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A PBE program will do its work, even with the wrong password. Suppose the wrong password were entered, the program would have no way of knowing it was an incorrect password. It would simply mix the bad value with the salt and produce a KEK. It wouldn t be the correct KEK, but the
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Password Salt Ray attacks password Session key
To: From : Sales quart er
If Pao-Chi uses PBE to protect bulk data, Ray can recover it by breaking the password. If PaoChi uses PBE to protect the session key, Ray must find the encrypted key
Encrypt engine
To: From : Sales quart er
Password
Salt
Ray has to find the PBE protected session key to attack the password
Ray attacks session key
To: From : Sales
Encrypt engine
program wouldn t know that; it just blindly follows instructions. It would then use that KEK to decrypt the session key. That would work; some value would come out as a result. It would be the wrong value, but there would be something there. Then the program would use this supposed session key to decrypt the ciphertext. The resulting data would be gibberish, but only then would it be possible to see that something went wrong.
Symmetric-Key Management
For this reason, it would have been more convenient if, when entering the password, there were some way to know immediately whether it s the correct password or not. That would be better than decrypting the entire bulk data before finding that out. One solution is to use the KEK to encrypt the session key along with something else, the something else being some recognizable value, such as the salt. Then when decrypting, the program checks this recognizable value first. If it s correct, continue using the session key to decrypt the bulk data. If not, the password was wrong and the process should start over. The overall process looks like this. To encrypt bulk data: 1. Generate a random or pseudo-random session key. Use this key to encrypt the data. 2. Enter the password, generate a salt, and mix the two together to produce the KEK. 3. Encrypt the salt and session key using the KEK. Store the encrypted data with the salt. 4. Store the encrypted session key, which is actually the session key and the salt (see Figure 3-6). To decrypt the data, follow these steps. 1. Collect the salt and password and mix the two together to produce what is presumably the KEK. 2. Using this KEK, decrypt the session key. The result is really the session key and the salt. 3. Check the decrypted salt. Is it correct a. If it is not correct, don t bother using the generated session key to decrypt the data; it s not the correct value. The user probably entered the wrong password. Go back to step 1. b. If it is correct, use the session key to decrypt the data. Instead of the salt, you can use a number of things as a check. For example, it could be an eight-byte number, the first four bytes being a random value and the second four, that random value plus 1. When decrypting, check the first eight bytes; if the second four bytes is the first four plus 1, it s the correct password. This may be more palatable than the salt, since if the salt is the check, there is now some known plaintext. Presumably, the cipher is immune to a known-plaintext attack, but nonetheless,
Figure 3-6
3
Use a KEK to encrypt the session key along with a recognizable value such as the salt. Entering the wrong password produces the wrong KEK/salt combination
Password
Salt
PBE Encrypt engine
Session key Encrypt engine Protected key and salt
Password
Salt = KEK salt
Protected key and salt
Session key Decrypt engine
some people might feel it is more secure without any known plaintext. Of course, it is possible to use the wrong password and get a KEK that decrypts the check into a different eight-byte value that by sheer coincidence passes the test. The chances of this happening are so small, it will probably never happen in a million years. Another check could be an algorithm identifier. This would be some sequence of bytes that represents the algorithm being used. Or it could be a combination of some of these values. In the real world, you ll probably find that engineers come up with complex procedures that include multiple checks. In these schemes, maybe one check accidentally passes, but not all of them.
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