Password Storage Cheat Sheet
DRAFT CHEAT SHEET - WORK IN PROGRESS
This article is focused on providing guidance to storing a password in order to help prevent password theft. Too often passwords are stored as clear text. Thus the password can be read directly by the database’s administrator, super users or via data theft by SQL Injection. Database backup media is also vulnerable to password theft via password storage. It is recommended that you avoid storing the clear text password or an encrypted version of the password.
Password Storage Rules
Passwords are secrets. There is no reason to decrypt them under any circumstances. It is crucial that passwords are stored in a way that they can be *verified* but not *reversed* in any way, even by insiders. To accomplish this, store the salted hashed value of the password. Preferably use a different random salt for each password hash instead of a constant long salt.
Use a modern hash algorithm
Use a long cryptographically random salt
If each password is simply hashed, identical passwords will have the same hash. There are two drawbacks to choosing to only storing the password’s hash:
- Due to the birthday paradox (http://en.wikipedia.org/wiki/Birthday_paradox), the attacker can find a password very quickly especially if the number of passwords the database is large.
- An attacker can use a list of precomputed hashed (http://en.wikipedia.org/wiki/Rainbow_table) to break passwords in seconds.
In order to solve these problems, a salt must be concatenated in front of the password before the digest operation.
A salt is a cryptographically random number of a fixed length. This salt must be different for each stored entry. Since rainbow tables are already passing 24 characters, a salt of 24 bytes or longer is the recommended minimum length.
Iterate the hash
To slow down the computation it is recommended to iterate the hash operation many times. While hashing the password many times does slow down hashing for both attackers and typical users, typical users don't really notice it being that hashing is such a small percentage of their total time interacting with the system. On the other hand, an attacker trying to crack passwords spends nearly 100% of their time hashing so hashing many times gives the appearance of slowing the attacker down by a factor of n while not noticeably affecting the typical user. A minimum of 1000 operations is recommended in RSA PKCS5 standard in 2000, a value that should be doubled every 2 years.
Cryptographic framework for password hashing is described in PKCS #5 v2.1: Password-Based Cryptography Standard. Specific secure password hashing algorithms exist such as bcrypt, scrypt. Implementations of secure password hashing exist for PHP (phpass), ASP.NET (ASP.NET 2.0 Security Practices), Java (OWASP Hashing Java).
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