php data encrypted with openssl_seal. How to decode in c#?

Question

I am passing some data (1-2MB) between a php app and a c# program. The info needs to be encrypted and I was using a Rijndael encryption, but encryption was very slow. I am trying to switch to openssl_seal on the php side and have that working fine:

PHP CODE

<!-- language: lang-php -->
    str = 123;
    $fp = fopen("/home/prod/publickey.pem", "r");
    $cert = fread($fp, 8192);
    fclose($fp);
    $pk1 = openssl_get_publickey($cert);
    openssl_seal($str, $sealed, $ekeys, array($pk1));
    openssl_free_key($pk1);
    
    $sealed = base64_encode($sealed);
    $Xevk = base64_encode($ekeys[0]);
    
    echo $Xevk."\n\n\n".$sealed;

But I am having issues trying to decrypt on the c# end.

C# Code

<!-- language: c# -->
    public static string DecryptSSL(string str) {
        string[] strs = System.Text.RegularExpressions.Regex.Split(str, "\n\n\n");
        X509Certificate2 myCert2 = null;
        RSACryptoServiceProvider rsa = null;

        try {
            myCert2 = new X509Certificate2(Properties.Resources.mycertkey, "");
            rsa = (RSACryptoServiceProvider)myCert2.PrivateKey;
        } catch(Exception e) {
            Console.WriteLine(e.Message); 
        }

        byte[] xkey = rsa.Decrypt(Convert.FromBase64String(strs[0]), false);

        byte[] content = Convert.FromBase64String(strs[1]);
        EncDec.RC4(ref content, xkey);

        return System.Convert.ToBase64String(content);
    }

    public static void RC4(ref Byte[] bytes, Byte[] key) {
        Byte[] s = new Byte[256];
        Byte[] k = new Byte[256];
        Byte temp;
        int i, j;

        for(i = 0; i < 256; i++) {
            s[i] = (Byte)i;
            k[i] = key[i % key.GetLength(0)];
        }

        j = 0;
        for(i = 0; i < 256; i++) {
            j = (j + s[i] + k[i]) % 256;
            temp = s[i];
            s[i] = s[j];
            s[j] = temp;
        }

        i = j = 0;
        for(int x = 0; x < bytes.GetLength(0); x++) {
            i = (i + 1) % 256;
            j = (j + s[i]) % 256;
            temp = s[i];
            s[i] = s[j];
            s[j] = temp;
            int t = (s[i] + s[j]) % 256;
            bytes[x] ^= s[t];
        }
    }

Where DecryptSSL is being called with an ascii string containing the response from the php page.

I never get the original string, '123' in this case, returned from the decrypt function. What else am I missing?

Answer 1

Found the issue here myself. In the c# code I was doing an extra encoding to base64 to get it back to a string rather than just converting the bytes to a string.

replacing

return System.Convert.ToBase64String(content);

with

return System.Text.Encoding.ASCII.GetString(content);

at the end of the DecryptSSL function got everything working!