Problem - Kevin Tafuro

To ensure that you choose the right cryptographic protocols for your
application, you need an understanding of these basics. However,
you’ll very rarely need to go all the way back to the primitive algorithms
we discuss in these chapters. Instead, you should focus on outof-the-box
protocols that are believed to be cryptographically strong.
While we therefore recommend that you thoroughly understand the
material in these chapters, we advise you to go to the recipes in
Chapter 9 to find something appropriate before you come here and
build something yourself. Don’t fall into the same trap that many of
Applied Cryptography’s readers have fallen into!
There are two classes of symmetric primitives, both of utmost importance. First are
symmetric encryption algorithms, which provide for data secrecy. Second are message
authentication codes (MACs), which can ensure that if someone tampers with data
while in transit, the tampering will be detected. Recently, a third class of primitives
has started to appear: encryption modes that provide for both data secrecy and message
authentication. Such primitives can help make the application of cryptography
less prone to disastrous errors.
In this chapter, we will look at how to generate, represent, store, and distribute symmetric-key
material. In Chapter 5, we will look at encryption using block ciphers
such as AES, and in Chapter 6, we will examine cryptographic hash functions (such
as SHA1) and MACs.
Towards the end of this chapter, we do occasionally forward-reference
algorithms from the next two chapters. It may be a good idea to
read Recipes 5.1 through 5.4 and 6.1 through 6.4 before reading Recipes
4.10 through 4.14.
4.1 Representing Keys for Use in Cryptographic
Algorithms
Problem
You need to keep an internal representation of a symmetric key. You may want to
save this key to disk, pass it over a network, or use it in some other way.
Solution
Simply keep the key as an ordered array of bytes. For example:
/* When statically allocated */
unsigned char *key[KEYLEN_BYTES];
/* When dynamically allocated */
unsigned char *key = (unsigned char *)malloc(KEYLEN_BYTES);
Representing Keys for Use in Cryptographic Algorithms | 117
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