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The study of breaking cryptographic systems is known as of cryptanalysis. Similar to the attacker, the cryptanalyst looks for weaknesses in algorithms. All algorithms can be broken; the good ones are simply the algorithms strong enough to withstand an attack for so long the break comes too late. So a cryptanalyst s job is to find weaknesses that may help someone break the algorithm faster. Attackers may use cryptanalytic techniques to do damage, but they may also use other tools. The cryptographer develops crypto systems; the cryptanalyst looks for weaknesses. It s important for the crypto community to know about the weaknesses because attackers are looking for them as well. Attackers are almost certainly not going to announce their discoveries to the world, so cryptanalysts perform a service, letting us all know what attackers probably know but won t tell us.
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The term key comes from the fact that the secret number you choose works in the same way that a conventional key works. To protect the contents of your house, you install a lock on the door. To operate the lock, you insert the key and turn it. The lock s tumblers and mechanisms work with the key in a prescribed way to activate a barrier that prevents the door from being opened. To unlock the door, you insert the key and turn it in the opposite direction. The tumblers and mechanisms work with the key to reverse the process and allow the door to be opened. In cryptography, to protect the contents of your files, you install a lock (an encryption algorithm) on your door (the computer). To operate the lock (encrypt the data), you insert the key (the secret number) and execute it (instead of turning this key, you operate the program by double-clicking, clicking OK, or pressing ENTER). The algorithm performs its steps using the key to alter the plaintext and convert it to ciphertext. To unlock the encrypted file, you insert the same key and execute. The algorithm reverses the steps and converts the ciphertext back to the original plaintext. Just as only the correct house key can open your front door, only the correct encryption key can decrypt data. In symmetric-key cryptography, the key that is used to encrypt data is the same key that is used to decrypt it. Symmetric essentially means the same on two sides, and that s what we have here: the same key on two sides of the encryption process. Figure 2-4, a picture you ll see quite a bit in this book, is the image we use to visualize cryptography.
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Figure 2-4 This encryption algorithm uses the key to convert plaintext to ciphertext. In symmetric-key cryptography, the key used for encryption is also necessary for decryption
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In this book we talk about some of the many different encryption algorithms you have to choose from, but remember that keys are not interchangeable among algorithms. For example, suppose that you encrypt data using the Triple Digital Encryption Standard (DES) algorithm (discussed later in the section titled Triple DES ). If you try to decrypt the data using the Advanced Encryption Standard (AES) cipher (discussed later in the section titled Advanced Encryption Standard ), even if you use the same key, you will not get the correct result.
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All computer crypto operates with keys. Why is a key necessary Why not create an algorithm that doesn t need a key As you saw in the memo example, if attackers can understand the algorithm, they can recover secret data simply by executing the algorithm. That s like installing a deadbolt on your front door with the lock on the outside. It s true that when the deadbolt is in place, the door cannot be opened. But anyone can open the door simply by turning the lock. It might seem that the solution is to keep the algorithm secret, but that approach has several problems. First, attackers always crack the algorithm (see Historical Note: They Always Figure Out The Algorithm, later in this chapter). What s more, suppose you do manage to keep the algorithm secret. Unless you are a cryptography expert and develop your own algorithms, you also must trust the company that wrote your algorithm never to reveal it deliberately or accidentally. Does anyone have that much trust in a corporate entity Here s the real question: Which would you trust more to keep secrets an algorithm that must be kept secret, or an algorithm that can do its job even if everyone in the world knows exactly how it works That s where keys come in. Keys relieve you of the need to worry about the algorithm used in your encryption scheme. If you protect your data with a key, you need protect only the key, something that s easier to do than protecting an algorithm. In this book you ll learn a lot about key protection. Also, if you use keys to protect your secrets, you can use different keys to protect different secrets. This means that if someone breaks one of your keys, your other secrets are still safe. If you re depending on a secret algorithm, an attacker who breaks that one secret gets access to all your secrets.
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