To set up an encrypted and authenticated pipe for sending email between two systems (in the author's case, from many systems around the internet to his central SMTP server, which then relays email to the rest of the world), one might run

dd if=/dev/urandom bs=32 count=1 of=keyfile spiped -d -s '[0.0.0.0]:8025' -t '[127.0.0.1]:25' -k keyfile

on a server and after copying keyfile to the local system, run

spiped -e -s '[127.0.0.1]:25' -t $SERVERNAME:8025 -k keyfile

at which point mail delivered via localhost:25 on the local system will be securely transmitted to port 25 on the server (which is configured to relay mail which arrives from 127.0.0.1 but not from other addresses).

You can also use spiped to protect SSH servers from attackers: Since data is authenticated before being forwarded to the target, this can allow you to SSH to a host while protecting you in the event that someone finds an exploitable bug in the SSH daemon -- this serves the same purpose as port knocking or a firewall which restricts source IP addresses which can connect to SSH. On the SSH server, run

dd if=/dev/urandom bs=32 count=1 of=/etc/ssh/spiped.key spiped -d -s '[0.0.0.0]:8022' -t '[127.0.0.1]:22' -k /etc/ssh/spiped.key

then copy the server's /etc/ssh/spiped.key to ~/.ssh/spiped_HOSTNAME_key on your local system and add the lines

Host HOSTNAME ProxyCommand spipe -t %h:8022 -k ~/.ssh/spiped_%h_key