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Unformatted text preview: Merkle’s Puzzles See: Merkle, Secrecy, Authentication, and Public Key Systems , UMI Research press, 1982 Merkle, Secure Communications Over Insecure Channels , CACM, Vol. 21, No. 4, pp. 294299, April 1978 c circlecopyrt Eli Biham  August 18, 2010 206 Merkle’s Puzzles (8) Merkle’s Puzzles Merkle’s puzzles ì÷øîìùú éúè éù 1. The first hint that two parties have computational advantage over attack ers 2. Exchanges keys over insecure channels 3. Uses puzzles c circlecopyrt Eli Biham  August 18, 2010 207 Merkle’s Puzzles (8) Puzzles • A Puzzle is a cryptogram, which is designed to be breakable • Breaking the cryptogram reveals the puzzle information hidden in the plaintext • A cryptogram can be encrypted using any secure cipher E . Examples: E = DES,E = AES • The complexity of solving the puzzle can be chosen by selecting the size of the puzzle keys. For example, for 2 20 complexity, 20bit puzzle keys can be used (the other key bits of E are fixed to some agreed value) • The plaintext of the puzzle should include redundancy to allow the users to solve it. Such redundancy is included by incorporating an agreed fixed value S, whose length suffices to ensure uniqueness of the solved puzzle key c circlecopyrt Eli Biham  August 18, 2010 208 Merkle’s Puzzles (8) Puzzles (cont.) Definition : A puzzle is E PK ( S bardbl ID bardbl K ) where • bardbl denotes concatenation • PK is an nbit puzzle key • S is an agreed fix value used in all the puzzles, whose length is at list n bits. It ensures uniqueness of the puzzle keys • ID is an nbit puzzle identifier, unique for each puzzle • K is a random value, whose size equals the size of the required common key — a keys of one of the puzzles will become the common key...
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 Spring '11
 YanivCarmeli
 Cryptography, Encryption, Merkle, Block cipher, Eli Biham

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