3 ms·

Yeah, salts mean you can't use rainbow tables. But you can still attack a single user. The question is what "computationally infeasible" actually means. How muc

by eridius 12y ago

Yeah, salts mean you can't use rainbow tables. But you can still attack a single user. The question is what "computationally infeasible" actually means. How much computing power would it take to crack a single user's password? How about if it's a weak one? A strong one? If the answer is "it would take $1000 worth of Amazon EC2 computing to crack a single weak password", well, that's certainly feasible to do if you have a specific target in mind.

azinman2 12y ago
And given th usernames and emails are out there, it reveals (a) all the companies using slack, (b) potentially very high value targets. I wonder who they had specifically emailed. My guess is those people aren't using slack anymore.
Xylakant 12y ago
The answer depends on the tuning parameters used for the bcrypt hashing (cost/work factor) and the length of the user password. Obviously a 1-character password will fall even with a high workfactor and a dictionary word will probably fall as well. This paper http://www.emsec.rub.de/media/crypto/veroeffentlichungen/2014/10/22/reconfig14_bcrypt.pdf http://www.emsec.rub.de/media/crypto/veroeffentlichungen/201... tags some numbers on breaking passwords with a low cost factor. Assuming a cost factor if 5 (12 would be more "real" world) and an 8 character password with an alphabet of 62 chars (uppercase, lowecase, 10 digits) the estimated cost to break a password within a month is in the millions with dedicated hardware designed for brcypt breaking (Fig 5). With a work factor of 12, an 8 character password will not break on EC2.
- eridius 12y ago
  I knew bcrypt was tunable, and I guess I was hoping someone from Slack would actually pop up and say how their bcrypt was tuned (which I guess means what the cost factor is). But you do have some good info, I wasn't aware of what expected cost factors were and how secure a single password would be at those cost factors. I'm assuming that your "8 character password" is assuming a randomly-generated password. I'm curious what the expected cost would be of breaking a particularly weak one (e.g. a human-generated password, based on dictionary words although perhaps with mnemonic devices or letter/number substitutions). The paper you linked says their hardware computed 6511 passwords per second at a cost factor of 5, and based on the cycle costs listed, I'm thinking it does 52 passwords per second at a cost factor of 12. Assuming a particularly weak password, I don't know how many passwords a brute-forcer would expect to have to try before hitting the correct one.

by eridius 12y ago

azinman2 12y ago

And given th usernames and emails are out there, it reveals (a) all the companies using slack, (b) potentially very high value targets. I wonder who they had specifically emailed. My guess is those people aren't using slack anymore.

Xylakant 12y ago

The answer depends on the tuning parameters used for the bcrypt hashing (cost/work factor) and the length of the user password. Obviously a 1-character password will fall even with a high workfactor and a dictionary word will probably fall as well. This paper http://www.emsec.rub.de/media/crypto/veroeffentlichungen/2014/10/22/reconfig14_bcrypt.pdf http://www.emsec.rub.de/media/crypto/veroeffentlichungen/201... tags some numbers on breaking passwords with a low cost factor. Assuming a cost factor if 5 (12 would be more "real" world) and an 8 character password with an alphabet of 62 chars (uppercase, lowecase, 10 digits) the estimated cost to break a password within a month is in the millions with dedicated hardware designed for brcypt breaking (Fig 5). With a work factor of 12, an 8 character password will not break on EC2.

eridius 12y ago
I knew bcrypt was tunable, and I guess I was hoping someone from Slack would actually pop up and say how their bcrypt was tuned (which I guess means what the cost factor is). But you do have some good info, I wasn't aware of what expected cost factors were and how secure a single password would be at those cost factors. I'm assuming that your "8 character password" is assuming a randomly-generated password. I'm curious what the expected cost would be of breaking a particularly weak one (e.g. a human-generated password, based on dictionary words although perhaps with mnemonic devices or letter/number substitutions). The paper you linked says their hardware computed 6511 passwords per second at a cost factor of 5, and based on the cycle costs listed, I'm thinking it does 52 passwords per second at a cost factor of 12. Assuming a particularly weak password, I don't know how many passwords a brute-forcer would expect to have to try before hitting the correct one.