 4130).
• Added direct links to IANA registry pages for clarity.

Updated Message Examples

• Appendix A: Updated Message Examples with newer algorithms.

Formatting and Editorial Updates (Technical Review) The following formatting and editorial improvements were made based on technical review feedback:

• Section 1.1 (Applicable RFCs):
  • Updated RFC references to reflect proper obsolescence chain (RFC 3851 -> RFC 5751 -> RFC 8551)
  • Added RFC 5751 and RFC 5652 to normative references
  • Added explicit explanation of when to use AuthEnvelopedData vs EnvelopedData based on encryption algorithm choice
• Section 1.3 (Algorithm Coverage):
  • Expanded hash function section to include encryption algorithms
  • Added key management algorithm requirements for ECDH
  • Added RFC 5753 to informative references
• Section 6 (AS2-Specific HTTP Headers):
  • Reformatted AS2-Version header descriptions to comply with RFC formatting requirements (72-character line limit)
  • Removed markdown artifacts {:format="none"} from all cross-references throughout the document (24 instances)
• RFC Reference Sections:
  • Moved RFC citations from section titles to first sentence of each section (9 sections in "Referenced RFCs and Their Contributions")
  • Example: "## RFC 2616 HTTP v1.1 " became "## RFC 2616 HTTP v1.1" with " specifies..." in text
• Capitalization:
  • Corrected "internet" to "Internet" (2 instances)
  • Ensured consistent capitalization throughout document
• Cross-References:
  • Standardized all internal section references to use plain markdown format without formatting directives
• Bullet Formatting:
  • Standardized all sections requiring bullets to use the same type and and same spacing and margins.

These updates improve RFC formatting compliance and document clarity while maintaining all technical content and backward compatibility requirements.