Show HN: Beamsplitter – a new possibly universal hash

You have a list of hash functions, and choose one at random, then hash a password. Later a hacker gets these hashed passwords, and has an extra hard time? But this wouldn't work for checking passwords because you wouldn't know what hash.

What is a real use case?

Alice is storing keys in a hash table. Since this is a hash-table, the hash (H) that Alice will choose must be fast. However, real-world hash-tables will use a relatively small number of bits from the output of H, because even if you have a table sized to 4 billion, that's only 32 bits.

Let's say that Alice does this by taking the lowest N bits of the output of H (this works in practice regardless of which bits Alice uses) where 2^N is the size of the table. N may change as elements are added

Eve wants to mess with Alice by sending a bunch of keys that all have the same bottom M bits, where M is the largest expected value for N. Since the hash H is very fast, this is very computationally cheap to brute-force, particularly if you have access to very parallel hardware like a GPU.

Now consider that instead of using hash H, Alice uses hash-family U. Whenever a hash table is created (or rehashed,) Alice selects a random hash from U. Eve can no longer easily generate keys that will collide in the hash table.

From what I can tell, for password hashing, this is not appreciably better than salting, if the size of the set of possible salts and the size of the set U are the same.

the funny thing is the very fears that are being promoted about this are in a way, sort of exactly the weaknesses that this parameterisible family of hash function was designed to secure against.

I mean people are afraid that there's somehow malevolent design floor but that could be true in any hash function with this you can use the structure to create your own hash function but bringing your nest box which to me at least greatly reduces the idear that there's some sort of exploit that could be persisting.

anyway, that unintended mystery is not bad at all in I'm my opinion. it's fun to watch people suspect byes I got from random.

it's also flattering because I think the skill required to create some sort of crazy exploitable sbox is way above me and way above the level of skill required to create a very good hash function.

people thinking that was my plan, hear this, it does not sound like a very smart plan to spend all that effort creating one amazing exploitable sbox that looks random but then at the same time say and even encourage people to use their own sbox.

I don't feel the suspicion of the sbox being bad actually requires any defense of it, because it seems just ridiculous to me, but I do think it's interesting to point out, like, that sort of a plan suspected doesn't really make sense.

I'm not saying the people who have such suspicions are ridiculous at all. they just haven't thought it through, I think and I understand the instinct to paranoia especially directed at works in this space. I think it's a fairly appropriate instinct. you just need to think things through.

the point was by using an s-box, you can bring your own s-box, to allay (or I guess create) such fears about exploitable designs, and create your own hash function.

some thoughts about how to do that I invite in the readme. I'm not prescribing rules. pick your own, pick whatever you like. The point is you can make your own hash function that will probably be a good hash function. I definitely think you should test it with smasher, or whatever, to make sure it doesn't have any kind of flaws. I'm fairly convinced, after testing a few random boxes, you'll be highly likely to make your own good hashes with this.

also good point about CPP I will change that.