mukan-sdk/docs/architecture/adr-036-arbitrary-signature.md

ADR 036: Arbitrary Message Signature Specification

Changelog

• 28/10/2020 - Initial draft

Authors

• Antoine Herzog (@antoineherzog)
• Zaki Manian (@zmanian)
• Aleksandr Bezobchuk (alexanderbez) [1]
• Frojdi Dymylja (@fdymylja)

Status

Draft

Abstract

Currently, in the Cosmos SDK, there is no convention to sign arbitrary message like on Ethereum. We propose with this specification, for Cosmos SDK ecosystem, a way to sign and validate off-chain arbitrary messages.

This specification serves the purpose of covering every use case, this means that cosmos-sdk applications developers decide how to serialize and represent Data to users.

Context

Having the ability to sign messages off-chain has proven to be a fundamental aspect of nearly any blockchain. The notion of signing messages off-chain has many added benefits such as saving on computational costs and reducing transaction throughput and overhead. Within the context of the Cosmos, some of the major applications of signing such data includes, but is not limited to, providing a cryptographic secure and verifiable means of proving validator identity and possibly associating it with some other framework or organization. In addition, having the ability to sign Cosmos messages with a Ledger or similar HSM device.

Further context and use cases can be found in the references links.

Decision

The aim is being able to sign arbitrary messages, even using Ledger or similar HSM devices.

As a result signed messages should look roughly like Cosmos SDK messages but must not be a valid on-chain transaction. chain-id, account_number and sequence can all be assigned invalid values.

Cosmos SDK 0.40 also introduces a concept of “auth_info” this can specify SIGN_MODES.

A spec should include an auth_info that supports SIGN_MODE_DIRECT and SIGN_MODE_LEGACY_AMINO.

Create the offchain proto definitions, we extend the auth module with offchain package to offer functionalities to verify and sign offline messages.

An offchain transaction follows these rules:

• the memo must be empty
• nonce, sequence number must be equal to 0
• chain-id must be equal to “”
• fee gas must be equal to 0
• fee amount must be an empty array

Verification of an offchain transaction follows the same rules as an onchain one, except for the spec differences highlighted above.

The first message added to the offchain package is MsgSignData.

MsgSignData allows developers to sign arbitrary bytes valid offchain only. Where Signer is the account address of the signer. Data is arbitrary bytes which can represent text, files, objects. It's applications developers decision how Data should be deserialized, serialized and the object it can represent in their context.

It's applications developers decision how Data should be treated, by treated we mean the serialization and deserialization process and the Object Data should represent.

Proto definition:

// MsgSignData defines an arbitrary, general-purpose, off-chain message
message MsgSignData {
    // Signer is the sdk.AccAddress of the message signer
    bytes Signer = 1 [(gogoproto.jsontag) = "signer", (gogoproto.casttype) = "github.com/cosmos/cosmos-sdk/types.AccAddress"];
    // Data represents the raw bytes of the content that is signed (text, json, etc)
    bytes Data = 2 [(gogoproto.jsontag) = "data"];
}

Signed MsgSignData json example:

{
  "type": "cosmos-sdk/StdTx",
  "value": {
    "msg": [
      {
        "type": "sign/MsgSignData",
        "value": {
          "signer": "cosmos1hftz5ugqmpg9243xeegsqqav62f8hnywsjr4xr",
          "data": "cmFuZG9t"
        }
      }
    ],
    "fee": {
      "amount": [],
      "gas": "0"
    },
    "signatures": [
      {
        "pub_key": {
          "type": "tendermint/PubKeySecp256k1",
          "value": "AqnDSiRoFmTPfq97xxEb2VkQ/Hm28cPsqsZm9jEVsYK9"
        },
        "signature": "8y8i34qJakkjse9pOD2De+dnlc4KvFgh0wQpes4eydN66D9kv7cmCEouRrkka9tlW9cAkIL52ErB+6ye7X5aEg=="
      }
    ],
    "memo": ""
  }
}

Consequences

There is a specification on how messages, that are not meant to be broadcast to a live chain, should be formed.

Backwards Compatibility

Backwards compatibility is maintained as this is a new message spec definition.

Positive

• A common format that can be used by multiple applications to sign and verify off-chain messages.
• The specification is primitive which means it can cover every use case without limiting what is possible to fit inside it.
• It gives room for other off-chain messages specifications that aim to target more specific and common use cases such as off-chain-based authN/authZ layers [2].

Negative

• Current proposal requires a fixed relationship between an account address and a public key.
• Doesn't work with multisig accounts.

Further discussion

• Regarding security in MsgSignData, the developer using MsgSignData is in charge of making the content laying in Data non-replayable when, and if, needed.
• the offchain package will be further extended with extra messages that target specific use cases such as, but not limited to, authentication in applications, payment channels, L2 solutions in general.

References

1. https://github.com/cosmos/ics/pull/33
2. https://github.com/cosmos/cosmos-sdk/pull/7727#discussion_r515668204
3. https://github.com/cosmos/cosmos-sdk/pull/7727#issuecomment-722478477
4. https://github.com/cosmos/cosmos-sdk/pull/7727#issuecomment-721062923