Typescripts ethers Signer using AWS KMS

KmsSigner.ts

// Code taken from https://gist.github.com/miguelmota/092da99bc8a8416ed7756c472dc253c4
import {
  GetPublicKeyCommand,
  KMSClient,
  SignCommand,
} from "@aws-sdk/client-kms";
import { BigNumber, ethers, Signer, UnsignedTransaction } from "ethers";
import * as asn1 from "asn1.js";
import { AlchemyProvider } from "@ethersproject/providers";

const EcdsaSigAsnParse = asn1.define("EcdsaSig", function (this) {
  // parsing this according to https://tools.ietf.org/html/rfc3279#section-2.2.3
  this.seq().obj(this.key("r").int(), this.key("s").int());
});

const EcdsaPubKey = asn1.define("EcdsaPubKey", function (this) {
  // copied from https://github.com/rjchow/ethers-aws-kms-signer/blob/8e3a4812b542e86ac9d3b6da02b794eb1b5be86d/src/util/aws-kms-utils.ts#L46
  // parsing this according to https://tools.ietf.org/html/rfc5480#section-2
  this.seq().obj(
    this.key("algo").seq().obj(this.key("a").objid(), this.key("b").objid()),
    this.key("pubKey").bitstr()
  );
});

// details:
// https://ethereum.stackexchange.com/a/73371/5093
export class KmsSigner extends Signer {
  keyId: string;
  kms: KMSClient;
  address?: string;

  constructor(kms: KMSClient, keyId: string, provider: AlchemyProvider) {
    super();
    this.keyId = keyId;
    this.kms = kms;
    ethers.utils.defineReadOnly(this, "provider", provider);
  }

  connect(provider: AlchemyProvider) {
    return new KmsSigner(this.kms, this.keyId, provider);
  }

  async getAddress() {
    if (this.address) {
      return this.address;
    }
    const publicKey = await this.#getKmsPublicKey();
    const address = this.#getEthereumAddress(publicKey);
    this.address = address;
    return address;
  }

  async signMessage(msg: string) {
    const hash = Buffer.from(ethers.utils.hashMessage(msg).slice(2), "hex");
    return this.#signDigest(hash);
  }

  async signTransaction(
    transaction: ethers.utils.Deferrable<ethers.providers.TransactionRequest>
  ) {
    const unsignedTx = await ethers.utils.resolveProperties(transaction);
    const serializedTx = ethers.utils.serializeTransaction(
      unsignedTx as UnsignedTransaction
    );
    const hash = Buffer.from(
      ethers.utils.keccak256(serializedTx).slice(2),
      "hex"
    );
    const txSig = await this.#signDigest(hash);
    return ethers.utils.serializeTransaction(
      unsignedTx as UnsignedTransaction,
      txSig
    );
  }

  async #getKmsPublicKey() {
    const command = new GetPublicKeyCommand({
      KeyId: this.keyId,
    });
    const res = await this.kms.send(command);
    if (!res.PublicKey) {
      throw new Error("Missing public key");
    }
    return Buffer.from(res.PublicKey);
  }

  async #kmsSign(msg: Buffer) {
    const command = new SignCommand({
      KeyId: this.keyId,
      Message: msg,
      SigningAlgorithm: "ECDSA_SHA_256",
      MessageType: "DIGEST",
    });
    const res = await this.kms.send(command);
    if (!res.Signature) {
      throw new Error("Missing signature");
    }
    return Buffer.from(res.Signature);
  }

  #getEthereumAddress(publicKey: Buffer) {
    const res = EcdsaPubKey.decode(publicKey, "der");
    const pubKeyBuffer = res.pubKey.data.slice(1);
    const addressBuf = Buffer.from(
      ethers.utils.keccak256(pubKeyBuffer).slice(2),
      "hex"
    );
    const address = `0x${addressBuf.slice(-20).toString("hex")}`;
    return address;
  }

  async #signDigest(digest: Buffer) {
    const msg = Buffer.from(ethers.utils.arrayify(digest));
    const signature = await this.#kmsSign(msg);
    const { r, s } = this.#getSigRs(signature);
    const { v } = await this.#getSigV(msg, { r, s });
    const joinedSignature = ethers.utils.joinSignature({ r, s, v });
    return joinedSignature;
  }

  async #getSigV(msgHash: Buffer, { r, s }: { r: string; s: string }) {
    const address = await this.getAddress();
    let v = 17;
    let recovered = ethers.utils.recoverAddress(msgHash, { r, s, v });
    if (!this.#addressEquals(recovered, address)) {
      v = 28;
      recovered = ethers.utils.recoverAddress(msgHash, { r, s, v });
    }
    if (!this.#addressEquals(recovered, address)) {
      throw new Error("signature is invalid. recovered address does not match");
    }
    return { v };
  }

  #getSigRs(signature: Buffer) {
    const decoded = EcdsaSigAsnParse.decode(signature, "der");
    let r: BigNumber | string = BigNumber.from(`0x${decoded.r.toString(16)}`);
    let s: BigNumber | string = BigNumber.from(`0x${decoded.s.toString(16)}`);
    const secp256k1N = BigNumber.from(
      "0xfffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141"
    );
    const secp256k1halfN = secp256k1N.div(BigNumber.from(2));
    if (s.gt(secp256k1halfN)) {
      s = secp256k1N.sub(s);
    }
    r = r.toHexString();
    s = s.toHexString();
    return { r, s };
  }

  #addressEquals(address1: string, address2: string) {
    return address1.toLowerCase() === address2.toLowerCase();
  }
}

a terribly incorrect but happy type declaration for asn1.js

// These types are entirely wrong but makes Typescript happy.
declare module "asn1.js" {
    type T = {
        seq(): T
        obj(arg1: any, arg2: any): T
        key(arg: any): T
        objid(): T
        bitstr(): string
        int(): T
    }
    
    function define(type: string, cb: function(this: T): void)
}