Re: CHALLENGE? Toto/signature attack w. unpublished public key

[Anonymous <nobody@replay.com>]

> > In addition it has to be that n is the right length based on the "s"
> > padding.  This limits it to an 8 bit range, in this case 1024-1031
> > bits.
> 
> The constraint I gave was that log(n) = 1024.  Bear in mind that the
> msbyte of s = 0x08, so we know that n > s, and I think we know that n
> < 2^1024 also based on the s padding from the signature. So based on
> this n would be in the range 1020 - 1024 bits, right?

Actually there is somewhat more flexibility than this.  You made up the
padding, it didn't come from the file, did it?  You could add perhaps 1
more byte of FF without stretching credibility too extremely.  Then n
could be 1020 - 1032 bits.  It's too bad that s started out so small
(assuming the "true" n was 1024 bits).

> > If you make n be a product of a bunch of small primes, so that you
> > can make signatures with it, then a third party can detect this and
> > know it is bogus.
> 
> He has to factor your n to determine that it is bogus though?  This
> would imply that he had more compute than you do.  (Not unreasonable
> threat model mind).

But didn't YOU have to factor n also?  That's what you showed, originally,
a large s^e which you factored down to get some small factors and a big
one.  If you manage to get a prime factorization you can combine factors
to get your n.  But it won't be any harder for him to factor than it was
for you.

> Resources available: one low end pentium based linux PC? :-) Just that
> the attack is clearly feasible is interesting though a demonstration
> would be perhaps more convincing to less technically aware IRS people.
> The biggest resource overhead is implementing that lot though, unless
> there exist packages or libraries which already do most of it for you.

The discrete log algorithm is similar to quadratic sieve and other modern
factoring algorithms.  You do multiple factorizations of other numbers
and combine them to generate the desired relationships.  The difference
is that the factor base is not just the primes, it uses special numbers.

There was a break a few years ago of the discrete log modulus being
used by Sun's RPC.  This was something like 200-300 bits.  Maybe that
discrete log code could be obtained from the authors.