Hacker News Reader

Comment by chpmrc | Hacker News Reader

The URLs of the images seem to be a combination of a MD5 hash and an ID (changing the ID will produce a different image). I guess the point is that only machines can reverse MD5 to get the actual "image name"?

View on HN

There is no reversing of an md5 hash. You can try to cause a hash collision, or brute force compute it, but you can't turn something like 40 bytes of data into 100 for example.

Rainbow table: https://en.wikipedia.org/wiki/Rainbow_table

This still isn't reversing a hash.

A one way function cannot be reversed by definition. I obviously meant finding a set of possible strings that produce that hash and one of them will likely be the image name. "Reverse" wasn't the perfectly accuarate word to use but sometimes a bit of intuition goes a long way.

Hashes are inherently lossy. Although a rainbow table can maybe tell you one possible input for a given hash, it cannot tell you exactly what was hashed.

but in the case of hashes -> URL there is a fairly reasonable rule set of what constitutes a plausible reversal. Therefore generated collisions could be reality checked, unlike other things (like a md5 of an encrypted file)

The sounds like a great research topic

Obviously I don't mean implementing a function that does f(md5(str)) => str in O(1) but rather performing a dictionary or brute force attack.

You can't reverse most hashes, you can just check if one thing's hash is the same as another thing's hash. If they are, they're probably the same thing.

If you can reverse a hash, its not a hash.

Theoretically, no, but in practice if you know that "password123" hashes to "blaHb1ah" then you get a DB of hashed passwords and see "blaHb1ah", you probably know that person's password is "password123". (which is why you use salts to fix that). For all intents and purposes I just reversed the hash in this context.

Kinda? But there's infinite number of other things that will hash to that same value.

So you can assume (probably with good certainty) that you've got the correct password, but you can't be sure.

So pedantically speaking, it's not really reversible.

> So you can assume (probably with good certainty) that you've got the correct password, but you can't be sure.

That's assuming no other constraints.

If the constraints on the password are strong enough (for example, must include letters, numbers, special characters, and be less than 30 characters) that there really may be only one input that satisfies those constraints and also hashes to the found value.

100% true.

True. However, in most circumstances, finding a input that hashes to that value is equivalent to reversing the hash. For example, if your password is "password", and that hashes to "blahblah", and I find that "foobar" also hashes to "blahblah", then I can log into your account with password "foobar" even though that wasn't your original password.

> However, in most circumstances, finding a input that hashes to that value is equivalent to reversing the hash.

In most circumstances, is guessing your smartphone's pin code is equivalent to hacking the security of your phone?

In both examples, the end result is the same, but the process is absolutely not. Reversing a hash is a general solution that can be reapplied in all circumstances, much like finding a security exploit to bypass the lockscreen of your phone.

Looking up a hash in a rainbow table that matches is much like guessing your PIN. It works for that one specific case. It is not a generalised solution across the board.

I appreciate that you specified "pedantically". A whole thread on an inaccurate use of the word "reverse". Oh boy!

This is true. If a "hash" is reversible, it's actually a cipher.

I'd also argue that a hash (SHA, MD5, etc) is also reversible IFF the bit length does not exceed the bitlength of the hash.

It's how many a password db is cracked. A hash may have infinite unhashed representations, but if the maxlength (in bits) is less than the hash type, then rainbow tables can relatively easily handle it.

Not really, since the hash length is the same. You can end up with infinitely many solutions to a hash, there's no guarantee that the smaller one is the correct one.

Reply on news.ycombinator.com