Aries RFC 0044: DIDComm File and MIME Types

• Authors: Daniel Hardman, Kyle Den Hartog
• Status: ACCEPTED
• Since: 2021-04-15
• Status Note: Revised as part of AIP 2.0 to ease the upcoming transition from DIDComm v1 to DIDComm v2.
• Start Date: 2018-11-13
• Tags: feature

Summary

Defines the media (MIME) types and file types that hold DIDComm messages in encrypted, signed, and plaintext forms. Covers DIDComm V1, plus a little of V2 to clarify how DIDComm versions are detected.

Motivation

Most work on DIDComm so far has assumed HTTP as a transport. However, we know that DID communication is transport-agnostic. We should be able to say the same thing no matter which channel we use.

An incredibly important channel or transport for messages is digital files. Files can be attached to messages in email or chat, can be carried around on a thumb drive, can be backed up, can be distributed via CDN, can be replicated on distributed file systems like IPFS, can be inserted in an object store or in content-addressable storage, can be viewed and modified in editors, and support a million other uses.

We need to define how files and attachments can contain DIDComm messages, and what the semantics of processing such files will be.

Tutorial

Media Types

Media types are based on the conventions of RFC6838. Similar to RFC7515, the application/ prefix MAY be omitted and the recipient MUST treat media types not containing / as having the application/ prefix present.

DIDComm v1 Encrypted Envelope (*.dee)

The raw bytes of an encrypted envelope may be persisted to a file without any modifications whatsoever. In such a case, the data will be encrypted and packaged such that only specific receiver(s) can process it. However, the file will contain a JOSE-style header that can be used by magic bytes algorithms to detect its type reliably.

The file extension associated with this filetype is dee, giving a globbing pattern of *.dee; this should be be read as "STAR DOT D E E" or as "D E E" files.

The name of this file format is "DIDComm V1 Encrypted Envelope." We expect people to say, "I am looking at a DIDComm V1 Encrypted Envelope", or "This file is in DIDComm V1 Encrypted Envelope format", or "Does my editor have a DIDComm V1 Encrypted Envelope plugin?"

Although the format of encrypted envelopes is derived from JSON and the JWT/JWE family of specs, no useful processing of these files will take place by viewing them as JSON, and viewing them as generic JWEs will greatly constrain which semantics are applied. Therefore, the recommended MIME type for *.dee files is application/didcomm-envelope-enc, with application/jwe as a fallback, and application/json as an even less desirable fallback. (In this, we are making a choice similar to the one that views *.docx files primarily as application/msword instead of application/xml.) If format evolution takes place, the version could become a parameter as described in RFC 1341: application/didcomm-envelope-enc;v=2.

A recipient using the media type value MUST treat it as if “application/” were prepended to any "typ" or "cty" value not containing a ‘/’ in compliance with the JWE /JWS family of specs.

The default action for DIDComm V1 Encrypted Envelopes (what happens when a user double-clicks one) should be Handle (that is, process the message as if it had just arrived by some other transport), if the software handling the message is an agent. In other types of software, the default action might be to view the file. Other useful actions might include Send, Attach (to email, chat, etc), Open with agent, and Decrypt to *.dp.

NOTE: The analog to this content type in DIDComm v2 is called a "DIDComm Encrypted Message." Its format is slightly different. For more info, see Detecting DIDComm Versions below.

DIDComm V1 Signed Envelopes (*.dse)

When DIDComm messages are signed, the signing uses a JWS signing envelope. Often signing is unnecessary, since authenticated encryption proves the sender of the message to the recipient(s), but sometimes when non-repudiation is required, this envelope is used. It is also required when the recipient of a message is unknown, but tamper-evidence is still required, as in the case of a public invitation.

By convention, DIDComm Signed Envelopes contain plaintext; if encryption is used in combination with signing, the DSE goes inside the DEE.

The file extension associated with this filetype is dse, giving a globbing pattern of *.dse; this should be be read as "STAR DOT D S E" or as "D S E" files.

