Why does Ruby's bcrypt lib include the salt in

问题:

This question already has an answer here:

• How can bcrypt have built-in salts? 3 answers
• Do I need to store the salt with bcrypt? 1 answer

I am using Coda Hale's Ruby bcrypt library. I noticed recently that it wasn't working like I thought it worked. I had thought that the proper procedure is:

1. Generate a salt
2. Obtain a password
3. Concatenate the salt and the password strings
4. Hash them through your hashing function

But when I look at the results of the bcrypt function it appears that the salt is concatenated to the hash not the password. That is the salt concatenation happens after step #4, not before. I'm assuming Coda Hale is doing this right, but I am wondering why it behaves like this.

Here's a brief IRB session to show what's weird (to me). Notice that in the results of the hash_secret function, the first 29 chars are the same as the salt. Any information as to why this is the case would be appreciated.

My only theory is that the salt is prepended as well as embedded in the hash, which eliminates the need to store the salt in a separate DB field (essentially a record packing strategy)?

irb#1(main):004:0> password_salt = BCrypt::Engine.generate_salt
=> "$2a$10$OrKdcWORLL8Gorhy9XR3UO"
irb#1(main):005:0> password='abc'
=> "abc"
irb#1(main):006:0> BCrypt::Engine.hash_secret(password, password_salt)
=> "$2a$10$OrKdcWORLL8Gorhy9XR3UOY8Sebzq92m7r02XPitzoazPdO7tmsEO"
irb#1(main):007:0> 

回答1:

There is no technical reason why this is the case. If you wanted to, you could store the salt and password separately. Heck, you could make the salt public if you wanted to. I've heard some people will use the user-id as the salt to save a few bits of storage in their database.

There would be no security gain by storing hashes and salts in different fields in the same database. All that really matters is that each salt is unique in order to thwart rainbow tables.

I imagine that the creator decided to concat the two strings simply to keep the salt and hash together in a single field in the database or application. Sometimes this can be useful, for example in languages that don't support multi-value returns.