In Depth: Rehashing Passwords

[InDepth#5] It sounds easy to rehash passwords, but is it really that easy?

The most secure way to store a password is to hash it and store only the hashed password. This ensures the original password can’t be easily extracted, while still allowing you to compare a password provided at login time with a stored hash to see if it matches. Modern password hashing algorithms also include a salt and a work factor, which add extra layers of security to protect the original password from being extracted and compromised.

If you’ve ever worked on a legacy application, you’ll know the database is full of legacy data structures¹, and passwords are no exception. Many legacy apps (and even some modern ones 😡) store passwords in plain-text, directly in the database. The next step after plain-text was to use MD5 and SHA1 to hash the passwords, which adds some obfuscation, but doesn’t offer much protection. Then we added salts in, to make it a bit harder to extract the passwords, and thus a multitude of custom variants of salted-hashed password algorithms. And now we’re up to algorithms like bcrypt and Argon2, which offer excellent password security with the right configuration.

What Is Hashing?