def scram_decrypt()

in scram/aes_scram.py [0:0]
42 lines of code
9 McCabe index (conditional complexity)

def scram_decrypt(K, N, A, C, X, Tag):
    """
    SCRAM Decryption

    Parameters:
            K: Key
            N: Nonce
            A: Additional Authenticated Data
            C: Ciphertext
            X: Encrypted Random value R and Padding Length
            Tag: Tag

    Returns:
            M_calculated: The decrypted Message
    """

    # Derive MAC key (KM)
    # S2 = N || 0x00 0x00 0x00 0x2 || 0^{8} || 0^{8} || 0^{16} || 0^{32}
    S2_calculated = N + byteStr(0x02, 4) + byteStr(0x0, 8) + byteStr(0x0, 8) + byteStr(0x0, 16) + byteStr(0x0, 32)
    U2_calculated = hmac.new(K, S2_calculated, hashlib.sha512).digest()
    KM_calculated = U2_calculated[0:32]

    # Derive T
    # T = GMAC (N, A||C, null)
    T_calculated = AES.new(key=KM_calculated, mode=AES.MODE_GCM, nonce=N).update(A + C).digest()

    # Derive one-time pad U3 from T_calculated,
    # S3 = N || 0x00 0x00 0x00 0x3 || 0^{8} || 0^{8} || T || 0^{32}
    S3_calculated = N + byteStr(0x03, 4) + byteStr(0x0, 8) + byteStr(0x0, 8) + T_calculated + byteStr(0x0, 32)
    U3_calculated = hmac.new(K, S3_calculated, hashlib.sha512).digest()

    # Decrypt R and PADDING_LEN, by xor'ing X and U3
    R_calculated = bytes(a ^ b for (a, b) in zip(U3_calculated[0:32], X[0:32]))
    PADDING_LEN_STR_calculated = bytes(a ^ b for (a, b) in zip(U3_calculated[32:34], X[32:34]))

    # Derive Message and Padding Lengths
    PADDING_LEN_calculated = int.from_bytes(PADDING_LEN_STR_calculated, ENDIANNESS)
    M_LEN_calculated = len(C) - PADDING_LEN_calculated

    # Authenticate R
    # S4 = N || 0x00 0x00 0x00 0x4 || A_LEN_STR || M_LEN_STR || T || R
    S4_calculated = N + byteStr(0x04, 4) + byteStr(len(A), 8) + \
        byteStr(M_LEN_calculated, 8) + T_calculated + R_calculated
    U4_calculated = hmac.new(K, S4_calculated, hashlib.sha512).digest()
    Tag_calculated = U4_calculated[0:16]

    if (Tag == Tag_calculated):
        print("PASSED: Authentication")
    else:
        print("FAILED: Authentication")
        return None

    # Now that Ciphertext and other parameters are authenticated, we can decrypt Ciphertext to get Plaintext
    # Derive Message Encryption key (KE)
    # S1 = N || 0x00 0x00 0x00 0x1 || 0^{8} || 0^{8} || 0^{16} || R
    S1_calculated = N + byteStr(0x01, 4) + byteStr(0x0, 8) + byteStr(0x0, 8) + byteStr(0x0, 16) + R_calculated
    U1_calculated = hmac.new(K, S1_calculated, hashlib.sha512).digest()
    KE_calculated = U1_calculated[0:32]

    # Decrypt Ciphertext
    PADDED_MSG_calculated = AES.new(key=KE_calculated, mode=AES.MODE_CTR, nonce=N).decrypt(C)

    # Strip off padding bytes
    M_calculated = PADDED_MSG_calculated[0:M_LEN_calculated]

    if DEBUG_ENABLED:
        print("\nDecryption Debug Info: ")
        debugByteStr("S1_calculated", S1_calculated)
        debugByteStr("S2_calculated", S2_calculated)
        debugByteStr("S3_calculated", S3_calculated)
        debugByteStr("S4_calculated", S4_calculated)
        debugByteStr("U1_calculated", U1_calculated)
        debugByteStr("U2_calculated", U2_calculated)
        debugByteStr("U3_calculated", U3_calculated)
        debugByteStr("U4_calculated", U4_calculated)
        debugByteStr("T_calculated", T_calculated)
        debugByteStr("R_calculated", R_calculated)
        debugByteStr("KE_calculated", KE_calculated)
        debugByteStr("KM_calculated", KM_calculated)
        debugByteStr("PADDED_MSG_calculated", PADDED_MSG_calculated)
        debugByteStr("M_calculated", M_calculated)

    return M_calculated