How to store passwords *correctly*?

问题:

An article that I stumbled upon here in SO provided links to other articles which in turn provided links to even more articles etc.

And in the end I was left completely stumped - so what is the best way to store passwords in the DB? From what I can put together you should:

• Use a long (at least 128 fully random bits) salt, which is stored in plaintext next to the password;
• Use several iterations of SHA-256 (or even greater SHA level) on the salted password.

But... the more I read about cryptography the more I understand that I don't really understand anything, and that things I had thought to be true for years are actually are flat out wrong. Are there any experts on the subject here?

Added: Seems that some people are missing the point. I repeat the last link given above. That should clarify my concerns.

https://www.nccgroup.trust/us/about-us/newsroom-and-events/blog/2007/july/enough-with-the-rainbow-tables-what-you-need-to-know-about-secure-password-schemes/

回答1:

You got it right. Only two suggestions:

1. If one day SHA1 becomes too weak and you want to use something else, it is impossible to unhash the old passwords and rehash them with the new scheme. For this reason, I suggest that attached to each password a "version" number that tells you what scheme you used (salt length, which hash, how many times). If one day you need to switch from SHA to something stronger, you can create new-style passwords while still having old-style passwords in the database and still tell them apart. Migrating users to the new scheme will be easier.

2. Passwords still go from user to system without encryption. Look at SRP if that's a problem. SRP is so new that you should be a little paranoid about implementing it, but so far it looks promising.

Edit: Turns out bcrypt beat me to it on idea number 1. The stored info is (cost, salt, hash), where cost is how many times the hashing has been done. Looks like bcrypt did something right. Increasing the number of times that you hash can be done without user intervention.

回答2:

In truth it depends on what the passwords are for. You should take storing any password with care, but sometimes much greater care is needed than others. As a general rule all passwords should be hashed and each password should have a unique salt.

Really, salts don't need to be that complex, even small ones can cause a real nightmare for crackers trying to gain entry into the system. They are added to a password to prevent the use of Rainbow tables to hack multiple account's passwords. I wouldn't add a single letter of the alphabet to a password and call it a salt, but you don't need to make it a unique guid which is encrypted somewhere else in the database either.

One other thing concerning salts. The key to making a password + salt work when hashing is the complexity of the combination of the two. If you have a 12 character password and add a 1 character salt to it, the salt doesn't do much, but cracking the password is still a monumental feat. The reverse is also true.

回答3:

Use:

1. Hashed password storage
2. A 128+ bit user-level salt, random, regenerated (i.e. you make new salts when you make new password hashes, you don't persistently keep the same salt for a given user)
3. A strong, computationally expensive hashing method
4. Methodology that is somewhat different (hash algorithm, how many hashing iterations you use, what order the salts are concatenated in, something) from both any 'standard implementation guides' like these and from any other password storage implementation you've written

回答4:

I think there no extra iteration on the password needed, juste make sure there is a salt, and a complexe one ;) I personnaly use SHA-1 combined with 2 salt keyphrases.

回答5:

The length of the salt doesnt really matter, as long as it is unique to a user. The reason for a salt is so that a given generated attempt at a hash match is only useful for a single row of your users table in the DB.

回答6:

Simply said, use a cryptographically secure hash algorithm and some salt for the passwords, that should be good enough for 99.99% of all use cases. The weak link will be the code that checks the password as well as the password input.