How to use the js-sha3.keccak_256.buffer function in js-sha3

async function signData(identity, signer, key, data) {
    const nonce = await didReg.nonce(signer);
    const paddedNonce = leftPad(Buffer.from([nonce], 64).toString("hex"));
    const dataToSign =
      "1900" +
      stripHexPrefix(didReg.address) +
      paddedNonce +
      stripHexPrefix(identity) +
      data;
    const hash = Buffer.from(sha3.buffer(Buffer.from(dataToSign, "hex")));
    const signature = ethutil.ecsign(hash, key);
    const publicKey = ethutil.ecrecover(
      hash,
      signature.v,
      signature.r,
      signature.s
    );
    return {
      r: "0x" + signature.r.toString("hex"),
      s: "0x" + signature.s.toString("hex"),
      v: signature.v
    };
  }

export function keccak (data) {
  return Buffer.from(keccak_256.buffer(data))
}