Question

I tried to encrypt data using this function in PHP and decrypt it with the other function in C#. But I don’t get the same string.

//php function
    public function onCrypt($text)
    {   
    $key=md5('DFDFDFDFDFDFDFDFDFDFDFDF',true);

    $crypttext = urldecode(trim(mcrypt_encrypt(MCRYPT_RIJNDAEL_128,$key, $text, MCRYPT_MODE_ECB, mcrypt_create_iv(mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_ECB), MCRYPT_RAND))));

    $text_crp =base64_encode($crypttext);


    return $text_crp;
            }

//c# function

//public static void DecryptFile
Parameters :
strKey : the key choosed in decryption .
PathPlainTextFile : path of the crypted file
PathPlainTextFile : the original file decrypted.

public static void DecryptFile(string strKey, string pathPlainTextFile, string pathCypheredTextFile)     
  {  

//crypt key with md5 function           
System.Security.Cryptography.MD5 alg = System.Security.Cryptography.MD5.Create();
System.Text.UTF8Encoding enc = new System.Text.UTF8Encoding();
byte[] md5val = alg.ComputeHash(enc.GetBytes(strKey));


StreamReader fsPlainTextFile = File.OpenText(pathPlainTextFile);    
FileInfo t = new FileInfo(pathCypheredTextFile);
StreamWriter Tex =t.CreateText();
string input = null;
while ((input = fsPlainTextFile.ReadLine()) != null)
{


        byte[] cipheredData = Convert.FromBase64String(input);

           RijndaelManaged rijndaeld = new RijndaelManaged();

            // define the used mode
            rijndaeld.Mode = CipherMode.ECB;

            // create the cipher AES - Rijndael
            ICryptoTransform decryptor = rijndaeld.CreateDecryptor(md5val,null);

             // Write the ciphered data in  MemoryStream
            MemoryStream ms= new MemoryStream(cipheredData);
            CryptoStream cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read);

                 // Insert  the ciphered data in a byte array
               byte[] plainTextData = new byte[cipheredData.Length];

            int decryptedByteCount = cs.Read(plainTextData, 0, plainTextData.Length);

            ms.Close();
            cs.Close();


   // Insert  the ciphered data in string encoded on Base64         
Tex.WriteLine (Encoding.UTF8.GetString(plainTextData, 0, decryptedByteCount));





}

Tex.Close();    


    }

Answer 1

At a quick glance, you’re not supplying an IV to the C# decryptor:

ICryptoTransform decryptor = rijndaeld.CreateDecryptor(md5val, null);

I’m not familiar with php, but it looks like you created an IV when you encrypted the content. You’ll need to have that same IV to decrypt it in the C# code (you’d need the same IV to decrypt it even if you were doing the decryption through php).