Re: A little OT: Diffie Hellman Exchange and Encryption on Cisco Routers

Here's a little note I wrote a long time back, that I keep around to remind myself of the DH details.

Bob knows b.

They both know g and n, where g has the mathematical property that it is primitive mod n.

Alice sends A (= g ^ a (mod n)) to Bob

Bob sends B (= g ^ b (mod n)) to Alice.

Alice does kA = B ^ a mod n
Bob does kB = A ^ b mod n = ((g ^ a) mod n) ^ (b mod n) = (g ^ a ^ b) mod n = ((g ^ b) mod n) ^ (a mod n) = B ^ a mod n = kA

i.e. both Alice and Bob have ended up with the final same number kA == kB, without sharing their secrets 'a' or 'b' with each other.

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For a 100-line technical summary of many of the mathematical concepts used in cryptography, here's a link. chew on it slowly :)

http://archives.neohapsis.com/archives/cisspstudy/2001-q4/0131.html

HTH,
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Ranjeet Shetye
Senior Software Engineer
Zultys Technologies
Ranjeet dot Shetye2 at Zultys dot com
http://www.zultys.com/
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The views, opinions, and judgements expressed in this message are solely those of the author. The message contents have not been reviewed or approved by Zultys.