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Posted 4 Aug 2009
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# RSA Private Key Encryption

, 4 Feb 2012
How to encrypt data using a private key in .NET.

## 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` (http://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)
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();

// 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.
6. Generally, more elegant code (I hope..!).

## Using the New Version

```string secret = "My secret message";
RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(512);  // Key bits length
/*
* 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);```

## History

• 4th August, 2009: Initial post.
• 5th August, 2009: Improved code to check the state of the uploaded keys before trying to make the encryption / decryption.
• 1st February, 2012: Improved and fixed new version.

## Share

 Software Developer WonderNet Israel
I speak three languages: C#, Hebrew and English...
Programming is my work, my hobby and my second love.
I work for an Israeli company named "WonderNet" that deal with digital signature solutions,
combined with Biometrics hand written signature (on a Wacom tablet).
We develop web application as well as client desktop ones,
using the .NET framwork: ASP.NET, C#, SQL-Server and more.
Cryptography is my "thing" since I lay my hand on Simon Singh work of art "The Code Book".

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 Having problems with decrypting a signature created by OpenSSL Member 876784112-Apr-19 7:00 Member 8767841 12-Apr-19 7:00
 I like how this shows the heart of asymmetric encryption using BigInteger. I knew it was possible but hadn't been able to find an example of it.
 My vote of 5 Member 136508367-Feb-19 0:18 Member 13650836 7-Feb-19 0:18
 public encryption / private decryption Member 1413119828-Jan-19 22:33 Member 14131198 28-Jan-19 22:33
 Reviewed Divyans Mahansaria14-Oct-18 3:55 Divyans Mahansaria 14-Oct-18 3:55
 how encrypt this file Member 1370002127-Feb-18 10:35 Member 13700021 27-Feb-18 10:35
 Re: how encrypt this file OriginalGriff27-Feb-18 10:39 OriginalGriff 27-Feb-18 10:39
 Re: how encrypt this file Member 1370002128-Feb-18 20:47 Member 13700021 28-Feb-18 20:47
 Would you please upload or provide System.Numerics.dll for this project? Member 135065146-Nov-17 9:28 Member 13506514 6-Nov-17 9:28
 try with another private key AZHAR SAYYAD12-Jun-17 21:53 AZHAR SAYYAD 12-Jun-17 21:53
 Starting the application Member 1232803419-Apr-17 2:49 Member 12328034 19-Apr-17 2:49
 Re: Starting the application Dudi Bedner19-Apr-17 6:04 Dudi Bedner 19-Apr-17 6:04
 It helped me a lot ashokrathore150320-Feb-17 18:33 ashokrathore1503 20-Feb-17 18:33
 AOEP Decode error Sergio Rius25-Feb-16 1:57 Sergio Rius 25-Feb-16 1:57
 Re: AOEP Decode error Dudi Bedner30-Mar-16 5:09 Dudi Bedner 30-Mar-16 5:09
 My vote of 5 Franc Morales18-Jan-16 16:16 Franc Morales 18-Jan-16 16:16
 Re: My vote of 5 Dudi Bedner30-Mar-16 5:04 Dudi Bedner 30-Mar-16 5:04
 What about large message Lets say 100 MB of data. saqibsabir2-Jan-15 5:29 saqibsabir 2-Jan-15 5:29
 Re: What about large message Lets say 100 MB of data. Dudi Bedner30-Mar-16 5:04 Dudi Bedner 30-Mar-16 5:04
 java,security.private key bindupv29-Dec-14 23:11 bindupv 29-Dec-14 23:11
 Why no more using BigInteger from own implementation Xmen Real 8-Dec-14 21:57 Xmen Real 8-Dec-14 21:57
 Re: Why no more using BigInteger from own implementation Dudi Bedner21-Dec-14 23:59 Dudi Bedner 21-Dec-14 23:59
 Re: Why no more using BigInteger from own implementation Xmen Real 22-Dec-14 22:39 Xmen Real 22-Dec-14 22:39
 Re: Why no more using BigInteger from own implementation Dudi Bedner23-Dec-14 0:13 Dudi Bedner 23-Dec-14 0:13
 Re: Why no more using BigInteger from own implementation Xmen Real 23-Dec-14 2:28 Xmen Real 23-Dec-14 2:28
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