For Debian, you'll need to install libpam-unix2. Replace the occurrences of pam_unix.so with pam_unix2.so in both /etc/pam.d/common-password and /etc/pam.d/common-auth. Additionally, in /etc/pam.d/common-password, change md5 to blowfish. New or changed passwords will be encrypted with Blowfish.