Encrypt in C# Decrypt AES CBC 256

Question

I want encrypt in C# and decrypt in c++ but I have some Encoding problem :

example variables :

Key : gOtIPaLver—vS5UAnJbPqsDZSf,yJ1 IVString : gOtIPaLver—vS5

my c# code :

public static string EncryptString(string message, string KeyString, string IVString)
    {
        byte[] Key = UTF8Encoding.UTF8.GetBytes(KeyString.Substring(0,32));
        byte[] IV = UTF8Encoding.UTF8.GetBytes(IVString);

        string encrypted = null;
        RijndaelManaged rj = new RijndaelManaged();
        rj.Key = Key;
        rj.IV = IV;
        rj.Mode = CipherMode.CBC;
        rj.Padding = PaddingMode.PKCS7;
        try
        {
            MemoryStream ms = new MemoryStream();

            using (CryptoStream cs = new CryptoStream(ms, rj.CreateEncryptor(Key, IV), CryptoStreamMode.Write))
            {
                using (StreamWriter sw = new StreamWriter(cs))
                {
                    sw.Write(message);
                    sw.Close();
                }
                cs.Close();
            }
            byte[] encoded = ms.ToArray();
            encrypted = Convert.ToBase64String(encoded);

            ms.Close();
        }
        catch (CryptographicException e)
        {
            Console.WriteLine("A Cryptographic error occurred: {0}", e.Message);
            return null;
        }
        catch (UnauthorizedAccessException e)
        {
            Console.WriteLine("A file error occurred: {0}", e.Message);
            return null;
        }
        catch (Exception e)
        {
            Console.WriteLine("An error occurred: {0}", e.Message);
        }
        finally
        {
            rj.Clear();
        }
        return encrypted;
    }

When I try encrypt and my Key have some not ASCII char - count of bytes is more than lenght of string .

How Encoding I shound use ? In C++ I use EVP (default padding is PKCS7) .

static string decryptEX(string KS, string ctext)
{
EVP_CIPHER_CTX* ctx;
ctx = EVP_CIPHER_CTX_new();
string IV = KS.substr(0, 16);
int rc = EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, (byte*)&KS[0], (byte*)&IV[0]);
std::string rtext;
rtext.resize(ctext.size());

int out_len1 = (int)rtext.size();
rc = EVP_DecryptUpdate(ctx, (byte*)&rtext[0], &out_len1, (const byte*)&ctext[0], (int)ctext.size());


int out_len2 = (int)rtext.size() - out_len1;
rc = EVP_DecryptFinal_ex(ctx, (byte*)&rtext[0] + out_len1, &out_len2);
try
{
    rtext.resize(out_len1 + out_len2);
}
catch (exception e)
{

}
return rtext;

}

Answer 1

Your call to

byte[] Key = UTF8Encoding.UTF8.GetBytes(KeyString.Substring(0,32));

fails because there are not 32 characters in that string. Yes, there would be 32 bytes if the string is encoded as UTF8. But Substring takes a count of characters.

Changing that line to

byte[] Key = UTF8Encoding.UTF8.GetBytes(KeyString);

gives a 32-byte long key.

Since you can't count on getting proper input for the KeyString, you could validate after this point that your key length is long enough.

byte[] Key = UTF8Encoding.UTF8.GetBytes(KeyString);
if (Key.Length < 32)
   throw new KeyLengthException("some useful info as to why this was thrown"); // or some other appropriate Exception type you create or resuse.

If I'm correct in assuming which key, IV, and block sizes you require, this should do what you need:

    public static string EncryptString(string message, string KeyString, string IVString)
    {
        byte[] Key = UTF8Encoding.UTF8.GetBytes(KeyString).Take(32).ToArray();
        if (Key.Length < 32)
            throw new Exception("KeyString not at least 32 bytes.");
        byte[] IV = UTF8Encoding.UTF8.GetBytes(IVString).Take(16).ToArray();
        if (Key.Length < 32)
            throw new Exception("IVString not at least 16 bytes.");
        string encrypted = null;
        RijndaelManaged rj = new RijndaelManaged();
        rj.KeySize = 128;
        rj.BlockSize = 128;
        rj.Key = Key;
        rj.IV = IV;
        rj.Mode = CipherMode.CBC;
        rj.Padding = PaddingMode.PKCS7;
        try
        {
            MemoryStream ms = new MemoryStream();

            using (CryptoStream cs = new CryptoStream(ms, rj.CreateEncryptor(Key, IV), CryptoStreamMode.Write))
            {
                using (StreamWriter sw = new StreamWriter(cs))
                {
                    sw.Write(message);
                    sw.Close();
                }
                cs.Close();
            }
            byte[] encoded = ms.ToArray();
            encrypted = Convert.ToBase64String(encoded);

            ms.Close();
        }
        catch (CryptographicException e)
        {
            Console.WriteLine("A Cryptographic error occurred: {0}", e.Message);
            return null;
        }
        catch (UnauthorizedAccessException e)
        {
            Console.WriteLine("A file error occurred: {0}", e.Message);
            return null;
        }
        catch (Exception e)
        {
            Console.WriteLine("An error occurred: {0}", e.Message);
        }
        finally
        {
            rj.Clear();
        }
        return encrypted;
    }