Davies-Meyer Construction

Asked Apr 10 '20 at 19:01

Active Apr 10 '20 at 19:01

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For implementing a hash function, we can use Davies-Meyer Construction. In Katz crypto book it is written:

Let $F$ be a block cipher with $n$-bit key length and $l$-bit block length. We can then define the compression function $h:\{0,1\}^{l+n} \rightarrow \{0,1\}^l$ by $h(k,x)=F_k(x)\oplus x$.

My question is what is the point of XORing $x$ with $F_k(x)$?

asked Apr 10 '20 at 19:01

Reza

Because in this way it can have proven security in the Ideal Cipher Model. – kelalaka Apr 10 '20 at 19:23
Is $k$ the secret key or it is part of the input? – Reza Apr 11 '20 at 14:33
The key is the message. There is no secret key on Hash functions except they are keyed Hash functions like HMAC. – kelalaka Apr 11 '20 at 14:35
Thanks, @kelalaka! If $k$ is not secret, then the above formula completely makes sense. – Reza Apr 11 '20 at 14:40

Davies-Meyer Construction

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