Cannot decrypt data with C# that was encrypted using PHP (Rijdael-128)

Question

I decrypt data using PHP with this code:

$content="1234"; $cp = mcrypt_module_open('rijndael-128', '', 'cbc', ''); $iv = mcrypt_create_iv(16, MCRYPT_RAND); $key = pack("H*",md5('a')); mcrypt_generic_init($cp, $key, $iv); $encrypted = mcrypt_generic($cp, $content); echo base64_encode($key)."\n"; echo base64_encode($iv)."\n"; echo base64_encode($encrypted)."\n"; mcrypt_generic_deinit($cp); mcrypt_module_close($cp);

$iv and $encrypted is then saved to file and read in the C# sample app:

var iv=...; var encrypted=...; var md5 = new MD5CryptoServiceProvider(); var key = md5.ComputeHash(Encoding.Default.GetBytes("a")); md5.Clear(); Console.WriteLine(Convert.ToBase64String(key)); Console.WriteLine(Convert.ToBase64String(iv)); Console.WriteLine(Convert.ToBase64String(encrypted));

The output here is exactly the same as the output from PHP, so I can assure there is no encoding error inbetween.

var rd = new RijndaelManaged { Key = key, IV = iv, Mode = CipherMode.CBC, KeySize = 128, Padding = PaddingMode.Zeros }; var buffer = new byte[encrypted.Length]; using(var ms = new MemoryStream(buffer)) { using(var cs = new CryptoStream(ms, rd.CreateDecryptor(), CryptoStreamMode.Write)) { cs.Write(encrypted, 0, encrypted.Length); ms.Read(buffer, 0, buffer.Length); Console.WriteLine(Encoding.Default.GetString(buffer)); } } rd.Clear();

The result of the decryption varies on every program start, even with exactly the same input data:

First run:
DMF1ucDxtqgxw5niaXcmYQ== <-Key
GoCeRkrL/EMKNH/BYeLsqQ== <-IV
UBE3DkgbJgj1K/TISugLxA== <-Encrypted
OlOB99yiCYRDoLx+0xxZxQ== <-"Decrypted"

Second run:
DMF1ucDxtqgxw5niaXcmYQ== <-Key
GoCeRkrL/EMKNH/BYeLsqQ== <-IV
UBE3DkgbJgj1K/TISugLxA== <-Encrypted
w5fcY5Fbb9KRgoHfhqAztA== <-"Decrypted"

Key, IV, Encrypted data are identical, but still the decrypted date varies and is always wrong. buffer should contain "1234" or "1234" plus 12 trailing zeros.

I don't see why the results vary and what is not working, but I have been staring at this darn piece of code for several hours now, and probably miss the obvious error...

Reversing the CryptoStream like this creates identically wrong results:

using(var ms = new MemoryStream(encrypted)) { using(var cs = new CryptoStream(ms, rd.CreateDecryptor(), CryptoStreamMode.Read)) { cs.Read(buffer, 0, buffer.Length); Console.WriteLine(Convert.ToBase64String(buffer)); } }

Help? Thanks! Alexander

Benjamin Podszun · Accepted Answer · 2010-02-11 16:32:51Z

Well, modifying an old sample of my sins of the past I ended up with this:

static string Decrypt() { byte[] keyBytes = Convert.FromBase64String("DMF1ucDxtqgxw5niaXcmYQ=="); byte[] iv = Convert.FromBase64String("GoCeRkrL/EMKNH/BYeLsqQ=="); byte[] cipherTextBytes = Convert.FromBase64String("UBE3DkgbJgj1K/TISugLxA=="); var symmetricKey = new RijndaelManaged { Mode = CipherMode.CBC, IV = iv, KeySize = 128, Key = keyBytes, Padding = PaddingMode.Zeros}; using (var decryptor = symmetricKey.CreateDecryptor()) using (var ms = new MemoryStream(cipherTextBytes)) using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read)) { var plainTextBytes = new byte[cipherTextBytes.Length]; int decryptedByteCount = cs.Read(plainTextBytes, 0, plainTextBytes.Length); return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount); } }

which gave "1234" with trailing \0 characters.. Did you just forget to convert the byte[] to a string again? What other difference am I missing?

The solution is very simple, yet mostly annoying... You have to set the "KeySize" Property before setting the "Key" property. I converted my code step by step to match your code, and as soon as I switched the two setters, it worked. In case someone from Microsoft is listening, you might include that bit of information into MSDN. I know it is obvious once you know it, but... Benjamin, Danke!

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