Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!eternal-september.org!feeder.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Ben Bacarisse Newsgroups: comp.programming Subject: Re: bike locks and encryption Date: Wed, 08 Oct 2014 01:34:07 +0100 Organization: A noiseless patient Spider Lines: 46 Message-ID: <87k34btn5s.fsf@bsb.me.uk> References: <144e68c8-e88b-47a4-af78-8129ad2557c7@googlegroups.com> <87sij0upte.fsf@bsb.me.uk> <0DPYv.572258$ck2.339445@fx17.am4> <87d2a3vr6e.fsf@bsb.me.uk> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: mx02.eternal-september.org; posting-host="017616aa25f81ec581c44d76d61ba2f3"; logging-data="13295"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19Gd/BfXMjcbXF8vAUZzQKGfH009KeeFxE=" Cancel-Lock: sha1:fsm3wMofN7rjIt0z10XryW/Y5MA= sha1:BLolrLGk2yQmYeycSa4w3hMlnFM= X-BSB-Auth: 1.6d433dd857184268df7a.20141008013407BST.87k34btn5s.fsf@bsb.me.uk Xref: csiph.com comp.programming:4818 Richard Heathfield writes: > Ben Bacarisse wrote: > >> Richard Heathfield writes: >> >>> Ben Bacarisse wrote: >>> >>>> RichD writes: >>>> >>>>> Something occurred to me recntly - bicycle locks >>>>> as examples of one way functions, so useful in >>>>> encryption. That is, easy to compute in encryption, >>>>> but intractable for decrytion, lacking the key. >>>> >>>> I don't think that's a good analogy for a one-way function. Although >>>> all one-way functions have, in some very abstract sense, a "key" (the >>>> function's inverse is, I suppose, a key) they are not, usually, >>>> invertable with some small amount of data acting as a key. >>> >>> I think RSA is a counter-example, isn't it? (More on that in a >>> second.) >> >> I think you (and the OP) might be confusing trap-door functions with >> one-way functions. > > My understanding of a trap-door function is that it's easy to compute in one > direction but hard to compute in the other without a crib (for example, > RSA). > > My understanding of a one-way function is that it's easy to compute in one > direction but hard to compute in the other without a crib (for example, > RSA). s/without a crib// > So yes, I may well be confusing them. ;-) All things are confusingly similar if you ignore the differences! Trap-door functions are a special case, and an analogy that is good for a special case is not always going to be a good one for the general case. The existence of a combination to open the padlock is such a huge special feature, that it makes the analogy more confusing then helpful. -- Ben.