RSA private key encryption

Question

Is there any way to perform private key encryption in C#?

I know about the standard RSACryptoServiceProvider in System.Security.Cryptography, but these classes provide only public key encryption and private key decryption. Also, they provide digital signature functionality, which uses internally private key encryption, but there are not any publicly accessible functions to perform private key encryption and public key decryption.

I've found this article on codeproject, which is a very good start point for performing this kind of encryption, however, I was looking for some ready-to-use code, as the code in the article can hardly encrypt arbitrary-long byte arrays containing random values (that means any values, including zeroes).

Do you know some good components (preferably free) to perform private key encryption?
I use .NET 3.5.

Note: I know this is generally considered as bad way of using asymmetric encryption (encrypting using private key and decrypting using public key), but I just need to use it that way.

Additional Explanation

Consider you have

var bytes = new byte[30] { /* ... */ };

and you want to use 2048bit RSA to ensure no one have changed anything in this array.

Normally, you would use digital signature (ie. RIPEMD160), which you then attach to the original bytes and send over to the receiver.

So, you have 30 bytes of original data, and additional 256 bytes of digital signature (because it is a 2048bit RSA), which is overall of 286 bytes. Hovewer, only 160 bits of that 256 bytes are actually hash, so there is exactly 1888 bits (236 bytes) unused.

So, my idea was this:

Take the 30 bytes of original data, attach to it the hash (20 bytes), and now encrypt these 50 bytes. You get 256 bytes long message, which is much shorter than 286 bytes, because "you were able to push the actual data inside the digital signature".

ECDSA Resources

MSDN
Eggheadcafe.com
c-plusplus.de
MSDN Blog
Wiki

DSA Resources

CodeProject
MSDN 1
MSDN 2
MSDN 3

Final Solution

If anyone is interested how I've solved this problem, I'm going to use 1024bit DSA and SHA1, which is widely supported on many different versions of Windows (Windows 2000 and newer), security is good enough (I'm not signing orders, I just need to ensure that some child can't crack the signature on his iPhone (:-D)), and the signature size is only 40 bytes long.

Answer 1

What you are trying to design is known as a "Signature scheme with message recovery".

Designing a new signature scheme is hard. Designing a new signature scheme with message recovery is harder. I don't know all the details about your design, but there is a good chance that it is susceptible to a chosen message attack.

One proposal for signature schemes with message recovery is RSA PSS-R. But unfortunately, this proposal is covered with a patent.

The IEEE P1363 standarization group, once discussed the addition of signature schemes with message recovery. However, I'm not sure about the current state of this effort, but it might be worth checking out.