Multisign

Two-party signature generation for Ed25519. This code allows two parties (a "client" and a "server") to jointly generate an Ed25519 signature under a key that neither one of them possesses in its entirety. The process consists of two phases. During the setup phase, the combined key is generated. During the signing phase, the signature is generated.

Setup phase:

Both the client and the server generate a regular Ed25519 key pair. The server may use the same key pair for all clients. Then, they exchange their public keys and call addPublicKeys to compute the combined public key. The combined public key is the key under which the signatures will validate.

Signing phase:

This phase proceeds in three steps.

• The client calls multiSignStep1. This produces some data that needs to be sent to the server, as well as a secret value that the client should keep locally.

• The server receives the data from the client and passes it to multiSignStep2. This produces some data that needs to be sent to the client.

• The client calls multiSignStep3, passing the data from the server as well as the local secret. This produces the signature.

sequenceDiagram
    Note over Client: generate Ed25519 key pair
    Note over Server: generate Ed25519 key pair
    Client->>Server: send key to
    Note over Server: apply addPublicKey with both keys
    Server->>Client: send combined key back
    Client->>Blockchain: add combined key
    Note over Client: multisignStep1
    Client->>Server: multisignStep1 data
    Note over Server: multisignStep2
    Server->>Client: multisignStep2 data
    Note over Client: multisignStep3 as signature
    Client->>Blockchain: request access key
    Blockchain->>Client: access key
    Note over Client: sign
    Client->>Blockchain: signed transaction

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See the comments above to specific methods for more details.

Methods

/**
 * Adds two public keys together, producing the combined key. The order of the keys does not matter.
 *
 * @param {Uint8Array} publicKey1 the first key
 * @param {Uint8Array} publicKey2 the second key
 *
 * @returns {Uint8Array | null} the combined key.
 * If the argument values are invalid, returns null or a meaningless value
 */
function addPublicKeys(
  publicKey1: Uint8Array,
  publicKey2: Uint8Array
): Uint8Array | null

/**
 * Performs the first step of the two-party signature generation algorithm.
 * This step is performed by the client.
 *
 * @returns {{
 *   data: Uint8Array,
 *   secret: unknown,
 * }}
 *
 * the "data" to be sent to the server. This is always STEP1_DATA_LEN bytes long.
 * the "secret" is an opaque value that the client should keep. This data should be stored IN MEMORY ONLY and only used ONCE.
 */
function multiSignStep1(): { data: Uint8Array; secret: Secret }

/**
 * Performs the second step of the two-party signature generation algorithm.
 * This step is performed by the server.
 *
 * @param {Uint8Array} step1data the data produced during step 1. Must be exactly STEP1_DATA_LEN bytes long.
 * @param {Uint8Array} msg the message to sign.
 * @param {Uint8Array} publicKey the combined public key (NOT the server's public key).
 * @param {Uint8Array} secretKey the server's secret key.
 *
 * @returns {Uint8Array|null} the data to be sent to the client. This is always STEP2_DATA_LEN bytes long.
 * If the argument values are invalid, returns null or a meaningless value.
 */
export function multiSignStep2(
  step1data: Uint8Array,
  msg: Uint8Array,
  publicKey: Uint8Array,
  secretKey: Uint8Array
): Uint8Array | null

/**
 * Performs the third step of the two-party signature generation algorithm. This step is performed by the client.
 *
 * @param {Uint8Array} step2data the data produced during step 2. Must be exactly STEP2_DATA_LEN bytes long.
 * @param {unknown} secret the value produced during step 1. THIS VALUE MUST NOT BE USED MORE THAN ONCE.
 * @param {Uint8Array} msg the message to sign, must be the same as during step 2.
 * @param {Uint8Array} publicKey the combined public key (NOT the client's public key).
 * @param {Uint8Array} secretKey the client's secret key.
 *
 * @returns {Uint8Array|null} the signature.
 * If the argument values are invalid, returns null or a meaningless value
 */
export function multiSignStep3(
  step2data: Uint8Array,
  secret: Secret,
  msg: Uint8Array,
  publicKey: Uint8Array,
  secretKey: Uint8Array
): Uint8Array | null