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*To*: [email protected]*Subject*: Re: subjective names and MITM*From*: Bill Stewart <[email protected]>*Date*: Fri, 06 Oct 1995 00:34:53 -0700*Sender*: [email protected]

At 10:24 PM 10/5/95 -0400, Adam Shostack <[email protected]> wrote: > The key does indeed have a high likelihood of being unique, >but dealing with 1024 bit identifiers could strain database systems, >especially when 100 well chosen bits would be than enough. If everybody in the world has a 1024-bit key, that's 750 GB; that's not bad at all by the time everybody in the world would have one, and you could do MD5s in about 100 GB, which is a little more convenient. Today, for 750 GB, you'd probably have tape in your system, or optical jukebox. But not everybody has one; on the other hand, I suppose individuals in well-heeled countries might end up with dozens of their own. On the other hand, the number of 1024-bit primes is something like 2^1023/log(2^1024) (if I remember right), or roughly 2^1013. The probability of two independent uniformly distributed primes matching is about 2^1023/2^32, which is still an astoundingly mindbogglingly LARGE number. If you've got 1024 bits of entropy in your input process, there will not _be_ any matches. (If people insist on using sources of randomness like clocks or user names, the chances are a lot higher....) Now, for 128-bit hashes, whether it's MD5 or just the near-bottom 128 key bits, by the time you get 2^64 of them together, you've got a 50% chance of a match. Not a problem, since you'll not likely need that many, even for the 2^33 people in the world. Still not a problem. #--- # Thanks; Bill # Bill Stewart, Freelance Information Architect, [email protected] # Phone +1-510-247-0664 Pager/Voicemail 1-408-787-1281 #---

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