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Aries RFC 0019: Encryption Envelope

Summary

There are two layers of messages that combine to enable interoperable self-sovereign agent-to-agent communication. At the highest level are DIDComm Plaintext Messages - messages sent between identities to accomplish some shared goal (e.g., establishing a connection, issuing a verifiable credential, sharing a chat). DIDComm Plaintext Messages are delivered via the second, lower layer of messaging - DIDComm Encrypted Envelopes. A DIDComm Encrypted Envelope is a wrapper (envelope) around a plaintext message to permit secure sending and routing. A plaintext message going from its sender to its receiver passes through many agents, and an encryption envelope is used for each hop of the journey.

This RFC describes the DIDComm Encrypted Envelope format and the pack() and unpack() functions that implement this format.

Motivation

Encryption envelopes use a standard format built on JSON Web Encryption - RFC 7516. This format is not captive to Aries; it requires no special Aries worldview or Aries dependencies to implement. Rather, it is a general-purpose solution to the question of how to encrypt, decrypt, and route messages as they pass over any transport(s). By documenting the format here, we hope to provide a point of interoperability for developers of agents inside and outside the Aries ecosystem.

We also document how Aries implements its support for the DIDComm Encrypted Envelope format through the pack() and unpack() functions. For developers of Aries, this is a sort of design doc; for those who want to implement the format in other tech stacks, it may be a useful reference.

Tutorial

Assumptions

We assume that each sending agent knows:

  • Its intended recipient(s).
  • What encryption (if any) is appropriate.
  • If encryption will be used, a public key of the receiving agent.
  • The physical endpoint to use for the receiver, and the appropriate transport protocol (https, zmq, etc.).

The assumptions can be made because either the message is being sent to an agent within the sending agent's domain and so the sender knows the internal configuration of agents, or the message is being sent outside the sending agent's domain and interoperability requirements are in force to define the sending agent's behaviour.

Example Scenario

The example of Alice and Bob's sovereign domains is used for illustrative purposes in defining this RFC.

Example Domains: Alice and Bob

In the diagram above:

  • Alice has
  • 1 Edge agent - "1"
  • 1 Routing agent - "2"
  • 1 Domain Endpoint - "8"
  • Bob has
  • 3 Edge Agents - "4", "5" and "6"
    • "6" is an Edge agent in the cloud, "4" and "5" are physical devices.
  • 1 Routing agent - "3"
  • 1 Domain Endpoint - "9"

For the purposes of this discussion we are defining the Encryption Envelope agent message flow to be:

1 → 2 → 8 → 9 → 3 → 4

However, that flow is just one of several that could match this configuration. What we know for sure is that:

  • 1 is the Sender agent in this case and so must send the first or original message.
  • 9 is the Domain Endpoint of Bob's domain and so must receive the message as an Encrypted Envelope.
  • 4 is the Receiver in this case and so must receive (and should be able to read) the first or original message.

Encrypted Envelopes

An encrypted envelope is used to transport any plaintext message from one agent directly to another. In our example message flow above, there are five encrypted envelopes sent, one for each hop in the flow. The process to send an encrypted envelope consists of the following steps:

  • Call the standard function pack() to wrap the plaintext message
  • Send the encrypted envelope using the transport protocol defined by the receiving endpoint
  • Receive the encrypted envelope
  • Call the standard function unpack() to retrieve the plaintext message (and possibly its provenance) from the encrypted envelope

This is repeated with each hop, but the encrypted envelopes are nested, such that the plaintext is never visible until it reaches its final recipient.

Implementation

We will describe the pack and unpack algorithms, and their output, in terms of Aries' initial implementation, which may evolve over time. Other implementations could be built, but they would need to emit and consume similar inputs and outputs.

The data structures emitted and consumed by these algorithms are described in a formal schema.

Authcrypt mode vs. Anoncrypt mode

When packing and unpacking are done in a way that the sender is anonymous, we say that we are in anoncrypt mode. When the sender is revealed, we are in authcrypt mode. Authcrypt mode reveals the sender to the recipient only; it is not the same as a non-repudiable signature. See the RFC about non-repudiable signatures, and this discussion about the theory of non-repudiation.

