#!/bin/bash
PUBKEY="$1"
USER="$2"
VALIDITY_SECONDS="$3"

CERTFILE="${PUBKEY/.pub/-cert.pub}"

ssh-keygen -s /home/alf/ca_key \
    -I "$USER@temp" \
    -n "$USER" \
    -V +0s:+${VALIDITY_SECONDS}s \
    -z $(date +%s) \
    "$PUBKEY" && echo "OK: Signed $CERTFILE"