I have below code written in .net framework to decrypt encrypted text and it is working fine.

private static string MD5(string testString)
{
    byte[] asciiBytes = ASCIIEncoding.ASCII.GetBytes(testString);
    byte[] hashedBytes = MD5CryptoServiceProvider.Create().ComputeHash(asciiBytes);
    string hashedString = BitConverter.ToString(hashedBytes).Replace("-", "").ToLower();
    return hashedString;
}

public static string DecryptString(string cypherText, string encryptionKey)
{
    byte[] cipherTextBytes = Convert.FromBase64String(cipherText);
    byte[] key = ASCIIEncoding.ASCII.GetBytes(MD5(encryptionKey));

    if (key == null || key.Length <= 0)
        throw new ArgumentNullException("key");

    string plaintext;
    using (var rijAlg = new RijndaelManaged())
    {
        rijAlg.BlockSize = 256;
        rijAlg.Key = key;
        rijAlg.Mode = CipherMode.CBC;
        rijAlg.Padding = PaddingMode.Zeros;
        rijAlg.IV = ASCIIEncoding.ASCII.GetBytes(MD5(MD5(_encryptionKey)));

        ICryptoTransform decryptor = rijAlg.CreateDecryptor(rijAlg.Key, rijAlg.IV);
        using (var msDecrypt = new MemoryStream(cipherTextBytes))
        using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
        using (var srDecrypt = new StreamReader(csDecrypt))
            plaintext = srDecrypt.ReadToEnd();
    }
    return plaintext;
}

Above code never worked and was returning some junk characters when the code was migrated to .net 5. With some research and help from fellow community member I got the below updated code that works.

public static string DecryptUsingBouncyCastle(string cypherText, string keyString)
        {
            byte[] key = ASCIIEncoding.ASCII.GetBytes(MD5(keyString));

            byte[] cipherTextBytes = Convert.FromBase64String(cypherText);
            if (key == null || key.Length <= 0)
                throw new ArgumentNullException("key");

            var ivStringBytes = ASCIIEncoding.ASCII.GetBytes(MD5(MD5(keyString)));

            var engine = new RijndaelEngine(256);
            var blockCipher = new CbcBlockCipher(engine);
            var cipher = new PaddedBufferedBlockCipher(blockCipher, new ZeroBytePadding());
            var keyParam = new KeyParameter(key);
            var keyParamWithIV = new ParametersWithIV(keyParam, ivStringBytes, 0, ivStringBytes.Length);
            cipher.Init(false, keyParamWithIV);
            var outputBytes = new byte[cipher.GetOutputSize(cipherTextBytes.Length)];
            var length = cipher.ProcessBytes(cipherTextBytes, outputBytes, 0);
            length += cipher.DoFinal(outputBytes, length);
            var resultText = Encoding.UTF8.GetString(outputBytes, 0, length);
            return resultText;
        }

I was cautioned that AES should be preferred over Rijndael with 256 bits block size. With that in mind I tried to write the decrypt method using AES but I could not succeed. It is not throwing any error but returning some garbage text.

 public static string DecryptStringFromAES(string cypherText, string keyString)
        {
            byte[] key = ASCIIEncoding.ASCII.GetBytes(MD5(keyString));
            byte[] cipherText = Convert.FromBase64String(cypherText);

            if (key == null || key.Length <= 0)
                throw new ArgumentNullException("key");

            string plaintext;
            using (var aes = Aes.Create())
            {
                aes.BlockSize = 128;
                aes.Key = key;
                aes.Mode = CipherMode.CBC;
                aes.Padding = PaddingMode.Zeros;
                aes.IV = new byte[16];

                ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
                using (var msDecrypt = new MemoryStream(cipherText))
                using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                using (var srDecrypt = new StreamReader(csDecrypt))
                    plaintext = srDecrypt.ReadToEnd();
            }
            return plaintext;
        }

I am not an expert in Cryptography and I don't really know what is wrong in the above code. My job demanded this task to decrypt some encrypted information from Active Directory. Encryption is done by some other process and I can't share the encryption key and don't have test code as we don't have a test environment.

Appreciate if someone could look at the code and see if I messed up anything with that code and suggest any correction that could possibly fix the issue. Thanks in advance.