RSA Introduction

The RSA (Rivest, Shamir, Adleman) encryption algorithm uses two Keys: Private and Public.

Scenario A

Suppose Alice wants to send a message to Bob (for his eyes only!). She can encrypt the message using the RSA algorithm with Bob’s Public Key, which is not a secret (that’s why they call it Public…). Once the message is encrypted, nobody can decrypt it, except the one holding the matching Private Key (that is Bob).

Scenario B

The reverse is also true: if Alice would encrypt the message using her own Private Key, Bob (and Eve, and everyone who can access this “encrypted” message) can decrypt it using Alice’s Public Key. So, if everybody can decrypt it, what’s the point in encrypting the message with a Private Key in the first place? Well, there is a point if Bob wants to make sure that the message has been written by Alice and not by someone else (Eve?).

.NET RSACryptoServiceProvider

The .NET Framework implements the RSA algorithm in the RSACryptoServiceProvider class. The instance of this class lets you create Key pairs, encrypt using a public key, decrypt using a private key (as in the first scenario), sign (sort of the second scenario, but not exactly), and verify the signature.

The Sign method accepts a message (as byte array) and creates a signature for this particular data. In the second scenario, Alice can write a message to Bob, and use this method to get a signature with her own private key. Then, she can send the message to Bob as is (unencrypted) with the signature. To verify the writer ID (Alice), Bob will use the Verify method with Alice’s public key as: Verify(aliceMessage, aliceSignature), and he will get “true” if this is the original message written and signed by Alice, or “false” if even one bit has been changed since. This is one useful implementation of private key encryption, but sometimes it’s just too complicated. You might want to send just a little message so the receiver can decrypt it and be sure it’s from you, without the need to sign and send him both components.

RSA Private Key Encryption

Unfortunately, the RSACryptoServiceProvider class does not provide you this option, so I wrote my own implementation of the RSA algorithm using the basics of the RSACryptoServiceProvider in conjunction with Chew Keong TAN’s class: BigInteger (https://www.codeproject.com/KB/cs/biginteger.aspx). At a low level, the RSA algorithm is about implementing mathematical equations on huge (huge) integers, so the BigInteger class is really essential. I couldn’t have done it myself.

Using the RSAEncryption Class

The class has six main methods:

void LoadPublicFromXml(string publicPath)
void LoadPrivateFromXml(string privatePath)
byte[] PrivateEncryption(byte[] data)
byte[] PublicEncryption(byte[] data)
byte[] PrivateDecryption(byte[] encryptedData)
byte[] PublicDecryption(byte[] encryptedData) 

I believe the method names are self explanatory. First, you have to create a private / public key pair, using the .NET RSACryptoServiceProvider class. To do that, you just create an instance of this class and then call the appropriate methods, like this:

void LoadPublicFromXml(string publicPath)
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider();
File.WriteAllText(@"C:\privateKey.xml", rsa.ToXmlString(true));  // Private Key
File.WriteAllText(@"C:\publicKey.xml", rsa.ToXmlString(false));  // Public Key

// Then, you can load those files to RSAEncryption instance:
RSAEncryption myRsa = new RSAEncryption();
myRsa.LoadPrivateFromXml(@"C:\privateKey.xml");
myRsa.LoadPublicFromXml(@"C:\publicKey.xml");

// Once the keys are loaded (if you load a private key, there is no need to
// load the public one) you can start Encrypt / Decrypt data
// using Private / Public keys.
byte[] message = Encoding.UTF8.GetBytes("My secret message");
byte[] encryptMsg = myRsa.PrivateEncryption(message);

byte[] decryptMsg = myRsa.PublicDecryption(encryptMsg);
string originalMsg = Encoding.UTF8.GetString(decryptMsg);
// returns "My secret message"

WinForms Tester Application

To help you get started with the RSAEncryption and the RSACryptoServiceProvider classes, I wrote a WinForms tester application that uses those classes. All you need to do is just play with it a little and read the code-behind.

Update: New Version

The new implementation of the RSA Private Encryption has a few advantages:

1. Bug fix: Added random padding to support 0 bytes prefix data.
2. Uses the new .NET 4 “BigInteger” struct for math support.
3. Extension methods implementation: the only class instance needed is RSACryptoServiceProvider.
4. Better Exceptions and error handling.
5. UnitTest project added.
6. Generally, more elegant code (I hope..!).

Using the New Version

string secret = "My secret message";
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(512);  // Key bits length
/*
* Skip the loading part for the RSACryptoServiceProvider will generate
* random Private / Public keys pair, that you can save later with
* rsa.ToXmlString(true);
* 
string key = "private or public key as xml string";
rsa.FromXmlString(key);
*/
// Convert the string to byte array
byte[] secretData = Encoding.UTF8.GetBytes(secret);

// Encrypt it using the private key:
byte[] encrypted = rsa.PrivareEncryption(secretData);

// Decrypt it using the public key
byte[] decrypted = rsa.PublicDecryption(encrypted);
string decString = Encoding.UTF8.GetString(decrypted);  // And back to string
Assert.AreEqual("My secret message", decString);