writing php MCrypt twofish for c# bouncycastle

问题:

I'm learning how to use the BouncyCastle c# cipher library for doing encryption. I'm not looking to send messages so I'm not thinking about security etc. I have written my c# code in Visual Studio.

Here is the problem. I have encrypted the text "Hello World!", using Twofish in CFB mode. The key is 1234567812345678. I've used phpfiddle http://phpfiddle.org/ online tool.

$algo = 'twofish';
$mode = 'cfb';

$cipher = mcrypt_module_open($algo,'',$mode,'');

$key = hex2bin('31323334353637383132333435363738'); //1234567812345678
$iv = hex2bin('00000000000000000000000000000000');

mcrypt_generic_init($cipher, $key, $iv);

$plaintext = utf8_encode('Hello World!');
$encrypted = mcrypt_encrypt($algo, $key, $plaintext, $mode, $iv);
printf("<br>Encrypted text: %s<br><br>",base64_encode($encrypted));

$decrypted = mcrypt_decrypt($algo, $key, $encrypted, $mode, $iv);
printf("<br>Decrypted text: %s (%s)<br><br>",$decrypted,bin2hex($decrypted));

mcrypt_generic_deinit($cipher);
mcrypt_module_close($cipher);

The result is as follows

cfdJ+M6MAzG4WJMb (Base64)

I have then created a c# version to decrypt the same text

// ASCII encoding and Zero padding
encoding = Encoding.ASCII
padding = IBlockCipherPadding.zeroBytePadding

// Set up the engine and cipher types
baseCipher = new TwofishEngine();
blockSize = baseCipher.GetBlockSize();    

modeCipher = new CfbBlockCipher(baseCipher, blockSize);
cipher = padding == null ? new PaddedBufferedBlockCipher(modeCipher) : new PaddedBufferedBlockCipher(modeCipher, padding);

// convert the strings to byte array and create a dummy 000000.. iv 

byte[] iv = new byte[blockSize];    // i.e. 16 bytes of zero
keyBytes = _encoding.GetBytes(key);   //1234567812345678
inputBytes = Convert.FromBase64String(inp);

// initiate the cipher with iv parameters
cipher.Init(true, new ParametersWithIV(new KeyParameter(keyBytes), iv));

// do the decryption
console.write(_encoding.GetString(cipher.DoFinal(inputBytes)) + "\n";)

But this gives me garbage out. I get HIl1oVW�rEdIp�

Close (H.l.o.W.r.d.) but every other letter is wrong!

ECB mode works fine so it must be something to do with the initialization vector.

Are there some differences between PHP and c# that I havn't learnt yet?

Where is my c# code incorrect in that case?

回答1:

OK. So I solved it finally. A basic error in my understanding of cipher block modes. CFB and OFB are Stream Ciphers not Padded Buffered block ciphers.

Therefore the cipher setup should have been

modeCipher = new CfbBlockCipher(baseCipher,8);      // Allways 8 bits for a stream cipher !!
cipher = new StreamBlockCipher(modeCipher);

(The cipher.doFinal was changed too but not shown here)

It now works fine.