did-jwt/src/signers/ES256KSigner.ts

Library for Signing and Verifying JWTs that use DIDs as issuers and JWEs that use DIDs as recipients

import { bytesToBase64url } from '../util.js'
import { Signer } from '../JWT.js'
import { sha256 } from '../Digest.js'
import { secp256k1 } from '@noble/curves/secp256k1.js'

/**
 *  Creates a configured signer function for signing data using the ES256K (secp256k1 + sha256) algorithm.
 *
 *  The signing function itself takes the data as a `Uint8Array` or `string` and returns a `base64Url`-encoded signature
 *
 *  @example
 *  ```TypeScript
 *  const sign: Signer = ES256KSigner(process.env.PRIVATE_KEY)
 *  const signature: string = await sign(data)
 *  ```
 *
 *  @param    {String}    privateKey   a private key as `Uint8Array`
 *  @param    {Boolean}   recoverable  an optional flag to add the recovery param to the generated signatures
 *  @return   {Function}               a configured signer function `(data: string | Uint8Array): Promise<string>`
 */
export function ES256KSigner(privateKey: Uint8Array, recoverable = false): Signer {
  const privateKeyBytes: Uint8Array = privateKey
  if (privateKeyBytes.length !== 32) {
    throw new Error(`bad_key: Invalid private key format. Expecting 32 bytes, but got ${privateKeyBytes.length}`)
  }

  return async (data: string | Uint8Array): Promise<string> => {
    const signature = secp256k1.sign(sha256(data), privateKeyBytes, {
      prehash: false,
      format: recoverable ? 'recovered' : 'compact',
    })
    if (recoverable) {
      const sigBytes = new Uint8Array(65)
      sigBytes.set(signature.slice(1, 65), 0)
      sigBytes[64] = signature[0]
      return bytesToBase64url(sigBytes)
    }
    return bytesToBase64url(signature)
  }
}