If the space of possible hashes is smaller than the space of possible inputs (e.g. the hash is represented with fewer bits than the input data is), which is typically the case, then you can use the Pigeonhole Principle to prove what John wrote:
https://en.wikipedia.org/wiki/Pigeonhole_principle On Friday, December 5, 2014 4:35:18 PM UTC-8, John Myles White wrote: > > This function is impossible to write in generality since hash functions > aren't one-to-one. > > -- John > > On Dec 5, 2014, at 4:32 PM, David Koslicki <[email protected] > <javascript:>> wrote: > > > Hello, > > > > Is there a built in function that will undo hash()? > > > > i.e. I am looking for a function "dehash()" such that > > dehash(hash("ACTG")) == "ACTG" > > > > I can't seem to find this anywhere (documentation, google, this user > group, etc). > > > > Thanks, > > > > ~David > >
