symetrix

speeding up the downward spiral

1. Chas 06.23.04 / 12pm

   Are you using a custom hashing algorithm, or a standard packaged deal? If it’s something that somebody else could use, they could create their own code and hash… Because you seem to be passing the hash via a hidden field, someone could then submit your form with their own hash and CAPTCHA values. If all your script is doing to verify is comparing the hashes, you may need to consider hiding that hash from the user (a session value, for example).
2. Mike 06.24.04 / 12pm

   The value in the hidden form field is the MD5 sum of TSH, where T is a timestamp of the day and hour, S is a secret string known only to the application, and H is the text contained in the image.

   It is possible for someone to reuse or resubmit a hash/solution pair, but it would only work for an hour. You could either rate limit the protected action (if it were protecting a signup form, only allow X signups per IP per hour) or expand the timestamp to include the minute.

   If anyones interested in using the code, drop me an email.
3. Chas 06.24.04 / 12pm

   Hmmm. A cracker wouldn’t necessarily be privy to this information, but knowing it would improve the chances of brute force cracking the S value. Once that was cracked, it would be trivial to reassemble or create a new TSH as you described. A good step would be using SHA, but I think expanding the timestamp to minutes (or even seconds) would solve the rest of the problem.

   In an application like a whois lookup (NetSol uses a captcha for theirs) limiting to seconds would be much more effective than minutes. Overall, looks good. I don’t have any experience with character recognition, or I’d try that for you.
4. Oliver 06.24.04 / 5pm

   I’m not an expert on this point, but using SHA-1 instead of MD5 wouldn’t strenghten the program. The cracker would just have to do 2^80 computations (for SHA-1) instead of 2^64 (for MD5): 2^64 is big enough. I even think it could have no effect at all if you just try to brute-force the program. By using SHA, you just garantee the number is unique.
   Expanding the timestamp to seconds would be a good solution.
   The best solution would be to show us the source code so we could analyze it!
5. Mike 06.25.04 / 1pm

   The reason I intergrated the ’secret’ into the computation was to allow a site owner to significantly change the hashing function with a few keystrokes.

   Assume for a second you do the 2^64 computations (thanks for doing the math Oliver) to find the secret. By changing a few characters in the secret, you again have to do 2^64 computations to find it.

   I do appricate the feedback on my methods though. Because of this dicussion i’m moving the hashing secret into an external file so a cron job could be setup to generate a new secret every day/week/month/etc.
6. Oliver 06.26.04 / 7am

   I’ve found a flaw: as long as you give the calculated hash in the answer page, I can re-submit the answer with the hash you’ve given me (same input, but your hash). With this method I don’t even need to load any picture at all!

   The first thing to do is remove the real hash on the answer page. I wrote a small script here to have a right answer every time.
7. Mike 06.27.04 / 2am

   I wouldn’t call it a flaw. The reason your given the secret key, submitted hash, and correct hash, is because this is just a demo. I have changed the output of the script to make this a little more obvious. Thanks.
8. Gerardo Capiel 01.04.05 / 6pm

   Are you making your source code available?