Pack Message

pack_message() interface

packed_message = pack_message(wallet_handle, message, receiver_verkeys, sender_verkey)

pack_message() Params:

  • wallet_handle: handle to the wallet that contains the sender's secrets.
  • message: the message (plaintext, or nested encrypted envelope) as a string. If it's JSON object it should be in string format first
  • receiver_verkeys: a list of recipient verkeys as string containing a JSON array
  • sender_verkey: the sender's verkey as a string. This verkey is used to look up the sender's private key so the wallet can put supply it as input to the encryption algorithm. When an empty string ("") is passed in this parameter, anoncrypt mode is used

pack_message() return value (Authcrypt mode)

This is an example of an outputted message encrypting for two verkeys using Authcrypt.

{
    "protected": "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",
    "iv": "ZqOrBZiA-RdFMhy2",
    "ciphertext": "K7KxkeYGtQpbi-gNuLObS8w724mIDP7IyGV_aN5AscnGumFd-SvBhW2WRIcOyHQmYa-wJX0MSGOJgc8FYw5UOQgtPAIMbSwVgq-8rF2hIniZMgdQBKxT_jGZS06kSHDy9UEYcDOswtoLgLp8YPU7HmScKHSpwYY3vPZQzgSS_n7Oa3o_jYiRKZF0Gemamue0e2iJ9xQIOPodsxLXxkPrvvdEIM0fJFrpbeuiKpMk",
    "tag": "kAuPl8mwb0FFVyip1omEhQ=="
}

The base64URL encoded protected decodes to this: 

{
    "enc": "xchacha20poly1305_ietf",
    "typ": "JWM/1.0",
    "alg": "Authcrypt",
    "recipients": [
        {
            "encrypted_key": "L5XDhH15Pm_vHxSeraY8eOTG6RfcE2NQ3ETeVC-7EiDZyzpRJd8FW0a6qe4JfuAz",
            "header": {
                "kid": "GJ1SzoWzavQYfNL9XkaJdrQejfztN4XqdsiV4ct3LXKL",
                "iv": "a8IminstXHi54_J-Je5IWlOcNgSwD9TB",
                "sender": "ftimwiiYRG7rRQbXgJ13C5aTEQIrsWDI_bsxDqiWbTlVSKPmw6418vz3HmMlelM8AuSiKlaLCmRDI4sDFSgZIcAVXo134V8o8lFoV1BdDI7fdKOZzrKbqCixKJk="
            }
        },
        {
            "encrypted_key": "eAMiD6GDmOtzREhI-TV05_Rhippy8jwOAu5D-2IdVOJgI8-N7QNSulYyCoWiE16Y",
            "header": {
                "kid": "HKTAiYM8cE2kKC9KaNMZLYj4GS8uWCYMBxP2i1Y92zum",
                "iv": "D4tNtHd2rs65EG_A4GB-o0-9BgLxDMfH",
                "sender": "sJ7piu4UDuL_o2pXb-J_JApxsaFrxiTmgpZjltWjYFTUir4b8MWmDdtzp0OnTeHLK9mFrhH4GVA1wVtnokUKogCdNWHscarQscQCRPZDKrW6boftwH8_EYGTL0Q="
            }
        }
    ]
}

pack output format (Authcrypt mode)

    {
        "protected": "b64URLencoded({
            "enc": "xchachapoly1305_ietf",
            "typ": "JWM/1.0",
            "alg": "Authcrypt",
            "recipients": [
                {
                    "encrypted_key": base64URLencode(libsodium.crypto_box(my_key, their_vk, cek, cek_iv))
                    "header": {
                          "kid": "base58encode(recipient_verkey)",
                           "sender" : base64URLencode(libsodium.crypto_box_seal(their_vk, base58encode(sender_vk)),
                            "iv" : base64URLencode(cek_iv)
                }
            },
            ],
        })",
        "iv": <b64URLencode(iv)>,
        "ciphertext": b64URLencode(encrypt_detached({'@type'...}, protected_value_encoded, iv, cek),
        "tag": <b64URLencode(tag)>
    }

Authcrypt pack algorithm

  1. generate a content encryption key (symmetrical encryption key)
  2. encrypt the CEK for each recipient's public key using Authcrypt (steps below)
    1. set encrypted_key value to base64URLencode(libsodium.crypto_box(my_key, their_vk, cek, cek_iv))
      • Note it this step we're encrypting the cek, so it can be decrypted by the recipient
    2. set sender value to base64URLencode(libsodium.crypto_box_seal(their_vk, sender_vk_string))
      • Note in this step we're encrypting the sender_verkey to protect sender anonymity
    3. base64URLencode(cek_iv) and set to iv value in the header
      • Note the cek_iv in the header is used for the encrypted_key where as iv is for ciphertext
  3. base64URLencode the protected value
  4. encrypt the message using libsodium.crypto_aead_chacha20poly1305_ietf_encrypt_detached(message, protected_value_encoded, iv, cek) this is the ciphertext.
  5. base64URLencode the iv, ciphertext, and tag then serialize the format into the output format listed above.

