code: function readHeader(buffer, header) {
    var idsz = buffer.readUIntBE(20, 1);
    header.salt = buffer.slice(0, KEY_LENGTH);
    header.rs = buffer.readUIntBE(KEY_LENGTH, 4);
    header.keyid = buffer.slice(21, 21 + idsz);
    return 21 + idsz;
  }
docs: /* Used when decrypting aes128gcm to populate the header values. Modifies the
  header values in place and returns the size of the header.