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

Cryptanalysts often attack both symmetric and public

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Unformatted text preview: hday attack, you should choose a hash-value twice as long as you otherwise might think you need. For example, if you want to drop the odds of someone breaking your system to less than 1 in 280, use a 160-bit one-way hash function. 7.5 How Long Should a Key Be? There’s no single answer to this question; it depends on the situation. To determine how much security you need, you must ask yourself some questions. How much is your data worth? How long does it need to be secure? What are your adversaries’ resources? A customer list might be worth $1000. Financial data for an acrimonious divorce case might be worth $10,000. Advertising and marketing data for a large corporation might be worth $1 million. The master keys for a digital cash system might be worth billions. In the world of commodities trading, secrets only need to be kept for minutes. In the newspaper business, today’s secrets are tomorrow’s headlines. Product development information might need to remain secret for a year or two. U.S. Census data are required by law to remain secret for 100 years. The guest list for your sister’s surprise birthday party is only interesting to your nosy relatives. Corporate trade secrets are interesting to rival companies. Military secrets are interesting to rival militaries. You can even specify security requirements in these terms. For example: The key length must be such that there is a probability of no more than 1 in 232 that an attacker with $100 million to spend could break the system within one year, even assuming technology advances at a rate of 30 percent per annum over the period. Table 7.10, taken partially from [150], estimates the secrecy requirements for several kinds of information: Future computing power is harder to estimate, but here is a reasonable rule of thumb: The efficiency of computing equipment divided by price doubles every 18 months and increases by a factor of 10 every five years. Thus, in 50 years the fastest computers will be 10 billion times faster t...
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