For a reference implementation, see https://github.com/hyperledger/indy-sdk/blob/master/libindy/src/commands/crypto.rs

pack_message() return value (Anoncrypt mode)

This is an example of an outputted message encrypted for two verkeys using Anoncrypt.

{
    "protected": "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",
    "iv": "M1GneQLepxfDbios",
    "ciphertext": "iOLSKIxqn_kCZ7Xo7iKQ9rjM4DYqWIM16_vUeb1XDsmFTKjmvjR0u2mWFA48ovX5yVtUd9YKx86rDVDLs1xgz91Q4VLt9dHMOfzqv5DwmAFbbc9Q5wHhFwBvutUx5-lDZJFzoMQHlSAGFSBrvuApDXXt8fs96IJv3PsL145Qt27WLu05nxhkzUZz8lXfERHwAC8FYAjfvN8Fy2UwXTVdHqAOyI5fdKqfvykGs6fV",
    "tag": "gL-lfmD-MnNj9Pr6TfzgLA=="
}

The protected data decodes to this:

{
    "enc": "xchacha20poly1305_ietf",
    "typ": "JWM/1.0",
    "alg": "Anoncrypt",
    "recipients": [
        {
            "encrypted_key": "XCN8V57Q1tgauLW1zgj1WQZQ0WDV0QwyEZFNNwF6DmiI49CK5s58dsV0d_NZK-SMNqe0iFXgXFvHpoc8kuViSM_K5lrpbMStj7CRPskvbS8=",
            "header": {
                "kid": "GJ1SzoWzavQYfNL9XkaJdrQejfztN4XqdsiV4ct3LXKL"
            }
        },
        {
            "encrypted_key": "hnOeL0Y9xOvcy5oFgtd1RVm9d73-0uGWNJCtG4lKswvYcWzSnDlhbbvjiHUCX5mMNYulilgCMFQvkvrRDnJI3E6ZjO1qRZuCQv4yT-w6oiA=",
            "header": {
                "kid": "2GXmuCN2JCxSqMRVftBHLxVJKSL5bXyzM8DsPzGqQoNj"
            }
        }
    ]
}

pack output format (Anoncrypt mode)

    {
         "protected": "b64URLencoded({
            "enc": "xchachapoly1305_ietf",
            "typ": "JWM/1.0",
            "alg": "Anoncrypt",
            "recipients": [
                {
                    "encrypted_key": base64URLencode(libsodium.crypto_box_seal(their_vk, cek)),
                    "header": {
                        "kid": base58encode(recipient_verkey),
                    }
                },
            ],
         })",
         "iv": b64URLencode(iv),
         "ciphertext": b64URLencode(encrypt_detached({'@type'...}, protected_value_encoded, iv, cek),
         "tag": b64URLencode(tag)
    }

Anoncrypt pack algorithm

  1. generate a content encryption key (symmetrical encryption key)
  2. encrypt the CEK for each recipient's public key using Anoncrypt (steps below)
    1. set encrypted_key value to base64URLencode(libsodium.crypto_box_seal(their_vk, cek))
      • Note it this step we're encrypting the cek, so it can be decrypted by the recipient
  3. base64URLencode the protected value
  4. encrypt the message using libsodium.crypto_aead_chacha20poly1305_ietf_encrypt_detached(message, protected_value_encoded, iv, cek) this is the ciphertext.
  5. base64URLencode the iv, ciphertext, and tag then serialize the format into the output format listed above.

For a reference implementation, see https://github.com/hyperledger/indy-sdk/blob/master/libindy/src/commands/crypto.rs

Unpack Message

unpack_message() interface

unpacked_message = unpack_message(wallet_handle, jwe)

unpack_message() Params

  • wallet_handle: wallet handle that contains the sender_verkey
  • jwe: a message which was returned from a pack_message() and follows the scheme format described below

