Secure Programming Cookbook for C and C++

6.1. Understanding the Basics of Hashes and MACs

Problem

You would like to understand the basic concepts behind hash functions as used in cryptography and message authentication codes (MACs).

Solution

See Section 6.1.3. Be sure to note the possible attacks on these constructs, and how to thwart them.

Discussion

One common thread running through the three types of primitives described in this chapter is that they take an arbitrary amount of data as an input, and produce a fixed-size output. The output is always identical given the exact same inputs (where inputs may include keys, nonces, and text). In addition, in each case, given random inputs, every output is (just about) equally likely.

Types of primitives

These are the three types of primitives:

Message authentication codes

MACs are hash functions that take a message and a secret key (and possibly a nonce) as input, and produce an output that cannot, in practice, be forged without possessing the secret key. This output is often called a tag . There are many ways to build a secure MAC, and there are several good MACs available, including OMAC, CMAC, and HMAC.

Cryptographic hash functions

These functions are the simplest of the primitives we’ll discuss (even though they are difficult to use securely). They simply take an input string and produce a fixed-size output string (often called a hash value or message digest ). Given the output string, there should be no way to determine the input string other than guessing (a dictionary attack). ...