applied cryptography - protocols, algorithms, and source code in c

008000008l 000000200l 008000208l 000020000l 000000008l

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Unformatted text preview: security. Fatal bugs may even be far-removed from the security portion of the software. 2. Ineffective protection against denial-of-service attacks. Some cryptographic protocols allow anonymity. It may be especially dangerous to deploy anonymous protocols if they increase the opportunities for unidentified vandals to disrupt service; anonymous systems therefore need to be especially resistant to denial-of-service attacks. Robust networks can more easily support anonymity; consider that hardly anyone worries very much about the millions of anonymous entry points to more robust networks like the telephone system or the postal service, where it’s relatively difficult (or expensive) for an individual to cause large-scale failures. 3. No place to store secrets. Cryptosystems protect large secrets with smaller ones (keys). Unfortunately, modern computers aren’t especially good at protecting even the smallest secrets. Multi-user networked workstations can be broken into and their memories compromised. Standalone, single-user machines can be stolen or compromised through viruses that leak secrets asynchronously. Remote servers, where there may be no user available to enter a passphrase (but see threat #5), are an especially hard problem. 4. Poor random-number generation. Keys and session variables need good sources of unpredictable bits. A running computer has a lot of entropy in it but rarely provides applications with a convenient or reliable way to exploit it. A number of techniques have been proposed for getting true random numbers in software (taking advantage of unpredictability in things like I/O interarrival timing, clock and timer skew, and even air turbulence inside disk enclosures), but all these are very sensitive to slight changes in the environments in which they are used. 5. Weak passphrases. Most cryptographic software addresses the key storage and key generation problems by relying on user-generated passphrase strings, which are presumed to be unpredictable enough to produce good key material and are also easy enough to remember that they do not require secure storage. While dictionary attacks are a well-known problem with short passwords, much less is known about lines of attack against user-selected passphrase-based keys. Shannon tells us that English text has only just over 1 bit of entropy per character, which would seem to leave most passphrases well within reach of brute-force search. Less is known, however, about good technique...
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