Unformatted text preview: hash function to transform an arbitrary-length text string into a pseudo-random-bit string. For example, the easy-to-remember text string: My name is Ozymandias, king of kings. Look on my works, ye mighty, and despair. might crunch into this 64-bit key: e6c1 4398 5ae9 0a9b Of course, it can be difficult to type an entire phrase into a computer with the echo turned off. Clever suggestions to solve this problem would be appreciated. If the phrase is long enough, the resulting key will be random. Exactly what “long enough” means is open to interpretation. Information theory tells us that standard English has about 1.3 bits of information per character (see Section 11.1). For a 64-bit key, a pass phrase of about 49 characters, or 10 normal English words, should be sufficient. As a rule of thumb, figure that you need five words for each 4 bytes of key. That’s a conservative assumption, since it doesn’t take into account case, spacing, and punctuation. This technique can even be used to generate private keys for public-key cryptography systems: The text string could be crunched into a random seed, and that seed could be fed into a deterministic system that generates public-key/private-key key pairs. If you are choosing a pass phrase, choose something unique and easy-to-remember. Don’t choose phrases from literature—the example from “Ozymandias” is a bad one. Both the complete works of Shakespeare and the dialogue from Star Wars are available on-line and can be used in a dictionary attack. Choose something obscure, but personal. Include punctuation and capitalization; if you can, include numbers and non-alphanumeric symbols. Poor or improper English, or even a foreign language, makes the pass phrase less susceptible to a dictionary attack. One suggestion is to use a phrase that is “shocking nonsense”: something offensive enough that you are likely to remember and unlikely to write down. Despite everything written here, obscurity is no substitute for true randomness. The best keys are r...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips