Is “double hashing” a password less secure than just hashing it once?

Question

Is hashing a password twice before storage any more or less secure than just hashing it once?

What I\'m talking about is doing this:

$hashed_password         


        

Answer 1

Yes, re-hashing reduces the search space, but no, it doesn't matter - the effective reduction is insignificant.

Re-hashing increases the time it takes to brute-force, but doing so only twice is also suboptimal.

What you really want is to hash the password with PBKDF2 - a proven method of using a secure hash with salt and iterations. Check out this SO response.

EDIT: I almost forgot - DON'T USE MD5!!!! Use a modern cryptographic hash such as the SHA-2 family (SHA-256, SHA-384, and SHA-512).