Unformatted text preview: an [713, 1074]. It could only be used for encryption, although Adi Shamir later adapted the system for digital signatures [1413]. Knapsack algorithms get their security from the knapsack problem, an NPcomplete problem. Although this algorithm was later found to be insecure, it is worth examining because it demonstrates how an NPcomplete problem can be used for publickey cryptography. The knapsack problem is a simple one. Given a pile of items, each with different weights, is it possible to put some of those items into a knapsack so that the knapsack weighs a given amount? More formally: Given a set of values M1, M2,..., Mn , and a sum S, compute the values of bi such that S = b1M1 + b2M2 + ...+ bnMn The values of bi can be either zero or one. A one indicates that the item is in the knapsack; a zero indicates that it isn’t. For example, the items might have weights of 1, 5, 6, 11, 14, and 20. You could pack a knapsack that weighs 22; use weights 5, 6, and 11. You could not pack a knapsack that weighs 24. In general, the time required to solve this problem seems to grow exponentially with the number of items in the pile. The idea behind the MerkleHellman knapsack algorithm is to encode a message as a solution to a series of knapsack problems. A block of plaintext equal in length to the number of items in the pile would select the items in the knapsack (plaintext bits corresponding to the b values), and the ciphertext would be the resulting sum. Figure 19.1 shows a plaintext encrypted with a sample knapsack problem. The trick is that there are actually two different knapsack problems, one solvable in linear time and the other believed not to be. The easy knapsack can be modified to create the hard knapsack. The public key is the hard knapsack, which can easily be used to encrypt but cannot be used to decrypt messages. The private key is the easy knapsack, which gives an easy way to decrypt messages. People who don’t know the private key are forced to try to solve the hard knapsack problem. Superincreasing Knapsacks
What is the easy knapsack problem? If the list of weigh...
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 Fall '10
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 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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