Encrypting in .Net decrypting in php

Question

I am writing some C# encrypting that is decrypted by php. They aren't working together very well. The php client has informed me of their configuration (RSSL_KEYGEN_PKCS5_V20, 256 bit AES in CBC mode, RSSL_PAD_ANSIX923). Here is what I put together:

public static byte[] Encrypt(byte[] plainText, string password,
         CipherMode cipherMode, int keySize, PaddingMode paddingMode)
    {
        byte[] salt = new byte[8];
        new RNGCryptoServiceProvider().GetBytes(salt);

        // PKCS#5 V2.0 PBKDF2 keyGenerator (uses default iteration count of 1000) 
        Rfc2898DeriveBytes keyGenerator = new Rfc2898DeriveBytes(password, salt);

        AesCryptoServiceProvider aes = new AesCryptoServiceProvider();
        aes.Mode = cipherMode;
        aes.KeySize = keySize;
        aes.Padding = paddingMode;
        aes.Key = keyGenerator.GetBytes(aes.KeySize / 8);

        // aes.Padding is left default PaddingMode.PKCS7 
        // aes.IV should be automatically set to some random value by .NET 
        // (as described in http://msdn.microsoft.com/en-us/library/5e9ft273(VS.71).aspx) 

        // Overestimate encrypted size requirements 
        byte[] encryptedDataBuffer = new byte[plainText.Length + 32 + 32 + 8];
        MemoryStream encryptedOutput = new MemoryStream(encryptedDataBuffer, true);
        CryptoStream encStream = new CryptoStream(encryptedOutput, aes.CreateEncryptor(),
                                                  CryptoStreamMode.Write);

        encryptedOutput.Write(salt, 0, salt.Length);
        encryptedOutput.Write(aes.IV, 0, aes.IV.Length);
        encStream.Write(plainText, 0, plainText.Length);

        encStream.FlushFinalBlock();
        byte[] encryptedData = new byte[encryptedOutput.Position];
        Array.Copy(encryptedDataBuffer, encryptedData, encryptedData.Length);
        encStream.Close();

        return encryptedData;
    }

I can encrypt and decrypt it on my side, but they cannot decrypt it. Is there something I am doing wrong here? I pass in something like this:

Encrypt(mytext, myPassword, CipherMode.CBC, 256, PaddingMode.ANSIX923);

Thanks.

Are you sure that they're using the same number of rounds (1000) in the key generation? — Peter Taylor
– Peter Taylor, Commented Jan 22, 2011 at 23:39
c# and PHP tagged in the same question? this can't be true... — kelloti
– kelloti, Commented Jan 23, 2011 at 3:06

DarylChymko · Accepted Answer · 2011-08-08 18:18:09Z

2

This was a challenge for myself due to the differing ways that .NET and mcrypt handle 'default' AES256 imlpementations. In some versions of PHP, m_crypt uses zero padding, so you'll have to setup the .NET end to do the same thing:

symmetricKey.Padding = PaddingMode.Zeros;

I posted a sample PHP & C# solution to github to help others who come across the same problem: https://github.com/dchymko/.NET--PHP-encryption

answered Aug 8, 2011 at 18:18

DarylChymko

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Chris · Accepted Answer · 2011-01-23 03:13:37Z

1

Narrow it down first: Are you generating the same key?

Once you're certain of that, then you can consider the encyption.

answered Jan 23, 2011 at 3:13

Chris

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Matthew Kruskamp · Accepted Answer · 2011-02-20 10:41:25Z

Using the CryptoServiceProvider did the trick.

 Encoding stringEncoding = new System.Text.UTF8Encoding();

        byte[] salt = new byte[8];

        new RNGCryptoServiceProvider().GetBytes(salt);

        AesCryptoServiceProvider aes = new AesCryptoServiceProvider();
        aes.Mode = cipherMode;
        aes.KeySize = keySize;

        aes.Key = stringEncoding.GetBytes(password);
        aes.IV = new byte[aes.IV.Length];

        // Overestimate encrypted size requirements 
        byte[] encryptedDataBuffer = new byte[plainText.Length + 32 + 32 + 8];
        MemoryStream encryptedOutput = new MemoryStream(encryptedDataBuffer, true);

        CryptoStream encStream = new CryptoStream(encryptedOutput, aes.CreateEncryptor(),
                                                  CryptoStreamMode.Write);

        encStream.Write(plainText, 0, plainText.Length);

        encStream.FlushFinalBlock();
        byte[] encryptedData = new byte[encryptedOutput.Position];
        Array.Copy(encryptedDataBuffer, encryptedData, encryptedData.Length);
        encStream.Close();

        return encryptedData;

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