5 ms·

I'm sorry if I'm completely misunderstanding, but couldn't a hacker generate a rainbow table once - of all the common passwords, say - and then, once they get a

by sorbus 17y ago

I'm sorry if I'm completely misunderstanding, but couldn't a hacker generate a rainbow table once - of all the common passwords, say - and then, once they get a copy of the database in which the password hashes are stored, simply compare them? In the same way that md5 rainbow tables exist, which greatly reduce the computations required when looking for passwords? Unless, of course, you salt them (which would turn it into 4.8 months to break all the common passwords in a table, building a rainbow table with the salt taken in mind - the only situation in which you're spending 4.8 months per user is if they're each salted differently, for example with the username)- but the article doesn't suggest salting bcrypt, so I'm ignoring that.

tptacek 17y ago
No. Bcrypted passwords have nonces in them, just like "salted" hashes. You don't have to think about it. You just use bcrypt.
- boundlessdreamz 17y ago
  Complete noob. Can you explain this more. if you bcrypt(12345) isn't the result always same ? If the hash varies then what does the inbuilt nonce depend on ?
  - tptacek 17y ago
    >> BCrypt::Password.create("hi,mom", :cost => 10) => "$2a$10$L/c.1uoZSh3oaU1fLrnYK.yyU4PiJXsIAzN22qnbU41liyLn5/of 2" >> BCrypt::Password.create("hi,mom", :cost => 10) => "$2a$10$3F0BVk5t8/aoS.3ddaB3l.fxg5qvafQ9NybxcpXLzMeAt.nVWn.NO" >> BCrypt::Password.create("hi,mom", :cost => 10) => "$2a$10$VEVmGHy4F4XQMJ3eOZJAUeb.MedU0W10pTPCuf53eHdKJPiSE8sMK" Despite the pages and pages and pages and pages of conversation about how to select and format "salts", adding random nonces to hashes is not one of the world's great CS problems.
    - boundlessdreamz 17y ago
      I don't understand it still. The cost factor and the string remains the same but different hashes are produced. So if a user inputs "hi,mom" and the app generates a hash, how is it going to be the same as the one in DB ? Or did you make a mistake and the cost factors are supposed to different ?
      - tptacek 17y ago
        Just do what the library tells you to do to check passwords and trust me that bcrypt hashes already have nonces built into them.
      - kasterma 17y ago
        I had the same question; I don't know the answer yet, but I found a location for the answer http://www.openbsd.org/papers/bcrypt-paper.ps http://www.openbsd.org/papers/bcrypt-paper.ps The documentation for py-bcrypt ( http://www.mindrot.org/projects/py-bcrypt/ http://www.mindrot.org/projects/py-bcrypt/) doesn't explain what happens, but shows use is very very easy. EDIT: current guess is that searching for the salt that was used might be part of what makes it hard.

by sorbus 17y ago

tptacek 17y ago

No. Bcrypted passwords have nonces in them, just like "salted" hashes. You don't have to think about it. You just use bcrypt.

boundlessdreamz 17y ago
Complete noob. Can you explain this more. if you bcrypt(12345) isn't the result always same ? If the hash varies then what does the inbuilt nonce depend on ?
- tptacek 17y ago
  >> BCrypt::Password.create("hi,mom", :cost => 10) => "$2a$10$L/c.1uoZSh3oaU1fLrnYK.yyU4PiJXsIAzN22qnbU41liyLn5/of 2" >> BCrypt::Password.create("hi,mom", :cost => 10) => "$2a$10$3F0BVk5t8/aoS.3ddaB3l.fxg5qvafQ9NybxcpXLzMeAt.nVWn.NO" >> BCrypt::Password.create("hi,mom", :cost => 10) => "$2a$10$VEVmGHy4F4XQMJ3eOZJAUeb.MedU0W10pTPCuf53eHdKJPiSE8sMK" Despite the pages and pages and pages and pages of conversation about how to select and format "salts", adding random nonces to hashes is not one of the world's great CS problems.
  - boundlessdreamz 17y ago
    I don't understand it still. The cost factor and the string remains the same but different hashes are produced. So if a user inputs "hi,mom" and the app generates a hash, how is it going to be the same as the one in DB ? Or did you make a mistake and the cost factors are supposed to different ?
    - tptacek 17y ago
      Just do what the library tells you to do to check passwords and trust me that bcrypt hashes already have nonces built into them.
    - kasterma 17y ago
      I had the same question; I don't know the answer yet, but I found a location for the answer http://www.openbsd.org/papers/bcrypt-paper.ps http://www.openbsd.org/papers/bcrypt-paper.ps The documentation for py-bcrypt ( http://www.mindrot.org/projects/py-bcrypt/ http://www.mindrot.org/projects/py-bcrypt/) doesn't explain what happens, but shows use is very very easy. EDIT: current guess is that searching for the salt that was used might be part of what makes it hard.