crc32 decrypt short string

Question

I am retrieving lists of crc32 hashes that contain names of files, not there contents.

I need to be able to decrypt the strings which are hashed names like "vacationplans_2010.txt"

which are less then 25 characters long.

is this possible?

Answer 1

it is one-way hash function. It can't be decrypted.

Answer 2

Despite what other users answered, CRC32 is not a cryptographic hash function; it is meant for integrity checks (data checksums). Cryptographic hash functions are often described as "one-way hash functions", CRC32 lacks the "one-way" part.

That being said, you should consider the following: since the set of all possible 25-characters-or-less filenames is more than 2^32, some file names are bound to have the same hash value. Therefore, it might be that for some of the CRC32 values you get - there will be several possible sources (file-names). You will need a way to determine the "real" source (i assume that human-decision would be the best choice, since our brain is a great pattern-recognition device, but it really depends on your scenario).

Several methods can be used to partially achieve what you are asking for. Brute-force is one of them (although, with 25 characters long file names, brute-force may take a while). A modified dictionary attack is another option. Other options are based on analysis of the CRC32 algorithm, and will require that you dive into the implementation details of the algorithm (otherwise you'll have a hard time understanding what you're implementing). For example, see this article, or this artice.

EDIT: definitions by Bruce Schneier (author of Applied Cryptography, among other things):

One-way functions are relatively easy to compute, but significantly harder to reverse. … . In this context, "hard" is defined as something like: It would take millions of years to compute x from f(x), even if all the computers in the worlds were assigned to the problem.

A hash function is a function, mathematical or otherwise, that takes a variable length input string and (called a pre-image) and converts it to a fixed length (generally smaller) output string (called a hash value).

The security of a one-way hash function is its one-wayness.