Unformatted text preview: random session key. Security of Public-Key Algorithms
Since a cryptanalyst has access to the public key, he can always choose any message to encrypt. This means that a cryptanalyst, given C = EK(P), can guess the value of P and easily check his guess. This is a serious problem if the number of possible plaintext messages is small enough to allow exhaustive search, but can be solved by padding messages with a string of random bits. This makes identical plaintext messages encrypt to different ciphertext messages. (For more about this concept, see Section 23.15.) This is especially important if a public-key algorithm is used to encrypt a session key. Eve can generate a database of all possible session keys encrypted with Bob’s public key. Sure, this requires a large amount of time and memory, but for a 40-bit exportable key or a 56-bit DES key, it’s a whole lot less time and memory than breaking Bob’s public key. Once Eve has generated the database, she will have his key and can read his mail at will. Public-key algorithms are designed to resist chosen-plaintext attacks; their security is based both on the difficulty of deducing the secret key from the public key and the difficulty of deducing the plaintext from the ciphertext. However, most public-key algorithms are particularly susceptible to a chosen-ciphertext attack (see Section 1.1). In systems where the digital signature operation is the inverse of the encryption operation, this attack is impossible to prevent unless different keys are used for encryption and signatures. Consequently, it is important to look at the whole system and not just at the individual parts. Good public-key protocols are designed so that the various parties can’t decrypt arbitrary messages generated by other parties—the proof-of-identity protocols are a good example (see Section 5.2). 19.2 Knapsack Algorithms
The first algorithm for generalized public-key encryption was the knapsack algorithm developed by Ralph Merkle and Martin Hellm...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips