(1) Create a file with command to execute, eg in file command.txt:
knock example.com 120:tcp 121:udp 122:tcp && sleep 1s && ssh email@example.com
(2) Encrypt the file with gpg:
gpg -a --symmetric --cipher-algo TWOFISH command.txt(cipher algorithm here is TWOFISH, but can be anything else). New file command.txt.asc should be created.
(3) Create a script which decrypt and execute above command:
#!/bin/bash command=`gpg -d command.txt.asc 2> /dev/null` eval "$command"This command will ask for the passphase and after successfull decryption will run desired command.