The name of this file format is "DIDComm V1 Signed Envelope." We expect people to say, "I am looking at a DIDComm V1 Signed Envelope", or "This file is in DIDComm V1 Signed Envelope format", or "Does my editor have a DIDComm V1 Signed Envelope plugin?"

As with *.dee files, the best way to hande *.dse files is to map them to a custom MIME type. The recommendation is application/didcomm-sig-env, with application/jws as a fallback, and application/json as an even less desirable fallback.

A recipient using the media type value MUST treat it as if “application/” were prepended to any "typ" or "cty" value not containing a ‘/’ in compliance with the JWE /JWS family of specs.

The default action for DIDComm V1 Signed Envelopes (what happens when a user double-clicks one) should be Validate (that is, process the signature to see if it is valid.

NOTE: The analog to this content type in DIDComm v2 is called a "DIDComm Signed Message." Its format is slightly different. For more info, see Detecting DIDComm Versions below.

DIDComm V1 Messages (*.dm)

The plaintext representation of a DIDComm message--something like a credential offer, a proof request, a connection invitation, or anything else worthy of a DIDComm protocol--is JSON. As such, it should be editable by anything that expects JSON.

However, all such files have some additional conventions, over and above the simple requirements of JSON. For example, key decorators have special meaning ( @id, ~thread, @trace , etc). Nonces may be especially significant. The format of particular values such as DID and DID+key references is important. Therefore, we refer to these messages generically as JSON, but we also define a file format for tools that are aware of the additional semantics.

The file extension associated with this filetype is *.dm, and should be read as "STAR DOT D M" or "D M" files. If a format evolution takes place, a subsequent version could be noted by appending a digit, as in *.dm2 for second-generation dm files.

The name of this file format is "DIDComm V1 Message." We expect people to say, "I am looking at a DIDComm V1 Message", or "This file is in DIDComm V1 Message format", or "Does my editor have a DIDComm V1 Message plugin?" For extra clarity, it is acceptable to add the adjective "plaintext", as in "DIDComm V1 Plaintext Message."

The most specific MIME type of *.dm files is application/json;flavor=didcomm-msg--or, if more generic handling is appropriate, just application/json.

A recipient using the media type value MUST treat it as if “application/” were prepended to any "typ" or "cty" value not containing a ‘/’ in compliance with the JWE /JWS family of specs.

The default action for DIDComm V1 Messages should be to View or Validate them. Other interesting actions might be Encrypt to *.dee, Sign to *.dse, and Find definition of protocol.

NOTE: The analog to this content type in DIDComm v2 is called a "DIDComm Plaintext Message." Its format is slightly different. For more info, see Detecting DIDComm Versions below.

As a general rule, DIDComm messages that are being sent in production use cases of DID communication should be stored in encrypted form (*.dee) at rest. There are cases where this might not be preferred, e.g., providing documentation of the format of message or during a debugging scenario using message tracing. However, these are exceptional cases. Storing meaningful *.dm files decrypted is not a security best practice, since it replaces all the privacy and security guarantees provided by the DID communication mechanism with only the ACLs and other security barriers that are offered by the container.

Native Object representation

This is not a file format, but rather an in-memory form of a DIDComm Message using whatever object hierarchy is natural for a programming language to map to and from JSON. For example, in python, the natural Native Object format is a dict that contains properties indexed by strings. This is the representation that python's json library expects when converting to JSON, and the format it produces when converting from JSON. In Java, Native Object format might be a bean. In C++, it might be a std::map<std::string, variant>...

There can be more than one Native Object representation for a given programming language.

Native Object forms are never rendered directly to files; rather, they are serialized to DIDComm Plaintext Format and then persisted (likely after also encrypting to DIDComm V1 Encrypted Envelope).

