Secure Programming Cookbook for C and C++

5.6. Using a Generic CBC Mode Implementation

Problem

You want a more high-level interface for CBC mode than your library provides. Alternatively, you want a portable CBC interface, or you have only a block cipher implementation and you would like to use CBC mode.

Solution

CBC mode XORs each plaintext block with the previous output block before encrypting. The first block is XOR’d with the IV. Many libraries provide a CBC implementation. If you need code that implements CBC mode, you will find it in the following discussion.

Discussion

Warning

You should probably use a higher-level abstraction, such as the one discussed in Recipe 5.16. Use a raw mode only when absolutely necessary, because there is a huge potential for introducing a security vulnerability by accident. If you still want to use CBC, be sure to use a message authentication code with it (see Chapter 6).

CBC mode is a way to use a raw block cipher and, if used properly, it avoids all the security risks associated with using the block cipher directly. CBC mode works on a message in blocks, where blocks are a unit of data on which the underlying cipher operates. For example, AES uses 128-bit blocks, whereas older ciphers such as DES almost universally use 64-bit blocks.

See Recipe 5.4 for a discussion of the advantages and disadvantages of this mode, as well as a comparison to other cipher modes.

CBC mode works (as illustrated in Figure 5-1) by taking the ciphertext output for the previous block, XOR’ing that with the plaintext ...