How to use MD5 in javascript to transmit a password

crypto-js is a rich javascript library containing many cryptography algorithms.

All you have to do is just call CryptoJS.MD5(password)

$.post(
  'includes/login.php', 
  { user: username, pass: CryptoJS.MD5(password) },
  onLogin, 
  'json' );

If someone is sniffing your plain-text HTTP traffic (or cache/cookies) for passwords just turning the password into a hash won't help - The hash password can be "replayed" just as well as plain-text. The client would need to hash the password with something somewhat random (like the date and time) See the section on "AUTH CRAM-MD5" here: http://www.fehcom.de/qmail/smtpauth.html

I would suggest you to use CryptoJS in this case.

Basically CryptoJS is a growing collection of standard and secure cryptographic algorithms implemented in JavaScript using best practices and patterns. They are fast, and they have a consistent and simple interface.

So In case you want calculate hash(MD5) of your password string then do as follows :

<script src="http://crypto-js.googlecode.com/svn/tags/3.0.2/build/rollups/md5.js"></script>
<script>
    var passhash = CryptoJS.MD5(password).toString();

    $.post(
      'includes/login.php', 
      { user: username, pass: passhash },
      onLogin, 
      'json' );
</script>

So this script will post hash of your password string to the server.

For further info and support on other hash calculating algorithms you can visit at:

http://code.google.com/p/crypto-js/