Password hashing and salting

2018-06-07 00:02:29

I'm trying to wrap my head around the logic of encrypting passwords with MD5/SHA combined with salting.

I understand the concept of a user proving a text password, and appending a random string (salt) to the text password, and hashing the final string via whatever encryption method you want.

This is where I lose the concept

Say in my database of users, I have usernames, and encrypted passwords generated with the random salt value

When the user goes to log into a system, and they enter their password, how do I obtain the correct salt to check the password validity?

If the salt is randomly generated to begin with, I can't recalculate it

Do I have to store the salt with the username/password record? If I query the database for the salt value by username, it would seem that defeats the purpose of having the salting.

How do I obtain the correct salt when it comes time to validate the supplied password?

From Wikipedia, Salt (cryptography)

A new salt is randomly generated for each password. In a typical setting, the salt and the password are concatenated and processed with a cryptographic hash function, and the resulting output (but not the original password) is stored with the salt in a database

You store it with the hash, to prevent dictionary attacks.

The salt is stored in the database, so you can use the same salt to verify the password. Todays libraries often will include the salt in the resulting hash-value like this (result of the PHP function password_hash()):

$2y$10$nOUIs5kJ7naTuTFkBy1veuK0kSxUFXfuaOKdOKf9xYT0KKIGSJwFa
 |  |  |                     |
 |  |  |                     hash-value = K0kSxUFXfuaOKdOKf9xYT0KKIGSJwFa
 |  |  |
 |  |  salt = nOUIs5kJ7naTuTFkBy1veu (22 characters)
 |  |
 |  cost-factor = 10 = 2^10 iterations
 |
 hash-algorithm = 2y = BCrypt

This 60 character string can be stored into a single field in the database. The verifying function can extract the salt from this string. The salt is not a secret, it fulfills its purpose even when it is known.

Please note that algorithms like MD5 and SHA-* are not appropriate to hash passwords, because they are too fast. Instead use an algorithm with a cost factor like BCrypt or PBKDF2. For more information you can have a look at my tutorial about safely storing passwords.

Yes, you store the salt. Salting is used to prevent pregenerated rainbow tables, it is not required to be secret, just unpredictable.

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