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A CBC Stream Cipher in C# (With wrappers for two open source AES implementations in C# and C)

, 4 Jul 2004
An article on .NET cryptography
A CBC Symetric Cipher implementation in C#

(Plus an implementation of Advanced Encryption Standard (AES) in .NET C# and C)

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Q: .NET already implements Rijndael in managed code why another implementation in C#?

A: .NET System.Security.Cryptography is quite good and its underlying Crypto API is great, but it lacks an important feature: Transparency. The only option you have is to trust it. The stream cipher here and the two AES libraries comes with complete source code.

Q: I checked the code. It does not implement any of the .NET System.Security.Cryptography interfaces. This makes it not suitable to use with existing code.

A: My intention was that this code had as little interaction with .NET namespaces as possible. The code can be easily ported to a not .NET language. This makes it very usable. The interface is so simple that integrating this code with the rest of .NET encryption should not be a problem.

Q: Sounds too good to be true, are there any drawbacks?

A: The C# AES implementation is slower that .NET (CryptoAPI) Rijndael implementation. However it is functionally the same as the C implementation. Use the C# wraper of the C aesc.dll in code. Use the pure C# implementation just to have an idea what the code does and to run verification tests.

Q: Any other issues I must know before using this code?

A: The original C# Aes.cs implementation is by James McCaffrey, and appeared in the MSDN magazine. Read EULA.doc for copyright issues. The C implementation is free for any kind of usage and is by Brian Gladman. The �*.cs� files have the names of the authors. I wrote only the CBC stream cipher library to make the code really usable in practice.

Q: How can I get started?

A: To get started see: cs\aes-example.cs

Q: Can I use your stream cipher with another block cipher?

A: Yes, and it is very easily. You need only to implement IBlockCipher.cs. See CAes.cs for an example.

CHANGES

02-JUL-2004 (a) Modified Decrypt() to fail silently
            (b) Removed some constans from code to make it generic

30-JUN-2004 (a) Modified padding to be compatible with .NET SymmetricAlgorithm

EOF

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vcepa

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