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.

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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)

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