Re: Algoritmic Complexity Attacks and 2.4.20 the dcache code

[Scott A Crosby <scrosby@cs.rice.edu>]

On Thu, 29 May 2003 23:24:40 -0700 (PDT), "David S. Miller" <davem@redhat.com> writes:

>    From: Scott A Crosby <scrosby@cs.rice.edu>
>    Date: 30 May 2003 00:04:24 -0500
>    
>    Have you seen the current dcache function?
>    
>    /* Linux dcache */
>    #define HASH_3(hi,ho,c)  ho=(hi + (c << 4) + (c >> 4)) * 11
>    
> Awesome, moving the Jenkins will actually save us some
> cycles :-)

Tricky though, because what if you want to hash more than 64 bits? You
have to somehow chain Jenkin's together.

Let H(a,b,c) be a jenkin's hash that does  '' mix(a,b,c) ; return a ''

Let a,b,c,d,e be inputs to be hashed, and R,S,T,U be random keys.


Its not safe to do anything like

 H(H(a,b,c),H(d,e,f),R)

Because an attacker can brute-force to find tuples (a,b,c),
(a',b',c'), ... so that H(a,b,c) == H(a',b',c') == ....

A better approach (which I make with no formal analysis of its
quality) might be to construct this:

 H(a,b,R) ^ H(c,d,S) ^ H(e,f,T)

Perhaps the best approach is to visit Usenix Security 2003 this August
and ask the experts there.

Scott