[Sip] question about "Computational Puzzles for SPAM Reduction in SIP"

Hello,

I'm a student from Belgium, and I'm very interested with the problem
of SPAM, especially in SIP. My english is not very good, sorry for my
mistakes...

I've tried to implement the algorithm given in the draft
"Computational Puzzles for SPAM Reduction in SIP"
(draft-jennings-sip-hashcash-02.txt), to see what's happening when the
"work" value defined in the document increases or decreases.

My problem is that my program CANNOT find solutions when "work" is too large.
When (work <= 8) my program always finds a solution to the puzzle.
When work==9 my program finds a solution with a probability of about 50%
With work==10 this probability falls: about 30%
With work==11... about 15%
...

I don't understand why, with larger values for "work", my program
finds more rarely the solution of the Puzzle...

The draft says that when the UAC reveives a "bad" puzzle and thus when
the UAC cannot find the solution of the puzzle, he must consider the
"419 Puzzle required" as an error message. But my errors are too
frequent to consider a large percentage of challenges as errors!!!

Does somebody has tried to implement this draft?
Do you get the same result as me?

Thank you very much, best regards,
Geoffrey Dawirs

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