As private keys are very sensitive, /usr/libexec/cjdns/privatetopublic reads them from stdin. If your shell, terminal, or other program keeps history, please avoid
echo 'key' | /usr/libexec/cjdns/privatetopublic, or similar constructs. A heredoc is suitable for use in scripts, and will avoid unwanted revealing of the key in process lists, like so:
/usr/libexec/cjdns/privatetopublic <<EOF key EOF
Key should be in hex form, maximum 64 characters. Extra characters will be silently ignored.