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The key used to encrypt the megabytes of information, or bulk data, is generally known as the session key. A session is simply an instance of encryption, possibly during an email exchange, a World Wide Web connection, or a database storage. In Pao-Chi s case, a session involves encrypting a file before storing it on his hard drive. Some systems generate a new key for each session; others use the same key from session to session. One way to store the session key securely is to encrypt it using a symmetric-key algorithm. Someone who finds the session key has really found the encrypted key. The attacker would have to break the encryption to get the key that protects the megabytes of information. Of course, the process of encrypting the session key itself needs a key. That is, the key needs a key. There s the session key and then the key encryption key, as shown in Figure 3-1. In the crypto literature, not surprisingly, the latter is often known as the KEK. You may be thinking that if Pao-Chi uses a KEK, he now has to store and protect it as well. Actually, he does not store the KEK, and therefore does not need to protect it. When he needs a KEK to encrypt, Pao-Chi will generate it, use it, and then throw it away. When he needs to decrypt the data, he generates the KEK again, uses it, and throws it away. He is able to generate the KEK a second time and produce the same value as before because it is based on a password. Pao-Chi uses an RNG or PRNG to gen-
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Figure 3-1 A session key protects data, and a key encryption key (KEK) protects the session key
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erate a session key, he uses password-based encryption (PBE) to build the KEK. It usually works like this (see Figure 3-2). 1. Enter the password. 2. Use an RNG or PRNG to generate a salt.
NOTE:
What s a salt We describe the salt and its purpose in a few paragraphs. 3. Using a mixing algorithm, blend the salt and password together. In most cases, the mixing algorithm is a message digest. And that s the second time we ve mentioned this tool the message digest. The first time was in discussing PRNGs. Remember, a digest is a blender, taking recognizable data and mixing it up into an unrecognizable blob. We ll talk more about message digests in 5. 4. The result of step 3 is a bunch of bits that look random. Take as many of those bits as needed for the KEK and use it with a symmetric-key algorithm to encrypt the session key. When the session key has been encrypted, throw away the KEK and the password. Save the salt. 5. When storing the now encrypted session key, be sure to store the salt along with it. It is necessary to decrypt. When it comes time to decrypt the data, here s the process. 1. Enter the password. 2. Collect the salt. The same salt used to encrypt is required (that s why you saved it with the encrypted session key). 3. Using the same mixing algorithm used to encrypt, blend the salt and password together. If one or more of the salt, password, or mixing algorithm is different, the result will be a KEK; however, it will be the wrong KEK. If all three elements are the same, the result is the correct KEK. 4. Use this KEK from step 3 along with the appropriate symmetric-key algorithm to decrypt the session key. You probably have four questions.
Figure 3-2
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In passwordbased encryption (PBE), (1) blend the password and the salt to form a KEK and then (2) use it to encrypt the session key. To decrypt the data, use the same password and salt
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