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

Question

I read the Wikipedia article about md5 hashes but I still can\'t understand how a hash can\'t be \"reconstituted\" back to the original text.

Could someone explain t

Answer 1

Think of a really basic hash - for the input string, return the sum of the ASCII values of each character.

hash( 'abc' ) = ascii('a')+ascii('b')+ascii('c')
              = 97 + 98 + 99
              = 294

Now, given the hash value of 294, can you tell what the original string was? Obviously not, because 'abc' and 'cba' (and countless others) give the same hash value.

Cryptographic hash functions work the same way, except that obviously the algorithm is much more complex. There are always going to be collisions, but if you know string s hashes to h, then it should be very difficult ("computationally infeasible") to construct another string that also hashes to h.