In general, there are three ways to attack cryptography:

Acquire or guess the secret key

Identify and exploit weaknesses in the cryptographic algorithms

Identify and exploit weaknesses in the software that implements the cryptographic solution

We discuss each of these techniques in the following sections, but the key point to remember is that cryptography is vulnerable to attack, and does not provide invincible security. The passage of time weakens cryptographic security. Once Eve has intercepted a message from Alice, she can attack the cryptography at her leisure; therefore, Alice and Bob must assume that at some point Eve will be able to read the contents of that message.

A brute force attack is the process of trying every possible key to determine which key Alice used. Modern cryptography is based on mathematical algorithms using numerical keys. The length of the key (the number of digits) determines how many possible keys there can be. If there are only 10 possible keys, Eve has to try to decrypt the message at most 10 times before she finds the key that Alice uses.

The cryptographic algorithms that we discuss in this book have different key sizes, some of them allowing ranges that are so big that it could take many hundreds of thousands of years to check all of the possible keys that Alice may have used. However, Eve has history on her side; the speed at which computers can test keys has increased enormously in recent years, and there is no reason why this should change in the future. Computing-performance increases will allow Eve to check more keys each day, or completely new techniques may be devised for attempting to guess keys, allowing her to test all of the combinations in a fraction of the time. Alternately, Eve may create a global network of computers and coordinate the guesses to speed up the attack using current technology.

Instead of using brute force, Eve may try to steal the key from Alice or Bob, either by tricking them into revealing the secret, or by intimidation and violence. While it is more likely that trickery would be used, we have to bear in mind that some information is of limitless value, and using violence to force Alice or Bob into revealing the key is not inconceivable.

Finally, Eve may just guess the key. Eve has to be lucky only once to be able to beat huge odds and guess the right key; the odds of her guessing correctly are extremely low, but that is the very nature of being lucky.

Cryptanalysis is the study of the algorithms that Alice and Bob have used, looking for weaknesses that can be exploited to reveal the messages or the keys that have been used. By the time that new cryptographic algorithms enter commercial use, they have been subjected to extensive cryptanalysis by security researchers and interested mathematicians, and are generally regarded to contain no obvious flaws.

However, just because there are no publicly known flaws does not mean that none will be discovered in the future or even that some flaws are already known, but they are being kept secret by people like Eve in order to lull Alice and Bob into a false sense of security. Cryptography is a dynamic area; new algorithms are developed and old ones are found to be insecure. Alice and Bob cannot rely on their selected algorithm to remain secure forever.

Even if Eve is unable to discover the key or find a weakness in the cryptographic algorithm, she may be able to exploit a bug in the software that Alice and Bob use to perform their cryptography.

Although new cryptographic theories are rigorously debated and tested before they are considered to be secure, the software that implements the theory is often untested and insecure. If Eve can find a weakness in the software, she may be able to exploit it to obtain the keys that Alice and Bob are using.

The most common problem with security software is that complex algorithms are not correctly implemented or are used in such a way as to make them much less secure than they should be, as in inadvertently discarding some of the key that Alice has created, thereby reducing the number of guesses that Eve has to make.

When Alice and Bob select cryptographic software, they must be confidant that their security will not be compromised by programming errors.