Question

I’m having a problem setting up RSA encryption/decryption mechanism between flex client and web service written in c#. The idea is this: I’ll encrypt some text from flex and then decrypt it from web service. I’m using as3crypto library from google. It is encrypting/decrypting text properly. I also have the code on the web service side to encrypt/decrypt properly. My problem is synchronizing them – basically sharing the public key to flex and keeping the private key to the web service.

My flex “encrypt” function takes modulus and exponent of RSA to do text encryption, so how do i get these modulus and exponent attributes from the web service’s RSACryptoServiceProvider, so they speak the same standard.
I tried the
RSAKeyInfo.Modulus
RSAKeyInfo.Exponent
from the web service and fed them to the flex client.
After doing encryption on flex I took the cipher text and fed it to decrypt method on web service, but it is giving me “bad data” error message.

System.Security.Cryptography.CryptographicException: Bad Data.

   at System.Security.Cryptography.CryptographicException.ThrowCryptogaphicException(Int32 hr)
   at System.Security.Cryptography.Utils._DecryptKey(SafeKeyHandle hPubKey, Byte[] key, Int32 dwFlags)
   at System.Security.Cryptography.RSACryptoServiceProvider.Decrypt(Byte[] rgb, Boolean fOAEP)
   at Microsoft.Samples.Security.PublicKey.App.RSADecrypt(Byte[] DataToDecrypt, RSAParameters RSAKeyInfo, Boolean DoOAEPPadding) in C:\Users
\Me\Desktop\After Release\5-24-2011-webServiceCrypto\publickeycryptography\CS\PublicKeyCryptography\PublicKey.cs:line 219
Encryption failed.

How do i make sure they are both using the same byte 64 or 128 byte encryption . ie the input from flex should fit to what is expected by the web service RSACryptoServiceProvider’s decrypt method.
(I’m assuming the size might be a problem, may be it’s not – i’m lost)

Here is the code, first flex client followed by web service c# code

private function encrypt():void {
                var rsa:RSAKey = RSAKey.parsePublicKey(getModulus(), getExponent());
                trace("Modulus Lenght: " + getModulus().length);
                trace("Exponent Lenght : " + getExponent().length);
                var data:ByteArray = getInput();  //returns byteArray of plainText
                var dst:ByteArray = new ByteArray;
                rsa.encrypt(data, dst, data.length);
                trace("Enc Data: " + dst.toString() );
                currentResult = Hex.fromArray(dst);
                encryptedText = currentResult;
                trace("Encrypted:: " + currentResult);
            }

            //For testing purposes
            private function decrypt():void {
                var rsa:RSAKey = RSAKey.parsePrivateKey(getModulus(), getExponent(), getPrivate(), getP(), getQ(), getDMP1(), getDMQ1(), getCoeff());
                var data:ByteArray = Hex.toArray(encryptedText);
                trace("Byte array: " + data.toString());
                var dst:ByteArray = new ByteArray;
                rsa.decrypt(data, dst, data.length);
                decryptedText = Hex.fromArray(dst);
                trace("Decrypted text: " + Hex.toString(decryptedText));
            }

And web service part is as follows:

       try
        {
            //Create a UnicodeEncoder to convert between byte array and string.
            UnicodeEncoding ByteConverter = new UnicodeEncoding();

            //Create byte arrays to hold original, encrypted, and decrypted data.
            byte[] dataToEncrypt = ByteConverter.GetBytes("Data to Encrypt");
            byte[] encryptedData;
            byte[] decryptedData;

            //Create a new instance of RSACryptoServiceProvider to generate
            //public and private key data.
            using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
            {
                //Pass the data to ENCRYPT, the public key information 
                //(using RSACryptoServiceProvider.ExportParameters(false),
                //and a boolean flag specifying no OAEP padding.
                encryptedData = RSAEncrypt(dataToEncrypt, RSA.ExportParameters(false), false);
                //Pass the data to DECRYPT, the private key information 
                //(using RSACryptoServiceProvider.ExportParameters(true),
                //and a boolean flag specifying no OAEP padding.
                decryptedData = RSADecrypt(encryptedData, RSA.ExportParameters(true), false);
                //Display the decrypted plaintext to the console. 
                Console.WriteLine("\n\nDecrypted plaintext: {0}", ByteConverter.GetString(decryptedData));
            }
        }

static public byte[] RSAEncrypt(byte[] DataToEncrypt, RSAParameters RSAKeyInfo, bool DoOAEPPadding)
    {
        try
        {
            byte[] encryptedData;
            //Create a new instance of RSACryptoServiceProvider.
            using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
            {

                //Import the RSA Key information. This only needs
                //toinclude the public key information.
                RSA.ImportParameters(RSAKeyInfo);

