scripts: add download-check-artifact.sh script

While checking wiki documentation about GPG signatures checking I found
this nice script and I've thought, that it would be nice to provide it
officially instead of some random internet site.

Usage example:

  $ ./scripts/download-check-artifact.sh https://downloads.openwrt.org/snapshots/targets/mediatek/filogic/openwrt-mediatek-filogic-openwrt_one-squashfs-sysupgrade.itb

  1) Downloading image file
  =========================
  ########################################### 100.0%

  2) Downloading checksum file
  ============================
  ########################################### 100.0%

  3) Downloading the GPG signature
  ================================
  ########################################### 100.0%

  4) Verifying GPG signature
  ==========================
  gpg: Signature made Tue 05 Nov 2024 05:21:50 PM UTC
  gpg:                using EDDSA key 92C561DE55AE6552F3C736B82B0151090606D1D9
  gpg: Good signature from "OpenWrt Build System (Nitrokey3) <contact@openwrt.org>" [ultimate]
  Primary key fingerprint: 8A8B C12F 46B8 36C0 F9CD  B36F 1D53 D187 7742 E911
       Subkey fingerprint: 92C5 61DE 55AE 6552 F3C7  36B8 2B01 5109 0606 D1D9

  5) Verifying SHA256 checksum
  ============================
  openwrt-mediatek-filogic-openwrt_one-squashfs-sysupgrade.itb: OK

  Verification done!
  ==================
  Downloaded artifact placed in '/var/home/ynezz/dev/openwrt/openwrt.git/openwrt-mediatek-filogic-openwrt_one-squashfs-sysupgrade.itb'

  Cleaning up.

Adding file in a state as downloaded from https://www.abitare.org/bin/download.sh

References: https://openwrt.org/docs/guide-quick-start/verify_firmware_checksum?s[]=gpg#linux
Signed-off-by: David S. H. Rosenthal <dshr@abitare.org>
Link: https://github.com/openwrt/openwrt/pull/16871
Signed-off-by: Petr Štetiar <ynezz@true.cz>

diff --git a/scripts/download-check-artifact.sh b/scripts/download-check-artifact.sh
new file mode 100755 (executable)
index 0000000..31868da
--- /dev/null
+++ b/scripts/download-check-artifact.sh
@@ -0,0 +1,154 @@
+#!/usr/bin/env bash
+# Script to perform verified file downloads.
+# Exit codes:
+#  0 - File downloaded successfully and verified
+#  1 - Failed to download requested file
+#  2 - Failed to download sha256sums file
+#  3 - Failed to download sha256sums.gpg file
+#  4 - GnuPG is available but fails to verify the signature (missing pubkey, file integrity error, ...)
+#  5 - The checksums do not match
+#  6 - Unable to copy the requested file to its final destination
+#  254 - The script got interrupted by a signal
+#  255 - A suitable download or checksum utility is missing
+
+[ -n "$1" ] || {
+       echo "Usage: $0 <url>" >&2
+       exit 1
+}
+
+finish() {
+       [ -e "/tmp/verify.$$" ] && {
+               echo "Cleaning up."
+               rm -r "/tmp/verify.$$"
+       }
+       exit $1
+}
+
+trap "finish 254" INT TERM
+
+destdir="$(pwd)"
+image_url="$1"
+image_file="${image_url##*/}"
+sha256_url="${image_url%/*}/sha256sums"
+gpgsig_url="${image_url%/*}/sha256sums.asc"
+keyserver_url="hkp://keyserver.ubuntu.com"
+
+# Find a suitable download utility
+if which curl >/dev/null; then
+       download() { curl --progress-bar -o "$1" "$2"; }
+elif which wget >/dev/null; then
+       download() { wget -O "$1" "$2"; }
+elif which fetch >/dev/null; then
+       download() { fetch -o "$1" "$2"; }
+else
+       echo "No suitable download utility found, cannot download files!" >&2
+       finish 255
+fi
+
+# Find a suitable checksum utility
+if which sha256sum >/dev/null; then
+       checksum() { sha256sum -c --ignore-missing "sha256sums"; }
+elif which shasum >/dev/null; then
+       checksum() {
+               local sum="$(shasum -a 256 "$image_file")";
+               grep -xF "${sum%% *} *$image_file" "sha256sums";
+       }
+else
+       echo "No SHA256 checksum executable installed, cannot verify checksums!" >&2
+       finish 255
+fi
+
+# Check for gpg availability
+if which gpg >/dev/null; then
+       runpgp() { gpg "$@"; }
+else
+       runpgp() {
+               echo "WARNING: No GnuPG installed, cannot verify digital signature!" >&2
+               return 0
+       }
+fi
+
+mkdir -p "/tmp/verify.$$"
+cd "/tmp/verify.$$"
+
+echo ""
+echo "1) Downloading image file"
+echo "========================="
+download "$image_file" "$image_url" || {
+       echo "Failed to download image file!" >&2
+       finish 1
+}
+
+echo ""
+echo "2) Downloading checksum file"
+echo "============================"
+download "sha256sums" "$sha256_url" || {
+       echo "Failed to download checksum file!" >&2
+       finish 2
+}
+
+echo ""
+echo "3) Downloading the GPG signature"
+echo "================================"
+download "sha256sums.gpg" "$gpgsig_url" || {
+       echo "Failed to download GPG signature!" >&2
+       finish 3
+}
+
+echo ""
+echo "4) Verifying GPG signature"
+echo "=========================="
+missing_key=$(runpgp --status-fd 1 --with-fingerprint --verify \
+       "sha256sums.gpg" "sha256sums" 2>/dev/null | sed -ne 's!^.* NO_PUBKEY !!p')
+
+if [ -n "$missing_key" ]; then
+       echo "The signature was signed by a public key with the id $missing_key" >&2
+       echo "which is not present on this system."                              >&2
+       echo ""                                                                  >&2
+
+       echo "Provide a public keyserver url below or press enter to accept the" >&2
+       echo "default suggestion. Hit Ctrl-C to abort the operation."            >&2
+       echo ""                                                                  >&2
+
+       while true; do
+               printf "Keyserver to use? [$keyserver_url] > "
+               read url; case "${url:-$keyserver_url}" in
+                       hkp://*)
+                               gpg --keyserver "${url:-$keyserver_url}" --recv-keys "$missing_key" || {
+                                       echo "Failed to download public key." >&2
+                                       finish 7
+                               }
+                               break
+                       ;;
+                       *)
+                               echo "Expecting a key server url in the form 'hkp://hostname'." >&2
+                       ;;
+               esac
+       done
+fi
+
+runpgp --with-fingerprint --verify "sha256sums.gpg" "sha256sums" || {
+       echo "Failed to verify checksum file with GPG signature!" >&2
+       finish 4
+}
+
+echo ""
+echo "5) Verifying SHA256 checksum"
+echo "============================"
+checksum || {
+       echo "Checksums do not match!" >&2
+       finish 5
+}
+
+cp "$image_file" "$destdir/$image_file" || {
+       echo "Failed to write '$destdir/$image_file'" >&2
+       finish 6
+}
+
+echo ""
+echo "Verification done!"
+echo "=================="
+echo "Firmware image placed in '$destdir/$image_file'."
+echo ""
+
+finish 0