Question

I want to implement a scenario where two endpoints can securely communicate with each other using public/private key encryption. The scenario is following:

For A to send a message to B:

A encrypts the message using A’s private key.

A encrypts the message using B’s public key.

A sends the message.

B receives the message.

B decrypts the message using A’s public key.

B decrypts the message using B’s private key.

B reads the message.

Here is what I have in C# using RSA encryption:

// Alice wants to send a message to Bob:

String plainText = "Hello, World!";
Byte[] plainData = Encoding.Default.GetBytes(plainText);
Byte[] cipherData = null;

RSACryptoServiceProvider alice = new RSACryptoServiceProvider();
RSACryptoServiceProvider bob = new RSACryptoServiceProvider();

var alicePrivateKey = alice.ExportParameters(true);
var alicePublicKey = alice.ExportParameters(false);

var bobPrivateKey = bob.ExportParameters(true);
var bobPublicKey = bob.ExportParameters(false);

RSACryptoServiceProvider messenger = new RSACryptoServiceProvider();
        
messenger.ImportParameters(alicePrivateKey);
cipherData = messenger.Encrypt(plainData, true);

messenger.ImportParameters(bobPublicKey);
cipherData = messenger.Encrypt(cipherData, true);

messenger.ImportParameters(alicePublicKey);
cipherData = messenger.Decrypt(cipherData, true);

messenger.ImportParameters(bobPrivateKey);
cipherData = messenger.Decrypt(cipherData, true);            

String result = Encoding.Default.GetString(alice.Decrypt(cipherData, true));

Clearly, there is something wrong with the following lines:

messenger.ImportParameters(bobPublicKey);
cipherData = messenger.Encrypt(cipherData, true);

Which throws System.Security.Cryptography.CryptographyException with message { "Bad Length" }.

As I can see it is not able to encrypt the data using just the public part of bob’s key.

Can someone throw some light on how to properly accomplish what I want to do in C#?

Answer 1

RSA is used to encrypt data which are smaller than the key. You use symmetric key to encrypt large amount of data and then use the RSA to share the symmetric key.

For further details you might refer to this question : how to use RSA to encrypt files (huge data) in C#