Secure Programming Cookbook for C and C++

5.5. Using a Raw Block Cipher

Problem

You’re trying to make one of our implementations for other block cipher modes work. They all use raw encryption operations as a foundation, and you would like to understand how to plug in third-party implementations.

Solution

Raw operations on block ciphers consist of three operations: key setup, encryption of a block, and decryption of a block. In other recipes, we provide three macros that you need to implement to use our code. In the discussion for this recipe, we’ll look at several desirable bindings for these macros.

Discussion

Warning

Do not use raw encryption operations in your own designs! Such operations should only be used as a fundamental building block by skilled cryptographers.

Raw block ciphers operate on fixed-size chunks of data. That size is called the block size . The input and output are of this same fixed length. A block cipher also requires a key, which may be of a different length than the block size. Sometimes an algorithm will allow variable-length keys, but the block size is generally fixed.

Setting up a block cipher generally involves turning the raw key into a key schedule . Basically, the key schedule is just a set of keys derived from the original key in a cipher-dependent manner. You need to create the key schedule only once; it’s good for every use of the underlying key because raw encryption always gives the same result for any {key, input} pair (the same is true for decryption).

Once you have a key schedule, you can ...