[Sussex] Q & A session from last nights presentation

[Colin Tuckley]

At my presentation last night Steve talked a bit about altering the text of
a signed message in such a way that the digest (and hence the signature) are
still valid.

I've done a bit more research this morning and have found the following:

1) The older message digest (hash) functions are indeed vulnerable. They are
no longer used because of this. The newer SHA-2 digest has so far proved to
secure.

2) The amount of computer resources required to generate a 'collision' even
in the older SHA-1 algorithm is very large (we are talking 2^69 operations
here). An example attack against the even more simple SHA-0 algorithm was
attempted. Finding the collision had complexity 2^51 and took about 80,000
CPU hours on a supercomputer with 256 Itanium 2 processors. So hardly practical!

So in theory Steve is correct, however in practice even governments have
trouble cracking the old algorithms and have not yet been able to crack the
newer ones.

regards,

Colin


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