function isHexDigit(code) {
  return (
    (code >= 48 && code <= 57) || // '0'-'9'
    (code >= 65 && code <= 70) || // 'A'-'F'
    (code >= 97 && code <= 102) // 'a'-'f'
  );
}
Decrypt a Sequence of Ciphertext Bytes to Produce the Original Sequence
of Plaintext Bytes. The function takes a key as a parameter which can be
for decrypting the eexec block or for decoding charStrings.