Comparable hashes

Question

i wasn't able to answer my question.

I need a hashing method that will generate a hash that can be compared with others and find out the fidelity,

let's say i have to 2 strings, "mother", "father" and when i compare the 2 hashes, it will say that there is a fidelity between them because of the "ther".

Is there any hashing method that it's able to do that?

thank you

Answer 1

PHP provides a function called similar_text which calculates similarity between two strings. You could also use the levenshtein function to calculate the distance between the two strings. Whilst these aren't hashing functions, I think they should provide the functionality I think you're after.

Answer 2

I'm not sure if you were looking for an answer specific to your specific case of 2 words, but there are definitely hash-style functions that are useful for comparing parts of a whole. A Hash Tree is a perfect example of one such structure. Hash trees are used to compare parts of a chunk of data and they aggregate for comparison of the entire chunk of data.

I'll also note that while others point out that most real world hash functions will not allow any information about the input to be derived from the output, they are talking about a Cryptographic Hash Function. The set of guarantees for a regular Hash Function is much less strict than those of a Cryptographic Hash Function. For instance, in Java you can override .hashCode() and return 4 for every object. This is perfectly valid, but not extremely useful. It is valid because collisions are ok in general hash functions, but they are considered failure in a cryptographic hash function.

Answer 3

I believe rot13, along with taking out all the vowels would qualify. Any real-world hash would not. That's kind of the point.

Answer 4

In short: There can't be in a universal sense of the word

This is why:

• One of the main functions of a hash is compression - apart from trivial usage (such as "mother" and "father") a hash will allways be shorter than the hashed information. E.g. a SHA1 (or even MD5) as a quick check, whether a a download of a 600MB ISO went without corruption will be much shorter than the file itself.
• Another main function of a hash is (very high grade) obfuscation. Were this not so, hashing a salted password would do nothing (or at least much less) to protect against a dicitionary attack, as similar passwords would result in similar hashes.