One-way compression function: Difference between revisions

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Revision as of 14:56, 6 January 2007

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The Davies-Meyer hash compression function feeds each block of the message (m_i) as the key to the block cipher. It feeds the previous hash value (H_i-1) as the cleartext to be encrypted. The output ciphertext is then also [[exclusive-or|XORed]] (<math>\oplus</math>) with the previous hash value (H_i-1) to produce the next hash value (H_i). In the first round when there is no previous hash value it uses a constant pre-specified initial value (H₀).

:<math>H_i = E_{m_i}{(H_{i-1})} \oplus {H_{i-1}}</math>

If the block cipher uses for instance 256-bit keys then each message block (m_i) is a 256-bit chunk of the message. If the same block cipher uses a block size of 128 bits then the input and output hash values in each round is 128 bits.

Variations of this method replace XOR with any other group operation, such as addition on 32-bit unsigned integers.

ThisIf constructionthe isused vulnerableblock tocipher anhas academicbeen attack,broken calledthen a so-called fixed point attack, thatcan be applied to this construction. According to Bruce Schneier saysthis "is not really worth worrying about"<ref>''Applied Cryptography'', second edition, page 448</ref>.

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