RSA Client Server Encryption Decryption

Question

Here is a simple code of RSA encryption/decryption .

How can I make it work for different application? In following example same RSA object is used for encryption and decryption. How can I make it work for 2 separate applications. So that data encrypted in one application is decrypted by other application.

using System;
using System.Security.Cryptography;
using System.Text;

class RSACSPSample
{

    static void Main()
    {
        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("Decrypted plaintext: {0}", ByteConverter.GetString(decryptedData));
            }
        }
        catch (ArgumentNullException)
        {
            //Catch this exception in case the encryption did 
            //not succeed.
            Console.WriteLine("Encryption failed.");

        }
    }

    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
        {
            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;
        }

    }
}

Answer 1

When you call

//Create a new instance of RSACryptoServiceProvider to generate 
//public and private key data. 
using (RSACryptoServiceProvider RSA = new RSACryptoServiceProvider())
....

new RSA keypair is generated as stated in you comment above the code. You need to share this keypair. You could do

RSA.ExportParameters(true)

or

RSA.ToXmlString(true)

Then you could include this output in configuration of both applications and at the beginning of execution you would have to call RSA.ImportParameters or RSA.FromXmlString.

Note that the above code extracts private part of keypair. If you want to only encrypt in application A and only decrypt in application B then application A needs only public part of keypair and application B needs private part.

Also note that encrypting big data by asymmetric cryptography is not recommended (for performance reasons) and you should consider using symmetric cryptography (ie AES256).