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Unformatted text preview: Hashing, OneTime Signatures, and MACs c circlecopyrt Eli Biham  August 18, 2010 157 Hashing, OneTime Signatures, and MACs (6) Digital Signatures A signature ( î éú ) is • Writing the name of a person, in his own hands, as a confirmation. • Commitment. Distinguish between • Identification: Assures the identity ( é ). • Commitment: Assures the commitment ( ú éú ). c circlecopyrt Eli Biham  August 18, 2010 158 Hashing, OneTime Signatures, and MACs (6) † Digital Signatures (cont.) It is possible to have identification without commitment, and vice versa: • An anonymous letter has neither. • A company letter has an identifying title. • A check is a commitment, even if it has no identification. c circlecopyrt Eli Biham  August 18, 2010 159 Hashing, OneTime Signatures, and MACs (6) Digital Signatures (cont.) A Digital signature S ( M ) ( ú éú øñî éú ): 1. Computable by the signer for any message M . 2. Everybody (and the receiver in particular) can verify its originality. 3. It is impossible to forge a signature. 4. The signer cannot claim that a message he signed is forged. c circlecopyrt Eli Biham  August 18, 2010 160 Hashing, OneTime Signatures, and MACs (6) One Way Functions Informal Definition : A one way function ( ú éðëéö÷ ð ) Y = f ( X ) is a function which is efficient to calculate but difficult to invert: for a given Y it is difficult to find any X such that Y = f ( X ). Note: There is no relationship between a one way function and an invertible function. Example : Y = f ( X ) = AES X (0) is a one way function, if there is no suc cessful attack on AES which finds the key X from the ciphertext Y . c circlecopyrt Eli Biham  August 18, 2010 161 Hashing, OneTime Signatures, and MACs (6) • Lamport and Diffie’s Signature Scheme Preparation : 1. A one way function Y = f ( X ) is selected. 2. Each user U chooses 2 n random values X ,X 1 ,...,X 2 n − 1 , and computes Y ,Y 1 ,...,Y 2 n − 1 by Y i = f ( X i ). 3. U publishes the vector Y = ( Y ,Y 1 ,...,Y 2 n − 1 ) in a public file under his name (i.e., in a newspaper, or in a public file maintained by a trusted center). 4. U publishes in advance as many vectors as the number of signatures he is expected to sign. c circlecopyrt Eli Biham  August 18, 2010 162 Hashing, OneTime Signatures, and MACs (6) Lamport and Diffie’s Signature Scheme (cont.) Signature generation : 1. A wants to sign an nbit message M to B ( M = m m 1 ...m n − 1 ). 2. A chooses one of his unused vectors from the public file, and sends it to B. 3. B verifies the existence of the vector in the public file. 4. A and B mark the vector as used in the public file. 5. A computes the signature S = S S 1 ...S n − 1 by S i = X 2 i , if m i = 0; X 2 i +1 , if m i = 1 and sends the signature S to B....
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 Spring '11
 YanivCarmeli
 Cryptographic hash function, Eli Biham, OneTime Signatures

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