Question

I’d like to know which algorithm is employed. I strongly assume it’s something simple and hopefully common. There’s no lag in generating the results, for instance.

Input: any string
Output: 5 hex characters (0-F)

I have access to as many keys and results as I wish, but I don’t know how exactly I could harness this to attack the function. Is there any method? If I knew any functions that converted to 5-chars to start with then I might be able to brute force for a salt or something.

I know for example that:
a=06a07
b=bfbb5
c=63447
(in case you have something in mind)

In normal use it converts random 32-char strings into 5-char strings.

Answer 1

The only way to derive a hash function from data is through brute force, perhaps combined with some cleverness. There are an infinite number of hash functions, and the good ones perform what is essentially one-way encryption, so it’s a question of trial and error.

It’s practically irrelevant that your function converts 32-character strings into 5-character hashes; the output is probably truncated. For fun, here are some perfectly legitimate examples, the last 3 of which are cryptographically terrible:

• Use the MD5 hashing algorithm, which generates a 16-character hash, and use the 10^th through the 14^th characters.
• Use the SHA-1 algorithm and take the last 5 characters.
• If the input string is alphabetic, use the simple substitution A=1, B=2, C=3, … and take the first 5 digits.
• Find each character on your keyboard, measure its distance from the left edge in millimeters, and use every other digit, in reverse order, starting with the last one.
• Create a stackoverflow user whose name is the 32-bit string, divide 113 by the corresponding user ID number, and take the first 5 digits after the decimal. (But don’t tell ’em I told you to do it!)