CODEWITHSUNDEEP
filehash
yask
yask

Reputation: 4268

How does hash functions (such as md5) verify if the files downloaded are not corrupt?

I wish to know how do software verify the downloaded files are not corrupt by using hash functions?

Upvotes: 0

Views: 276

Answers (2)

Michael Coleman
Michael Coleman

Reputation: 3398

Consider password hash verfication process....
you signup to "www.example.com" and they ask for your password

"your-secret-password" >> gets hashed and becomes gn234hs (for example) You now have a "reference" hash

you come back a month later and as long as you provide same password the hash function will produce the same output gn234hs - which matches the original and verifies that what you entered is the same as what was entered last time.

No big insights there....

what if, instead of feeding in a password - someone feed a binary representation of a file or a collection of text files into the hashing function.

[010101001010101... huge number] >> hash function
hash function produces 32j4h234j234k23j4h23k4h23kj423kj4h3 you now have a "reference hash" for that file.

Now you get a file off the internet

If you run the file through the same hashing function and you get 32j4h234j234k23j4h23k4h23kj423kj4h3 - same as for a password - you know the file is a bit for bit representation of the original.

So the question is, I get how a hash can represent a password thats only a few characters , but how can a hash represent an unbelievably huge binary sequence or text file, be "sensitive" enough to detect changes and still have a unique quality?
Basically, because of the "randomness" of the output of cryptographic hash functions (as distinct from ordinary hashes) and the number of possible combinations a hash can have is so huge, that whilst its possible for different permutations of the items being hashed to result in the same hash - its so small as to be considered statistically insignificant.

Its a bit oversimplified, but hopefully that helps. There (obviously) is tons of info on the subject if you google it, e.g. the wiki article linked to already.

Upvotes: 1

committedandroider
committedandroider

Reputation: 9251

You should read http://en.wikipedia.org/wiki/File_verification,

"Hash-based verification ensures that a file has not been corrupted by comparing the file's hash value to a previously calculated value. If these values match, the file is presumed to be unmodified." That's how

Upvotes: 1

Related Questions