Proof of work - an unobtrusive alternative to captchas

Re:Proof of work - an unobtrusive alternative to captchas by tomgallard on Friday May 06, 2011 @05:25AM (#2520454)

This wouldn't work would it? If the server sends the client a number (n), surely, I (as a spammer), could simply pre-calculate the largest integer (p) where the hash starts with 0000. The server cannot verify this is the wrong answer, without calculating all the hashes between n and p.

So I can send back p as my answer for all values of n, without needing to do any client side calculation

[ Parent | on HN ]

Re:Proof of work - an unobtrusive alternative to captchas by dchest on Friday May 06, 2011 @07:25AM (#2520625)

You're right, the server needs to send additional unique information, as in http://en.wikipedia.org/wiki/Hashcash

    Server:

    uniq_id = long random string
    salt = long random string
    work_number = random integer
    
    storage[uniq_id] = salt
    send(uniq_id, salt, work_number)


    Client: 

    new_number = work_number

    while hash[:4] != "0000"
      hash = hashfunc(salt | new_number++)

    send(uniq_id, new_number)
    

    Server:
    
    salt = storage[uniq_id]
    if no salt
         deny

    delete from storage[uniq_id]

    if hashfunc(salt | new_number)[:4] != "0000" 
         deny
    else
         allow

May also add timestamps.

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