CODEWITHSUNDEEP
algorithmsecurityhashcryptographycryptographic-hash-function
grit639
grit639

Reputation: 346

In Hashing, can't we find AT LEAST one original text hashing to the given hash value

I have a basic question about hashing. It is said that hashing is one way. I have a doubt that if we simply reverse the steps in program/algorithm/logic then can't we find at least one input which hashes to the given output hash value?.

I found 2 related posts, but I am still not completely clear:

How is one way hashing possible?

How do one-way hash functions work? (Edited)

I have the same question as the comment to the accepted answer in the first post:

"Well, but if I want to bypass a password check it suffices to find one string that hashes to the same value as the original password". Does this comment hold water?.

Upvotes: 0

Views: 959

Answers (3)

Artjom B.
Artjom B.

Reputation: 61952

The operations in a cryptographic hash function are so complex and there are so many of them that reversing the function (compute at least one valid input for a given output) is incredibly infeasible. It doesn't matter if you do that reversing by hand or with the help of some sort of algorithmic solver. This is called (first) preimage resistance and this is what cryptographers are attacking when a new hash function is proposed. If the hash function stood the test of time, it is considered secure.

On the other hand it is much easier to just generate a bunch of candidate passwords and run the known hash function over them to check for equality with the given output. Humans are pretty bad at generating good passwords or passphrases. Have a look at this talk.

In Hashing, can't we find AT LEAST one original text hashing to the given hash value

In that context, "finding" as in brute forcing the input space is easier than attacking the hash function itself.

Upvotes: 1

PMF
PMF

Reputation: 17288

There's a very simple way of giving a hash function that is not reversible:

int GetHashCode(byte[] myData)
{
    return 1;
}

This is a perfectly valid hash function, as it maps the contents of an arbitrary data set to a much smaller domain (int in this case). It satisfies the condition that the same input data gives the same output data.

It is obvious that this function is not reversible.

(Of course, this hash function is not suitable for securing anything, but that's only one application of hash functions)

Upvotes: 0

datenwolf
datenwolf

Reputation: 162297

What you're thinking of is called "hash collisions".

And you're right to think, that if one could find an efficient method to determined inputs for a given hash functions that produce a desired output, this would break a lot of systems (https://en.wikipedia.org/wiki/Preimage_attack)

That's there the bones and meat of cryptographically secure hash functions come in. Those are built in a way, that it is very, very difficult to find a preimage that produces a desired hash.

Over time mathamaticians and cryptologists are chipping away on those hashes and quite a number of hash functions that were used for securing thing have been broken (MD4, MD5, SHA-1).

Also it's important to differentiate between hashes that are intended to check the integrity of messages, and hashes that are intended to protect secrets.

For integrety checking you want fast hashes, so that you can put a lot of data through them with minimal effort. MD5, SHA-1, SHA-2 are such hashes.

For secret keeping you want SLOW -er than molasses hashes, so that one can't easily brute force through dictionaries of other predicable patterns of a secret. SCrypt, BCrypt, Argon and many-round PBKDF schemes are such hashes.

Upvotes: 3

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