On Tuesday, 15 November, 2016 15:30, R Smith wrote: > >> On 2016/11/15 10:38 PM, Jens Alfke wrote: > >> >> On Nov 15, 2016, at 11:35 AM, Quan Yong Zhai <q...@msn.com> wrote: > >> >> Create a custom function MD5 , >> > If you’re going to go to this trouble, at least use SHA256! >> > >> > MD5 is broken. These days no one should be using it for anything, >> > except >> when needed for compatibility with legacy formats/protocols. >> >> This strikes me as one of those statements that, upon hearing that a >> rocket failed to land upright, cries out: "OMG, this is the end of >> Space exploration as we know it!". >> >> MD5 is not broken. What would broken mean? It still hashes a byte >> array just as good as it always did producing an easy key to check and >> compare against. Sure, for ultimate safety systems you /should/ note >> that the hashes /could/ possibly be brute-forced to, in the minimum >> case, reproduce a duplicate answer for a different byte-list. > >All hash functions that take an input that is larger (longer) that the output >must by definition have collisions. There is no if, and, or but about it. If >you input every 513 bit sequence >into SHA-512 you WILL WITH ABSOLUTE >CERTAINTY obtain the same 512 bit hash result at least twice. > This is the pigeon(hole) principle, attributed to the German mathematician Peter Gustav Lejeune Dirichlet, who is noted to have used this argument in 1834.
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