Password scheme for websites: How wll would this work, and has it already been done?

[irongeek at irongeek.com (Adrian Crenshaw)]

I'm sure by now the many of you here have heard of the asshatery that is
zero for 0wned (zf05.txt) and it's started me thinking about password
management across websites.

Remembering a unique password for each and every site is hard to manage.
Now, what I currently do is have one password for finance stuff, another for
website related stuff and yet another for forums I've visited, sort of by
level of how much I care if they get compromised. Still, it's a pain to go
around changing passwords when you hear Binrev or Hak5 got hacked and your
not sure if they got your credintials.

I was wondering if this schem is workable from a security standpoint, and if
someone has already implemented it into a Firefox plugin. Lets say you do
this, take a password you use everywhere, conatinate it with the domain name
of the site you are making a password for, then take the md5 hash and use it
as your password.For example, if my password was "mypassword" and I were
using it on Pauldotcom.com:


md5("mypasswordpauldotcom.com") = "4b7958e4302cae2836f1c05532f835f4"

This way, it's still easy to remeber, but even if an attacker gets the plain
text from what is store on the site (4b7958e4302cae2836f1c05532f835f4 in
this case), they can't use it to compromise account on other sites since
your password would be different, for example:

md5("mypasswordirongeek.com") = "1c96d14e6e048924cabf3009b064958f"

Do you see any major weaknesses in this scheme? Anyone know how to implement
a Firefox plugin to simplify it?

Thanks,
Adrian
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