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| Date | Wed, 23 Jan 2013 19:53:28 -0500 |
| From | Dave Angel <d@davea.name> |
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| Subject | Re: Uniquely identifying each & every html template |
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On 01/23/2013 07:39 PM, Chris Angelico wrote: > On Thu, Jan 24, 2013 at 11:09 AM, Dave Angel <d@davea.name> wrote: >> I certainly can't disagree that it's easy to produce a very long hash that >> isn't at all secure. But I would disagree that longer hashes >> *automatically* reduce chances of collision. > > Sure. But by and large, longer hashes give you a better chance at > avoiding collisions. > > Caveat: I am not a cryptography expert. My statements are based on my > own flawed understanding of what's going on. I use the stuff but I > don't invent it. > >> Wikipedia - http://en.wikipedia.org/wiki/Cryptographic_hash_function >> >> seems to say that there are four requirements. >> it is easy to compute the hash value for any given message >> it is infeasible to generate a message that has a given hash >> it is infeasible to modify a message without changing the hash >> it is infeasible to find two different messages with the same hash >> >> Seems to me a small hash wouldn't be able to meet the last 3 conditions. > > True, but the definition of "small" is tricky. Of course the one-byte > hash I proposed isn't going to be difficult to break, since you can > just brute-force a bunch of message changes until you find one that > has the right hash. > > But it's more about the cascade effect - that any given message has > equal probability of having any of the possible hashes. Make a random > change, get another random hash. So for a perfect one-byte hash, you > have exactly one chance in 256 of getting any particular hash. > > By comparison, a simple/naive hash that just XORs together all the > byte values fails these checks. Even if you take the message 64 bytes > at a time (thus producing a 512-bit hash), you'll still be insecure, > because it's easy to predict what hash you'll get after making a > particular change. > > This property of the hash doesn't change as worldwide computing power > improves. A hashing function might go from being "military-grade > security" to being "home-grade security" to being "two-foot fence > around your property", while still being impossible to predict without > brute-forcing. But when an algorithm is found that generates > collisions faster than the hash size indicates, it effectively reduces > the hash size to the collision rate - MD5 is 128-bit, but (if I > understand the Wikipedia note correctly) a known attack cuts that to > 20.96 bits of "real hash size". So MD5 is still better than a perfect > 16-bit hash, but not as good as a perfect 32-bit hash. (And on today's > hardware, that's not good enough.) > > http://en.wikipedia.org/wiki/Collision_resistant > > ChrisA > Thanks. I've read a lot about encryption and data compression (two overlapping fields), and done some amateurish work (first time was 1975) that was just to keep something secret, not to be especially secure. I find the field fascinating, but have never needed to do anything particularly secure for a real product. -- DaveA
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Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-18 12:48 -0800
Re: Uniquely identifying each & every html template John Gordon <gordon@panix.com> - 2013-01-18 20:59 +0000
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-18 14:12 -0800
Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-18 17:09 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-19 00:39 -0800
Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-19 04:00 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 23:08 -0800
Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-21 18:20 +1100
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 01:19 -0800
Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-21 20:31 +1100
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 04:06 -0800
Re: Uniquely identifying each & every html template Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-21 12:39 +0000
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 04:55 -0800
Re: Uniquely identifying each & every html template rusi <rustompmody@gmail.com> - 2013-01-21 19:24 -0800
Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-22 15:39 +1100
Re: Uniquely identifying each & every html template Tom P <werotizy@freent.dd> - 2013-01-22 00:01 +0100
Re: Uniquely identifying each & every html template Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-01-21 23:43 +0000
Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-22 11:04 +1100
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-22 17:36 -0800
Re: Uniquely identifying each & every html template Joel Goldstick <joel.goldstick@gmail.com> - 2013-01-21 07:47 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:00 -0800
Re: Uniquely identifying each & every html template Michael Torrie <torriem@gmail.com> - 2013-01-22 16:55 -0700
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:12 -0800
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-23 01:37 -0800
Re: Uniquely identifying each & every html template Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-01-23 09:49 +0000
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 02:29 -0800
Re: Uniquely identifying each & every html template Joel Goldstick <joel.goldstick@gmail.com> - 2013-01-23 07:03 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 04:26 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 04:26 -0800
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-23 06:25 -0800
Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-23 07:38 -0500
Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-24 10:25 +1100
Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-23 19:09 -0500
Re: Uniquely identifying each & every html template Chris Angelico <rosuav@gmail.com> - 2013-01-24 11:39 +1100
Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-23 19:53 -0500
Re: Uniquely identifying each & every html template Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-01-23 16:01 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:12 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:00 -0800
Re: Uniquely identifying each & every html template Dave Angel <d@davea.name> - 2013-01-21 17:26 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 04:06 -0800
Re: Uniquely identifying each & every html template Tim Roberts <timr@probo.com> - 2013-01-21 19:57 -0800
Re: Uniquely identifying each & every html template Tim Roberts <timr@probo.com> - 2013-01-21 20:04 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 22:49 -0800
Re: Uniquely identifying each & every html template Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-01-22 16:08 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:15 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-23 01:15 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 01:19 -0800
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 04:56 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:03 -0800
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 15:35 -0800
Re: Uniquely identifying each & every html template Piet van Oostrum <piet@vanoostrum.org> - 2013-01-21 21:48 +0100
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 22:38 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:07 -0800
Re: Uniquely identifying each & every html template alex23 <wuwei23@gmail.com> - 2013-01-21 15:36 -0800
Re: Uniquely identifying each & every html template rusi <rustompmody@gmail.com> - 2013-01-21 20:18 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-21 07:07 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 23:08 -0800
Re: Uniquely identifying each & every html template Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-01-19 16:32 -0500
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 22:52 -0800
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-20 22:52 -0800
Re: Uniquely identifying each & every html template John Gordon <gordon@panix.com> - 2013-01-22 16:35 +0000
Re: Uniquely identifying each & every html template Ferrous Cranus <nikos.gr33k@gmail.com> - 2013-01-19 00:39 -0800
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