?splints for broken hash functions

[Hal Finney]

John Kelsey critiques the proposal from Practical Cryptography:
> >"We do not know of any literature about how to fix the hash functions, 
> >but here is what we came up with when writing this book. ... Let h be 
> >one of the hash functions mentioned above. Instead of m->h(m), we use 
> >m->h(h(m) || m) as hash function.
>
> I believe this falls to a generalization of the Joux attack, as well.  (Someone may have already noticed this.)  
>
> a.  I build a 2^{80} multicollision on h(m) using Joux' attack, requiring 80*2^{80} work.  
>
> b.  I now have 2^{80} different messages which are being hashed with the same IV.  I expect one pair of them to give me a collision.  

You did 80*2^80 work to create a collision.  But you could create a
collision without all this effort in only 2^80 work, by a conventional
birthday attack.  Just ignore the internal structure and treat it as
a 160 bit hash function and you can collide in 2^80 work.  You worked
harder than you needed to, so this is not a break.

Hal

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