#!/usr/bin/env bash
#
# Helper script to properly download an OpenPGP key from a remote location.
# Inspired by https://gitlab.torproject.org/tpo/onion-services/onionprobe/-/blob/main/scripts/get-tor-debian-key
#

# Parameters
BASENAME="`basename $0`"
URL="$1"
FINGERPRINT="$2"
DEST="$3"
CANDIDATE="`mktemp`"

# Ensure the candidate file is remove upon exit
trap "rm -rf $CANDIDATE" INT TERM EXIT

# Check syntax
if [ -z "$3" ]; then
  echo "usage: $BASENAME <url> <fingerprint> <dest-file>"
  exit 1
fi

# Download the OpenPGP directly from a remote location.
#
# Advantage: handles any upstram updates in the key, like renewed expiration.
wget -qO- "$URL" | gpg --dearmor | tee "$CANDIDATE" > /dev/null || exit 1

# Get the actual fingerprint after downloading, since we cannot assume that the
# remote file has the correct fingerprint.
#
# Then we need to check the actual key fingerprint.
KEY_FPR="`cat $CANDIDATE | gpg --with-fingerprint --with-colons 2> /dev/null | grep '^fpr' | cut -d : -f 10 | head -1`"

# Compare the actual fingerprint with the one we're looking for
if [ "$KEY_FPR" == "$FINGERPRINT" ]; then
  echo "$BASENAME: downloaded $URL key matches the expected fingerprint $FINGERPRINT"

  if [ ! -z "$DEST" ]; then
    echo "$BASENAME: saving key $FINGERPRINT on $DEST..."
    touch "$DEST"              || exit 1
    chmod 644 "$DEST"          || exit 1
    cat "$CANDIDATE" > "$DEST" || exit 1
  else
    echo "$BASENAME: saving key $FINGERPRINT on $FINGERPRINT.asc..."
    cp "$CANDIDATE" "$FINGERPRINT.asc"
  fi
else
  echo "$BASENAME: error: downloaded $URL key does not matche the expected fingerprint $FINGERPRINT (got $KEY_FPR instead)"
  exit 1
fi