lib/pack-codec.js
function $checksum(byte) {
    checksum += toHex(byte);
    if (++offset < 20) return $checksum;
    var actual = sha1sum.digest();
    if (checksum !== actual) throw new Error("Checksum mismatch: " + actual + " != " + checksum);
  }
function $checksum(byte) {
    checksum += toHex(byte);
    if (++offset < 20) return $checksum;
    var actual = sha1sum.digest();
    if (checksum !== actual) throw new Error("Checksum mismatch: " + actual + " != " + checksum);
  }
20 byte checksum