I think I found a way to do it with ~8GB. As you mentioned you only need 2^30 tries due to birthday paradox.

But, instead of using a hash table to store (seed, hash) pairs, we can just use a list of only hashes. To generate the next hash we use current hash. i.e. next_hash = hash("example.com/" + string(int64(cur_hash))). So now, we just store 60-bit hash values which takes up 2^30 * 60 bits = 8gigs.

The only other problem is detecting if a hash already exists, which I think can be done using a space efficient probabilistic hashset like a bloom filter (and when the bloom filter says the hash already exists, we do a linear scan to find where it exists and we have a possible collision).

Actually, using that method, you'd need much less than 8GB, since a collision would mean you'd hit a cycle, so you could store something like 1 value for every 2^10 iterations. When you found a collision with one of those, you could examine the interval more carefully to find exactly where the collision was.

Ahh yes, that's very clever. Thanks for sharing :)

This is essentially the idea behind rainbow tables, which let you make a configurable time/space tradeoff when trying to find a hash collision vs a fixed known dictionary/space.

Hellman (yes as in Diffie-Hellman) invented this idea (chaining a reduction function and a hash together over and over) for the improved time/space tradeoff. Hellman's idea actually predates the terrible Microsoft cryptosystems which are famously vulnerable to it, because too many software engineers don't realise Computer Science and thus Mathematics are applicable to their work whether they like it or not.

Oechslin improved on Hellman's idea to make "Rainbow Tables" by sightly perturbing the reduction function with the depth, so that collisions are fleeting unless they also coincidentally happen in the same position on a hash chain, this makes his Rainbow Table far more efficient/ reliable for the same pre-computation effort and it produces the "rainbow" picture in his slide deck.

For my purpose Oechslin's improvement was irrelevant of course because I actually wanted collisions.

But memory fades, it is possible either that I got lucky, that I actually had 16GB of RAM, or that I made some minor efficiency improvements that were enough to make it work.

Thanks for the info. I didn't know the history of that at all. And yeah, the head in the sand effect of many devs in relation to crypto is frustrating.

Interesting, that's similar to rainbow tables.