BlockSize must be 128 in this implementation

Question

I'm using the following code to decrypt some strings I reicive but I'm getting the following error: BlockSize must be 128 in this implementation.

I would like to know if there is any way and how it would be possible to decrypt the strings I get without decreasing the size since I can't change my key and IV because the strings were encrypted with then.

       private string Decrypt(string text)
       {
           var inputByteArray = Convert.FromBase64String(text);

           var rm = new RijndaelManaged();
           rm.BlockSize = 256;
           rm.IV = Encoding.UTF8.GetBytes("11111111111111111111111111111111");
           rm.KeySize = 256;
           rm.Key = new SHA256Managed().ComputeHash(Encoding.UTF8.GetBytes("11111111-1111-1111-1111-111111111111"));

           var decryptor = rm.CreateDecryptor();
           var msDecrypt = new MemoryStream(inputByteArray);
           var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read);
           var srDecrypt = new StreamReader(csDecrypt);
           return srDecrypt.ReadToEnd();
       }

I'm using .netcore 5

Answer 1

to solve this problem i followed Topaco suggestion and used the BouncyCastle/C# library and that solved the error i was getting, below I leave the solution I used based on other forums.

public static string Decrypt(string cipherText)
        {
    var ivStringBytes = Encoding.UTF8.GetBytes("11111111111111111111111111111111");
            var cipherTextBytes = Convert.FromBase64String(cipherText);
            var keyBytes = new SHA256Managed().ComputeHash(Encoding.UTF8.GetBytes("11111111-1111-1111-1111-111111111111"));
            var engine = new RijndaelEngine(256);
            var blockCipher = new CbcBlockCipher(engine);
            var cipher = new PaddedBufferedBlockCipher(blockCipher, new Pkcs7Padding());
            var keyParam = new KeyParameter(keyBytes);
            var keyParamWithIV = new ParametersWithIV(keyParam, ivStringBytes, 0, 32);
            cipher.Init(false, keyParamWithIV);
            var finalBytes = cipher.DoFinal(cipherTextBytes);
            var final = Encoding.UTF8.GetString(finalBytes);
            return final;
}

And this is the package that i used:

<PackageReference Include="BouncyCastle.NetCore" Version="1.8.10" />

Answer 2

As far as I remember you must use a package to accomplish this easily because the Rijndael class of .net Core caps at 128. Something like Rijandael256 or Rijndael256.Core by 2Toad github here can be very useful.

Here is some example code for how to decrypt a string using that specific package.

using Rijndael256;

...

string example = "nonsense cipher text here";
string password = "dont code a password like this";
string example2 = Rijndael.Decrypt(example, password, KeySize.Aes256);