Re^4: MD5-based Unique Session ID Generator

in reply to Re^3: MD5-based Unique Session ID Generator
in thread MD5-based Unique Session ID Generator

Double hashing without adding something else to it gains you nothing. If you get a collision with the first hash, you'll always get a collision with the second as well.

double hash just costs you processing time.

Comment on Re^4: MD5-based Unique Session ID Generator

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Re^5: MD5-based Unique Session ID Generator by ctilmes (Vicar) on Aug 20, 2004 at 12:12 UTC
I would even go as far as to say that double hashing can actually make more collisions. If you get a collision with the first hash, you absolutely will get a collision with the second hash, but if you don't get a collision with the first hash, you still have a chance of getting a collision with the second hash. say you start with X and Y. hash(X) = X' hash(Y) = Y' hash(X') = X'' hash(Y') = Y'' if X' = Y' (collision with first hash), then X'' = Y'' (collision with second hash) if X' != Y' (no collision with first hash), then X'' may = Y'' (possible collision with second hash)	[reply]

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Re^5: MD5-based Unique Session ID Generator
by ctilmes (Vicar) on Aug 20, 2004 at 12:12 UTC

say you start with X and Y.

hash(X) = X'
hash(Y) = Y'

hash(X') = X''
hash(Y') = Y''

if X' = Y' (collision with first hash), then X'' = Y'' (collision with second hash)
if X' != Y' (no collision with first hash), then X'' may = Y'' (possible collision with second hash)

[reply]

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