pgp isn't open source. Use gpg instead.
Gnu Privacy Guard
http://www.gnupg.org/download/

# -c symmetric cipher only (w/passphrase)
# -a ascii armor
gpg -ca mystuff.txt

# decrypt to mystuff.txt
gpg mystuff.txt.asc

# decrypt to stdout instead of to a file
gpg -d mystuff.txt.asc