C# RSA Encrpytion -> Laravel phpseclib decrypt()

I’m using a key pair generated by phpseclib and then I use it to encrypt in C# a message and decrypt in PHP.

The public key is:

-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAvh4OvlS5+7Skk5Nij296
rDAKQsxZrz+gV/MInvWqe/SAOlOf8vqs6xJsn7sd523X8m+b0Kd751UbeeDQRGjC
mbQc9iwQHUFrNa0bSVnpyoc27K+cuxNdtoPG1XXRCFLKKqVJKSyB5ZPu+oQr+OIo
6zYhU5jfCEYbpzPdyYqfDGJPpJv/r8F11MCBrdbvD5Z42ysTDqSxFGIfROWtDOP5
ShSNcVGRQYfX3E/v8j+qy0yu2PD/W1UqNd3m6TTkUj/u1XukUMWp+tfXEdEp7IEU
askvGnBfHgJcsBp2c6kT9WxIU0ai4G8mo/2OsHprw42PBWLcJwfUeUgY3itSczni
WQIDAQAB
-----END PUBLIC KEY-----

The conversion to XML is done via https://superdry.apphb.com/tools/online-rsa-key-converter

Here is how the message is encrypted in C#:

public void Synch()
    {
        string messageToSend = "123";
        
        var encryptedData = EncryptString(messageToSend);
        
        var request = new HTTPRequest(new Uri(APIUrl), HTTPMethods.Post, OnRequestFinished);

        request.AddField("message", encryptedData);
        request.FormUsage = BestHTTP.Forms.HTTPFormUsage.Multipart;

        request.Send();
    }

public byte[] EncryptBytes(string message)
    {
        // https://superdry.apphb.com/tools/online-rsa-key-converter
        string XML = "<RSAKeyValue><Modulus>vh4OvlS5+7Skk5Nij296rDAKQsxZrz+gV/MInvWqe/SAOlOf8vqs6xJsn7sd523X8m+b0Kd751UbeeDQRGjCmbQc9iwQHUFrNa0bSVnpyoc27K+cuxNdtoPG1XXRCFLKKqVJKSyB5ZPu+oQr+OIo6zYhU5jfCEYbpzPdyYqfDGJPpJv/r8F11MCBrdbvD5Z42ysTDqSxFGIfROWtDOP5ShSNcVGRQYfX3E/v8j+qy0yu2PD/W1UqNd3m6TTkUj/u1XukUMWp+tfXEdEp7IEUaskvGnBfHgJcsBp2c6kT9WxIU0ai4G8mo/2OsHprw42PBWLcJwfUeUgY3itSczniWQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>";
        

        //get the object back from the stream
        RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
        rsa.FromXmlString(XML);
        
        byte[] bytesMessage = Encoding.UTF8.GetBytes(message);
        var bytesCipherText = rsa.Encrypt(bytesMessage, false);

        return bytesCipherText;
    }

public string EncryptString(string message)
    {
        string encryptedText = Convert.ToBase64String(EncryptBytes(message));
        return encryptedText;
    }

What is sent (encrypted message) by C# to PHP (changes everytime):

DE9e82l/lP211YQ0gDhvIcwU1dIXaN7re9+VpUYEB62yRYYnmzZfHxv+WeMHtvg5Yq15Lr4rhvQpTiMGyktxkR5wPQRQhDX74gonPIKgYYIfoqF0mswkulaiLReagcroVyd4ZFEfPxKScLXqjo3yIFLaGWVMGoh81ZwRoOUEGLCPkhsRMp1CiHecGZEmZm7VNoiOEeA5xRdO3asl5/jrbnpXeNXcU4OFV4n1hfdw8W04VHyzDU3AmkDnpfO3AHnZluGPOYpl3OYVn9bsmuqL9d02gBr0UP72vp5GxNoDZAy3tzymhdNZArN9sfBXX+QPOlK2eaFwSypGIa/SUn+erg==

On the decryption side in PHP:

Route::post('/decrypt', function (Request $request) {
    $encoded_message = $request->input('message');
    if (!empty($encoded_message)) {
        $private = RSA::loadFormat('PKCS8', file_get_contents(resource_path() . '/rsa/key.pem'), $password = false);

        try {
            $message = $private->decrypt(base64_decode($encoded_message));
            echo $message;
            }
        } catch (Exception $e) {
            return response()->json([
                'message' => 'Failed to process: '.$e->getMessage().$e->getTraceAsString(),
            ]);
        }

The output is like this:

Response: {"message":"Failed to process: Decryption error#0 C:\www\laravel\vendor\phpseclib\phpseclib\phpseclib\Crypt\RSA\PrivateKey.php(469):

What am I missing here?

Answer

In the C# code PKCS#1 v1.5 padding is applied (2nd parameter of Encrypt() is false), phpseclib uses OAEPadding by default (here).

Decryption works if PKCS#1 v1.5 padding is explicitly specified in the PHP code:

$message = $private->withPadding(RSA::ENCRYPTION_PKCS1)->decrypt(base64_decode($encoded_message));

---

Alternatively, OAEPadding could be used in the C# code. But be careful, if in Encrypt() the 2nd parameter is set to true, .NET uses SHA-1 for both digests by default. However, phpseclib applies SHA-256 for both digests by default. Therefore, in the C# code, for compatibility with the PHP code, both digests must be explicitly set to SHA256 (this is not possible with RSACryptoServiceProvider, but with RSACng; also, the overload of Encrypt() is needed, with which the digest can be set explicitly).