This is the 4th and final article in a series on cryptography with .NET. So far in this series we have looked at some of the built in cryptographic primitives build into .NET including, Symmetric Encryption with AES, Asymmetric encryption using RSA, Cryptographic random number generation and hashing.

What I want to do in this final part is talk about using these different cryptographic primitives to do what is called Hybrid Encryption.

What is Hybrid Encryption?

So, what is hybrid encryption? Let’s start off with Wikipedia’s definition.

In cryptography, public-key cryptosystems are convenient in that they do not require the sender and receiver to share a common secret in order to communicate securely (among other useful properties). However, they often rely on complicated mathematical computations and are thus generally much more inefficient than comparable symmetric-key cryptosystems. In many applications, the high cost of encrypting long messages in a public-key cryptosystem can be prohibitive. A hybrid cryptosystem is one which combines the convenience of a public-key cryptosystem with the efficiency of a symmetric-key cryptosystem.

A hybrid cryptosystem can be constructed using any two separate cryptosystems:

• a key encapsulation scheme, which is a public-key cryptosystem, and
• a data encapsulation scheme, which is a symmetric-key cryptosystem.

The hybrid cryptosystem is itself a public-key system, who’s public and private keys are the same as in the key encapsulation scheme.

Note that for very long messages the bulk of the work in encryption/decryption is done by the more efficient symmetric-key scheme, while the inefficient public-key scheme is used only to encrypt/decrypt a short key value.

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This is the 3^rd part in a short series on cryptography in .NET. In the previous 2 articles I covered using Symmetric algorithms like AES and Asymmetric algorithms like RSA. In this section I want to cover random number generation and hashing. This will lead into the final article which will be about combining cryptographic primitives to create hybrid encryption protocols.

The primitive I want to discuss is generating cryptographically strong random numbers. This is useful if you want to generate random session keys for AES for example. To generate a random number you use the RNGCryptoServiceProvider class in .NET. Once you have constructed the object you just call GetBytes() and pass in the length in bytes of the random number you want to generate.

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