                //Encrypt the passed byte array and specify OAEP padding.  
                //OAEP padding is only available on Microsoft Windows XP or
                //later.  
                encryptedData = RSA.Encrypt(DataToEncrypt, DoOAEPPadding);
            }
            return encryptedData;
        }
        //Catch and display a CryptographicException  
        //to the console.
        catch (CryptographicException e)
        {
            Console.WriteLine(e.Message);

            return null;
        }

    }


 static public byte[] RSADecrypt(byte[] DataToDecrypt, RSAParameters RSAKeyInfo, bool DoOAEPPadding)
    {
        try
        {
            Console.WriteLine("Modulus Lenghth :" + RSAKeyInfo.Modulus.Length);
            Console.WriteLine("Exponent Length :" + RSAKeyInfo.Exponent.Length);
            byte[] decryptedData;
            //Create a new instance of RSACryptoServiceProvider.
            using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
            {
                //Import the RSA Key information. This needs
                //to include the private key information.
                RSA.ImportParameters(RSAKeyInfo);

                //Decrypt the passed byte array and specify OAEP padding.  
                //OAEP padding is only available on Microsoft Windows XP or
                //later.  
                decryptedData = RSA.Decrypt(DataToDecrypt, DoOAEPPadding);
            }
            return decryptedData;
        }
        //Catch and display a CryptographicException  
        //to the console.
        catch (CryptographicException e)
        {
            Console.WriteLine(e.ToString());

            return null;
        }

    }

I’m not quite sure if this RSA set up is the way to go…
Any kinda comment / advice/ or recommended solution is welcome,
thanks guys

Answer 1

Eureka! Eureka! I got it.

The problem was after decryption from web service, the encrypted byte array missed 0’s in between, so that when changed to string it gets unreadable ‘????????’ text. So I just put paddWithZeros() function to pad the decrypted byte array with 0’s between bytes and it worked.

Thanks Kevin, your solution gave me an insight into what things I should consider. So during decrypting I specify parameter fOAEP as false, so it would use PKCS#1 for padding (making both libraries use the same standard).

RSA.Decrypt(DataToDecrypt, DoOAEPPadding); // DoOAEPPadding = false

another error that i was getting is Bad Data exception. This was fixed when i shared the RSA cryptoServiceProvider’s parameters (modulus and exponent) to actionScript methods.

I also changed the byte[] array of c# RSA attributes (like Modulus n, Exponent e, Private d..etc) to hexa string so that I’d be able to share with as3crypto library.

I’d love to share what worked for me; save others some time.

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
               xmlns:s="library://ns.adobe.com/flex/spark" 
               xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600">

    <fx:Script>
        <![CDATA[
            import com.hurlant.crypto.Crypto;
            import com.hurlant.crypto.rsa.RSAKey;
            import com.hurlant.crypto.symmetric.ICipher;
            import com.hurlant.crypto.symmetric.IPad;
            import com.hurlant.util.Hex;

            private var currentResult:String;
            private var encryptedText:String;
            private var decryptedText:String;

            private function encrypt(plainText:String):String {
                var rsa:RSAKey = RSAKey.parsePublicKey(getModulus(), getExponent());
                var data:ByteArray = Hex.toArray(Hex.fromString(plainText));  //returns byteArray of plainText
                var dst:ByteArray = new ByteArray;
                rsa.encrypt(data, dst, data.length);
                currentResult = Hex.fromArray(dst);
                encryptedText = currentResult;
                trace ("Cipher: " + currentResult);
                return currentResult;
            }

            private function getInput():ByteArray {
                return null;
            }

            private function getModulus():String {
                return "b6a7ca9002b4df39af1ed39251a5d"; //read this value from web service.
            }

            private function getExponent():String {
                return "011"; //read this value from web service.
            }

            //For debugging and testing purposes
            // private function decrypt(cipherText:String):String {
                // var rsa:RSAKey = RSAKey.parsePrivateKey(getModulus(), getExponent(), getPrivate(), getP(), getQ(), getDMP1(), getDMQ1(), getCoeff());
                // var data:ByteArray = Hex.toArray(cipherText);
                // var dst:ByteArray = new ByteArray;
                // rsa.decrypt(data, dst, data.length);
                // decryptedText = Hex.fromArray(dst);
                     //trace('decrypted : ' + decryptedText);
                // return Hex.toString(decryptedText);
            // }

        ]]>
    </fx:Script>

    <fx:Declarations>
        <!-- Place non-visual elements (e.g., services, value objects) here -->
    </fx:Declarations>
    <mx:VBox >
        <s:Button label="Encrypt Text" click="encrypt('my plain text')" />
        <s:Button label="Decrypt Text" click="decrypt({encryptedText})" />
    </mx:VBox>
</s:Application>

And the web service part of decryption looks like this:

 static public string RSADecrypt(string cipherText)
    {
        UnicodeEncoding ByteConverter = new UnicodeEncoding();
        byte[] DataToDecrypt = StringToByteArray(cipherText);
        bool DoOAEPPadding = false;
        try
        {
            byte[] decryptedData;
            using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
            {
                KeyInfo keyInfo = new KeyInfo();
                RSAParameters RSAKeyInfo = keyInfo.getKey();
                RSA.ImportParameters(RSAKeyInfo); 
                decryptedData = RSA.Decrypt(DataToDecrypt, DoOAEPPadding);
            }
            byte[] paddedOutput = paddWithZeros(decryptedData); //to sync with as3crypto
            return (ByteConverter.GetString(paddedOutput));

        }catch (CryptographicException e)
        {
            //handle error
            return null;
        }
    }

I’ll do some reading about padding schemes for RSA, see if there is any misconception.

Thanks