2010/04/27

Blowfish password hashing

Ever since I read this article on password hashing, which was more than two years ago, I wanted to bundle a proper password hashing library to Gauche. I finally managed to put bcrypt in, as crypt.bcrypt module. (ref:crypt.bcrypt : This link will be valid after 0.9.1 release).

The bcrypt library does password encryption with adaptive hashing, using blowfish algorithm. (Do not confuse it with another program also called bcrypt, which also uses blowfish but it encrypts files.) They say it is the default password hashing used on OpenBSD. Most popular lightweight languages have an extension package to use bcrypt password hashing, e.g. bcrypt-ruby for Ruby, py-bcrypt for Python, or Crypt::Eksblowfish::Bcrypt for Perl.

I decided to include crypt.bcrypt in the Gauche distribution instead of making it an optional extension package. Gauche has sys-crypt (ref:sys-crypt) which is a direct interface to system's crypt(3) function, and also a few popular hash functions such as MD5 and SHA1/SHA2 (ref:rfc.md5, ref:rfc.sha). If crypt.bcrypt is something you have to install separately, it would be very tempting to use one of those coming with Gauche by default instead. I confess I have used MD5 before for an web app.

I'm totally convinced by the Thomas Ptacek's article I linked at the top of this post; original unix crypt(3) or even MD5 or SHA shouldn't be used for password hashing anymore. It's not so much because MD5 is a weak hash; it's because password hashing requires different property than data hashing schemes such as SHA. See the article for more details. But even if somebody doesn't agree or doesn't care, there's not much reason to use weaker schemes if the proper hashing library also comes by default.

I took bcrypt code from http://www.openwall.com/crypt/ , which is provided in public domain. So it doesn't add any external dependency.

Using bcrypt is very easy. You only need to remember just one function, bcrypt-hashpw. To calculate a password hash, call it with the password string:

  (bcrypt-hashpw password-string)  => hashed-string

The library automatically takes care of salting. To check if the given password is correct, pass the given password and hashed string. If returned string matches the provided hashed string, the password is correct.

  (bcrypt-hashpw password-string hashed-string) => hashed-string

There's also a function that you can calcurate a secure salt if you want.

Tags: 0.9.1, crypt.bcrypt