Function cryptography_symmetric_decrypt

st2common/st2common/util/crypto.py:293–351

Decrypt the provided ciphertext which has been encrypted using symmetric_encrypt() method (it assumes input is in hex notation as returned by binascii.hexlify). NOTE 1: This function assumes ciphertext has been encrypted using symmetric AES crypto from keyczar library. Underneath i

(decrypt_key, ciphertext)

def cryptography_symmetric_decrypt(decrypt_key, ciphertext):
    """
    Decrypt the provided ciphertext which has been encrypted using symmetric_encrypt() method (it
    assumes input is in hex notation as returned by binascii.hexlify).

    NOTE 1: This function assumes ciphertext has been encrypted using symmetric AES crypto from
    keyczar library. Underneath it uses crypto primitives from cryptography library which is Python
    3 compatible.

    NOTE 2: This function is loosely based on keyczar AESKey.Decrypt() (Apache 2.0 license).
    """
    if not isinstance(decrypt_key, AESKey):
        raise TypeError(
            "Decrypted key needs to be an AESKey class instance"
            f" (was {type(decrypt_key)})."
        )
    if not isinstance(ciphertext, (six.text_type, six.string_types, six.binary_type)):
        raise TypeError(
            "Ciphertext needs to either be a string/unicode or bytes"
            f" (was {type(ciphertext)})."
        )
    aes_key_bytes = decrypt_key.aes_key_bytes
    hmac_key_bytes = decrypt_key.hmac_key_bytes

    if not isinstance(aes_key_bytes, six.binary_type):
        raise TypeError(f"AESKey is not bytes (it is {type(aes_key_bytes)}).")
    if not isinstance(hmac_key_bytes, six.binary_type):
        raise TypeError(f"HMACKey is not bytes (it is {type(hmac_key_bytes)}).")

    # Convert from hex notation ASCII string to bytes
    ciphertext = binascii.unhexlify(ciphertext)

    data_bytes = ciphertext[KEYCZAR_HEADER_SIZE:]  # remove header

    # Verify ciphertext contains IV + HMAC signature
    if len(data_bytes) < (KEYCZAR_AES_BLOCK_SIZE + KEYCZAR_HLEN):
        raise ValueError("Invalid or malformed ciphertext (too short)")

    iv_bytes = data_bytes[:KEYCZAR_AES_BLOCK_SIZE]  # first block is IV
    ciphertext_bytes = data_bytes[
        KEYCZAR_AES_BLOCK_SIZE:-KEYCZAR_HLEN
    ]  # strip IV and signature
    signature_bytes = data_bytes[-KEYCZAR_HLEN:]  # last 20 bytes are signature

    # Verify HMAC signature
    backend = default_backend()
    h = hmac.HMAC(hmac_key_bytes, hashes.SHA1(), backend=backend)
    h.update(ciphertext[:-KEYCZAR_HLEN])
    h.verify(signature_bytes)

    # Decrypt ciphertext
    cipher = Cipher(algorithms.AES(aes_key_bytes), modes.CBC(iv_bytes), backend=backend)

    decryptor = cipher.decryptor()
    decrypted = decryptor.update(ciphertext_bytes) + decryptor.finalize()

    # Unpad
    decrypted = pkcs5_unpad(decrypted)