Negotiating Compatibility

When parties want to communicate via DIDComm, a number of mechanisms must align. These include:

1. The type of service endpoint used by each party
2. The key types used for encryption and/or signing
3. The format of the encryption and/or signing envelopes
4. The encoding of plaintext messages
5. The protocol used to forward and route
6. The protocol embodied in the plaintext messages

Although DIDComm allows flexibility in each of these choices, it is not expected that a given DIDComm implementation will support many permutations. Rather, we expect a few sets of choices that commonly go together. We call a set of choices that work well together a profile. Profiles are identified by a string that matches the conventions of IANA media types, but they express choices about plaintext, encryption, signing, and routing in a single value. The following profile identifiers are defined in this version of the RFC:

Defined Profiles

• didcomm/aip1: The encryption envelope, signing mechanism, plaintext conventions, and routing algorithms embodied in Aries AIP 1.0, circa 2020.
• didcomm/aip2;env=rfc19: The signing mechanism, plaintext conventions, and routing algorithms embodied in Aries AIP 2.0, circa 2021 -- with the old-style encryption envelope from Aries RFC 0019. This legal variant of AIP 2.0 minimizes differences with codebases that shipped AIP 1.0 support.
• didcomm/aip2;env=rfc587: The signing mechanism, plaintext conventions, and routing algorithms embodied in Aries AIP 2.0, circa 2021 -- with the new-style encryption envelope from Aries RFC 0587. This legal variant of AIP 2.0 lays the foundation for DIDComm v2 support by anticipating the eventual envelope change.
• didcomm/v2: The encryption envelope, signing mechanism, plaintext conventions, and routing algorithms embodied in the DIDComm messaging spec.

Profiles are named in the accept section of a DIDComm service endpoint and in an out-of-band message. When Alice declares that she accepts didcomm/aip2;env=rfc19, she is making a declaration about more than her own endpoint. She is saying that all publicly visible steps in an inbound route to her will use the didcomm/aip2;env=rfc19 profile, such that a sender only has to use didcomm/aip2;env=rfc19 choices to get the message from Alice's outermost mediator to Alice's edge. It is up to Alice to select and configure mediators and internal routing in such a way that this is true for the sender.

Detecting DIDComm Versions

Because media types differ from DIDComm V1 to V2, and because media types are easy to communicate in headers and message fields, they are a convenient way to detect which version of DIDComm applies in a given context:

Nature of Content	V1	V2
encrypted	application/didcomm-envelope-enc DIDComm V1 Encrypted Envelope *.dee	application/didcomm-encrypted+json DIDComm Encrypted Message *.dcem
signed	application/didcomm-sig-env DIDComm V1 Signed Envelope *.dse	application/didcomm-signed+json DIDComm Signed Message *.dcsm
plaintext	application/json;flavor=didcomm-msg DIDComm V1 Message *.dm	application/didcomm-plain+json DIDComm Plaintext Message *.dcpm

It is also recommended that agents implementing Discover Features Protocol v2 respond to queries about supported DIDComm versions using the didcomm-version feature name. This allows queries about what an agent is willing to support, whereas the media type mechanism describes what is in active use. The values that should be returned from such a query are URIs that tell where DIDComm versions are developed:

Version	URI
V1	https://github.com/hyperledger/aries-rfcs
V2	https://github.com/decentralized-identity/didcomm-messaging

What it means to "implement" this RFC

For the purposes of Aries Interop Profiles, an agent "implements" this RFC when:

• If it encounters references to DIDComm V1 media types (e.g., in HTTP headers, in attachments, or in the fields of DIDComm messages where a media type is expected), it imputes the meaning documented here.
• If it characterizes DIDComm V1 content in its own outbound communication or UI, it does so with the media type strings, friendly names, and file extensions documented here.

Reference

The file extensions and MIME types described here are also accompanied by suggested graphics. Vector forms of these graphics are available.

Implementations

The following lists the implementations (if any) of this RFC. Please do a pull request to add your implementation. If the implementation is open source, include a link to the repo or to the implementation within the repo. Please be consistent in the "Name" field so that a mechanical processing of the RFCs can generate a list of all RFCs supported by an Aries implementation.

Name / Link	Implementation Notes