How do I install pgcrypto in PostgreSQL 8.4?

I'm using Ubuntu Server 10.10 and I have installed PostgreSQL 8.4 using apt-get install postgresql. I would like to use the built-in sha1() function, but it seems that I have to install pgcrypto first. But I don't know how to install it. There is no pgcrypto if I try to install it using apt-get install pgcrypto and I don't find any files starting with pgcrypto in my system (I tried find / -name "pgcrypto*"). How do I install pgcrypto so I can use the digest('word-to-hash','sha1') function in my database queries? ------------- **Update:** I'm struggling to install pgcrypto on another Ubuntu machine. After installing the package using sudo apt-get install postgresql-contrib-8.4 how do I install it to my current PostgreSQL database?