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Suppose that someone figured out that with a given algorithm, every 14th bit of a given ciphertext is the same as every 12th bit of its plaintext. In other words, if the 14th bit of ciphertext is 1, the 12th bit of plaintext is 1, the 28th bit of ciphertext is 0, the 24th of plaintext is 0, and so on, no matter what the key. Furthermore, the attacker sees that if certain combinations of bits appear in certain locations in the ciphertext, a corresponding portion of the plaintext must be another pattern. If an algorithm had such weaknesses, an attacker could look at the ciphertext and decipher parts of the plaintext even without knowing the key. This knowledge might be enough to enable the attacker to recover enough of the original message to do damage (see Figure 2-9).
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Figure 2-9 If an algorithm has a weakness, an attacker might figure out portions of plaintext without the key, reconstructing most or all of the message
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Here s another possible weakness. Suppose the attacker knows what some of the plaintext and its corresponding ciphertext is. And suppose this attacker is able to therefore deduce the key. But if the attacker knows what the plaintext is, why bother figuring out the key The answer is that the attacker might know, or be able to guess, only a portion of the plaintext. Recall the memo at the beginning of the chapter. An attacker might see the ciphertext, realize it s a Word for Windows document, and guess some of the control characters at the beginning.
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Furthermore, the attacker guesses the document is a memo from the conventional TO: , FROM: , and, RE: In short, if someone can compute the key from a chunk of ciphertext and its corresponding plaintext, the rest of the message will follow. This is known as a known-plaintext attack. Obviously, you don t want to use an algorithm that might be susceptible to such an attack.
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How long will your secret remain secret The answer is, as long as it takes the attacker to break it. The attacker has two kinds of tools: the bruteforce attack and attacks that exploit weaknesses in your algorithm. In analyzing the security of your message, a key question is how long would a successful brute-force attack take. There s no rigid, specified time, since the attacker may get lucky and find it early or may get unlucky and find it later, but as shown in Table 2-1, you can estimate the variables based on worst-case scenarios. In general, the bigger the key, the longer a brute-force attack will take. But if the algorithm is weak, it doesn t matter how long the key is. The statement Longer keys mean more security doesn t apply to a weak algorithm. The point is this: If you pick a weak algorithm, you have no control over how strongly your secret is protected. So the best strategy is to pick an algorithm that is not weak and further deter an attacker by using a longer key. That statement may seem so obvious that it s not worthwhile even to mention it. If you re curious about what happens when people overlook these obvious protections, however, read Crypto Blunders in the accompanying CD for a couple of stories on using weak algorithms and small keys.
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Virtually all symmetric ciphers use the key to build a key table, which is usually a pseudo-random array of a particular size in a particular format. This process is known as key setup, or initialization. It s the key table that does the encryption. Why have a key table One reason is that you might want to use keys of varying lengths depending on the application. The algorithm needs a
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