Unpack Algorithm

  1. seralize data, so it can be used
    • For example, in rust-lang this has to be seralized as a struct.
  2. Lookup the kid for each recipient in the wallet to see if the wallet possesses a private key associated with the public key listed
  3. Check if a sender field is used.
    • If a sender is included use auth_decrypt to decrypt the encrypted_key by doing the following:
      1. decrypt sender verkey using libsodium.crypto_box_seal_open(my_private_key, base64URLdecode(sender))
      2. decrypt cek using libsodium.crypto_box_open(my_private_key, sender_verkey, encrypted_key, cek_iv)
      3. decrypt ciphertext using libsodium.crypto_aead_chacha20poly1305_ietf_open_detached(base64URLdecode(ciphertext_bytes), base64URLdecode(protected_data_as_bytes), base64URLdecode(nonce), cek)
      4. return message, recipient_verkey and sender_verkey following the authcrypt format listed below
    • If a sender is NOT included use anon_decrypt to decrypt the encrypted_key by doing the following:
      1. decrypt encrypted_key using libsodium.crypto_box_seal_open(my_private_key, encrypted_key)
      2. decrypt ciphertext using libsodium.crypto_aead_chacha20poly1305_ietf_open_detached(base64URLdecode(ciphertext_bytes), base64URLdecode(protected_data_as_bytes), base64URLdecode(nonce), cek)
      3. return message and recipient_verkey following the anoncrypt format listed below

NOTE: In the unpack algorithm, the base64url decode implementation used MUST correctly decode padded and unpadded base64URL encoded data.

For a reference unpack implementation, see https://github.com/hyperledger/indy-sdk/blob/master/libindy/src/commands/crypto.rs

unpack_message() return values (authcrypt mode)

{
    "message": "{ \"@id\": \"123456780\",\"@type\":\"https://didcomm.org/basicmessage/1.0/message\",\"sent_time\": \"2019-01-15 18:42:01Z\",\"content\": \"Your hovercraft is full of eels.\"}",
    "recipient_verkey": "HKTAiYM8cE2kKC9KaNMZLYj4GS8uWCYMBxP2i1Y92zum",
    "sender_verkey": "DWwLsbKCRAbYtfYnQNmzfKV7ofVhMBi6T4o3d2SCxVuX"
}

unpack_message() return values (anoncrypt mode)

{
    "message": "{ \"@id\": \"123456780\",\"@type\":\"https://didcomm.org/basicmessage/1.0/message\",\"sent_time\": \"2019-01-15 18:42:01Z\",\"content\": \"Your hovercraft is full of eels.\"}",
    "recipient_verkey": "2GXmuCN2JCxSqMRVftBHLxVJKSL5bXyzM8DsPzGqQoNj"
}

Additional Notes

  • All kid values used currently are base58 encoded ed25519 keys. If other keys types are used, say secp256k1, base58 encoding should also be used here for interoperability.

  • All algorithm APIs which use libsodium are from sodiumoxide rust wrapping of the original C implementation.

Drawbacks

The current implementation of the pack() message is currently Hyperledger Aries specific. It is based on common crypto libraries (NaCl), but the wrappers are not commonly used outside of Aries. There's currently work being done to fine alignment on a cross-ecosystem interoperable protocol, but this hasn't been achieved yet. This work will hopefully bridge this gap.

Rationale and alternatives

As the JWE standard currently stands, it does not follow this format. We're actively working with the lead writer of the JWE spec to find alignment and are hopeful the changes needed can be added.

We've also looked at using the Message Layer Security (MLS) specification. This specification shows promise for adoption later on with more maturity. Additionally because they aren't hiding metadata related to the sender (Sender Anonymity), we would need to see some changes made to the specification before we could adopt this spec.

Prior art

The JWE family of encryption methods.

Unresolved questions

  • How transport protocols (https, zmq, etc.) will be be used to send Encrypted Envelopes?
    • These will need to be defined using separate RFCs. For example, HTTP might POST a message and place it in the body of the HTTP POST.
  • How will the encrypted envelopes work with routing tables to pass a message through a domain, potentially over various transport protocols?
    • There's not much certainty whether routing tables or some other mechanism will be used. This needs to be defined in another RFC.
  • If the encryption envelope protocol fails, how is that failure passed back to those involved in the transmission?

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
Indy Cloud Agent - Python Reference agent implementation contributed by Sovrin Foundation and Community
Aries Framework - .NET .NET framework for building agents of all types
Streetcred.id Commercial mobile and web app built using Aries Framework - .NET
Aries Cloud Agent - Python Contributed by the government of British Columbia.
Aries Static Agent - Python Useful for cron jobs and other simple, automated use cases.
Aries Framework - Go For building agents, hubs and other DIDComm ../../features in GoLang.
Aries Protocol Test Suite