# ICS 030: Cosmos Signed Messages

TODO: Replace with valid ICS number and possibly move to new location.

• Changelog
• Abstract
• Preliminary
• Specification
• Future Adaptations
• API
• References

# Status

Proposed.

# Changelog

# Abstract

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.

A standardized protocol for hashing, signing, and verifying messages that can be implemented by the Cosmos SDK and other third-party organizations is needed. Such a standardized protocol subscribes to the following:

• Contains a specification of human-readable and machine-verifiable typed structured data
• Contains a framework for deterministic and injective encoding of structured data
• Utilizes cryptographic secure hashing and signing algorithms
• A framework for supporting extensions and domain separation
• Is invulnerable to chosen ciphertext attacks
• Has protection against potentially signing transactions a user did not intend to

This specification is only concerned with the rationale and the standardized implementation of Cosmos signed messages. It does not concern itself with the concept of replay attacks as that will be left up to the higher-level application implementation. If you view signed messages in the means of authorizing some action or data, then such an application would have to either treat this as idempotent or have mechanisms in place to reject known signed messages.

# Preliminary

The Cosmos message signing protocol will be parameterized with a cryptographic secure hashing algorithm SHA-256 and a signing algorithm S that contains the operations sign and verify which provide a digital signature over a set of bytes and verification of a signature respectively.

Note, our goal here is not to provide context and reasoning about why necessarily these algorithms were chosen apart from the fact they are the defacto algorithms used in Tendermint and the Cosmos SDK and that they satisfy our needs for such cryptographic algorithms such as having resistance to collision and second pre-image attacks, as well as being deterministic and uniform.

# Specification

Tendermint has a well established protocol for signing messages using a canonical JSON representation as defined here.

An example of such a canonical JSON structure is Tendermint's vote structure:

type CanonicalJSONVote struct {
    ChainID   string               `json:"@chain_id"`
    Type      string               `json:"@type"`
    BlockID   CanonicalJSONBlockID `json:"block_id"`
    Height    int64                `json:"height"`
    Round     int                  `json:"round"`
    Timestamp string               `json:"timestamp"`
    VoteType  byte                 `json:"type"`
}

With such canonical JSON structures, the specification requires that they include meta fields: @chain_id and @type. These meta fields are reserved and must be included. They are both of type string. In addition, fields must be ordered in lexicographically ascending order.

For the purposes of signing Cosmos messages, the @chain_id field must correspond to the Cosmos chain identifier. The user-agent should refuse signing if the @chain_id field does not match the currently active chain! The @type field must equal the constant "message". The @type field corresponds to the type of structure the user will be signing in an application. For now, a user is only allowed to sign bytes of valid ASCII text (see here). However, this will change and evolve to support additional application-specific structures that are human-readable and machine-verifiable (see Future Adaptations).

Thus, we can have a canonical JSON structure for signing Cosmos messages using the JSON schema specification as such:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "$id": "cosmos/signing/typeData/schema",
  "title": "The Cosmos signed message typed data schema.",
  "type": "object",
  "properties": {
    "@chain_id": {
      "type": "string",
      "description": "The corresponding Cosmos chain identifier.",
      "minLength": 1
    },
    "@type": {
      "type": "string",
      "description": "The message type. It must be 'message'.",
      "enum": [
        "message"
      ]
    },
    "text": {
      "type": "string",
      "description": "The valid ASCII text to sign.",
      "pattern": "^[\\x20-\\x7E]+$",
      "minLength": 1
    }
  },
  "required": [
    "@chain_id",
    "@type",
    "text"
  ]
}

e.g.

{
  "@chain_id": "1",
  "@type": "message",
  "text": "Hello, you can identify me as XYZ on keybase."
}

# Future Adaptations

As applications can vary greatly in domain, it will be vital to support both domain separation and human-readable and machine-verifiable structures.

Domain separation will allow for application developers to prevent collisions of otherwise identical structures. It should be designed to be unique per application use and should directly be used in the signature encoding itself.

Human-readable and machine-verifiable structures will allow end users to sign more complex structures, apart from just string messages, and still be able to know exactly what they are signing (opposed to signing a bunch of arbitrary bytes).

Thus, in the future, the Cosmos signing message specification will be expected to expand upon it's canonical JSON structure to include such functionality.

# API

Application developers and designers should formalize a standard set of APIs that adhere to the following specification:

# cosmosSignBytes

Params:

• data: the Cosmos signed message canonical JSON structure
• address: the Bech32 Cosmos account address to sign data with

Returns:

• signature: the Cosmos signature derived using signing algorithm S

# Examples

Using the secp256k1 as the DSA, S:

data = {
  "@chain_id": "1",
  "@type": "message",
  "text": "I hereby claim I am ABC on Keybase!"
}

cosmosSignBytes(data, "cosmos1pvsch6cddahhrn5e8ekw0us50dpnugwnlfngt3")
> "0x7fc4a495473045022100dec81a9820df0102381cdbf7e8b0f1e2cb64c58e0ecda1324543742e0388e41a02200df37905a6505c1b56a404e23b7473d2c0bc5bcda96771d2dda59df6ed2b98f8"

# References