licensing for libpam-pwfile (ITP)

[Antonio Radici <antonio@dyne.org>]

Hi there,
I'm adopting libpam-pwfile and while checking the licenses in the source
package I've found that part of it is a derivative work of this code:
http://www.multicians.org/thvv/gpw.html

The fact that it is a derivative work is clear in the README of the upstream
package, checking the license of GWP I've found that there is no 'common'
license for the code but just a statement that says:

---
You are welcome to use the source of the password generator, if you

    * Share your source with others freely
    * Let me know you're using it
    * Give me credit, and all the other pioneers, if you use the data or algorithms
---

Is this a proper "license"?
I've sent a mail upstream asking if he could adopt a more descriptive license which
states clearly how to handle derivative work but I haven't got any reply so far.

What do you think?

Cheers
Antonio