Connect to ssh servers with temporary keys (for public servers like terminal.shop)

sshtmp.sh

#!/bin/zsh

REMOTE_SERVER=$1
REPLACEMENT="anon@ubuntu"
LOGIN="$USER@$HOSTNAME"
WD=`mktemp -d`
pushd $WD > /dev/null
    ssh-keygen -t rsa -b 4096 -q -f "$WD/id_rsa" -N ""
    sed -i -e "s/$LOGIN/$REPLACEMENT/g" "$WD/id_rsa.pub"
    chmod 400 "$WD/id_rsa"
    chmod 400 "$WD/id_rsa.pub"
    ssh -T -t -o "UserKnownHostsFile=/dev/null" -o "StrictHostKeyChecking=no" -o "IdentitiesOnly=yes" -i "$WD/id_rsa.pub" $REMOTE_SERVER < /dev/tty
popd > /dev/null
cleanup() {
    rm -rf "$WD"
}
trap cleanup EXIT INT QUIT TERM

$HOSTNAME should be $HOST on macOS, it seems

I also get invalid key on macOS, so I'm not sure what's wrong.