How to convert AES/GCM-256 encryption in C# to python?

Question

I am working on converting the encryption function of AES/GCM-256 in C# to Python. i found the following code in Python and i am using it, but the problem is that my data in the Python function is the same as the C# data. The output string (encoded) produced by my Python function is not the same as the output of the C# function, although my inputs (key, iv and data) are the same in both functions.

I would be grateful if someone could help me

My keyis: b'4fda3c622e966e0839441401bbd3b8f191d4267bf5f19b40812a34b212fd3ed9'

My iv is: b'4fda3c622e966e0839441401bbd3b8f191d4267bf5f19b40812a34b212fd3ed9'

C# function

        public static string AesEncrypt(byte[] payload, byte[] key, byte[] iv)
        {
            var cipher = new GcmBlockCipher(new AesEngine());

            byte[] baPayload = new byte[0];
            cipher.Init(true, new AeadParameters(new KeyParameter(key), 128, iv, baPayload));
            var cipherBytes = new byte[cipher.GetOutputSize(payload.Length)];
            int len = cipher.ProcessBytes(payload, 0, payload.Length, cipherBytes, 0);
            cipher.DoFinal(cipherBytes, len);
            return Convert.ToBase64String(cipherBytes);
        }

It can be converted to bytes using the following function PASSPHRASE in C#:

 public static byte[] StringToByteArray(string hex)
        {
            return Enumerable.Range(0, hex.Length)
                            .Where(x => x % 2 == 0)
                            .Select(x => Convert.ToByte(hex.Substring(x, 2), 16))
                            .ToArray();
        }

python:

PASSPHRASE= b'a4b42ed2702cb1b00a14f39a88c719cb04e5e8b29e2479634c990258e327483'

        def AES_encrypt(data,iv):
           global PASSPHRASE
           data_json_64 = data
           key = binascii.unhexlify(PASSPHRASE)
           cipher = AES.new(key, AES.MODE_GCM, iv)
           x = cipher.encrypt(data)
           return x

Because all my data is the same, I expect my output to be the same, but it is not

My input test string to both C# and Python is:: "In publishing and graphic design, Lorem ipsum is a"

My iv to both C# and Python is: '4fda3c622e966e0839441401bbd3b8f191d4267bf5f19b40812a34b212fd3ed9'

The encoded output in C# is: 02Em9Vve6fWtAcVNesIXzagoB327EmskwMZdRippAAaxqAzkp0VeGSjctbaguqA/01CnPHB2PkRDDOxjgZ9pAfu2

The encoded output in Python is: HudpKzIov7lNt4UNng+a9P/FLXrzdenwDBT4uFYhIUc3XOS7TpaCzxja8I+zHCdXnvk=

Answer 1

After implementing suggested changes from your comments and some minor tweaks to your code, it is possible to reproduce your C# result. The main problem was the handle of a ciphertext and a tag. As @Topaco has stated in the comments, C# implicitly concatenates ciphertext and tag as ciphertext|tag, which will mean you need to do the same in your code. If you follow the GCM documentation you will see that you need to use encrypt_and_digest() function in order to obtain the ciphertext and the tag. Here is what you need to change:

• cipher = AES.new(key, AES.MODE_GCM, iv) → cipher = AES.new(key, AES.MODE_GCM, binascii.unhexlify(iv))
• x = cipher.encrypt(data) → x, key = cipher.encrypt_and_digest(data)
• return x → return x + key

These changes should help you fix your code. I also have some other suggestions. I wouldn't use global variables in your code. For starters, this will mean that your function should be AES_encrypt(data, key, iv) and not AES_encrypt(data, iv). Why are global variables a bad idea? In this particular case it makes no sense you use a global variable for password as you shouldn't use the same password for encryption of everything. Additionally, when talking about encryption, you will see that every algorithm is using a key which is not the same as a password. Here you are implementing the password based encryption. However, you are not doing it correctly, as you are using a password (passphrase) as a cryptographic key. The idea behind a password based encryption is to use a good digest/key stretching technique to create a key from your password. Finally, you should take a look at the NIST Special Publication 800-38D to learn more about the GCM and NIST's suggested parameters when it comes to implementing a safe encryption.