Mini Shell

#!/usr/bin/bash

# Copyright 2018 B. Persson, Bjorn@Rombobeorn.se
#
# This material is provided as is, with absolutely no warranty expressed
# or implied. Any use is at your own risk.
#
# Permission is hereby granted to use or copy this shellscript
# for any purpose, provided the above notices are retained on all copies.
# Permission to modify the code and to distribute modified code is granted,
# provided the above notices are retained, and a notice that the code was
# modified is included with the above copyright notice.


function print_help {
    cat <<'EOF'
Usage: gpgverify --keyring=<pathname> --signature=<pathname> --data=<pathname>

gpgverify is a wrapper around gpgv designed for easy and safe scripting. It
verifies a file against a detached OpenPGP signature and a keyring. The keyring
shall contain all the keys that are trusted to certify the authenticity of the
file, and must not contain any untrusted keys.

The differences, compared to invoking gpgv directly, are that gpgverify accepts
the keyring in either ASCII-armored or unarmored form, and that it will not
accidentally use a default keyring in addition to the specified one.

Parameters:
  --keyring=<pathname>    keyring with all the trusted keys and no others
  --signature=<pathname>  detached signature to verify
  --data=<pathname>       file to verify against the signature
EOF
}


fatal_error() {
    message="$1"  # an error message
    status=$2     # a number to use as the exit code
    echo "gpgverify: $message" >&2
    exit $status
}


require_parameter() {
    term="$1"   # a term for a required parameter
    value="$2"  # Complain and terminate if this value is empty.
    if test -z "${value}" ; then
        fatal_error "No ${term} was provided." 2
    fi
}


check_status() {
    action="$1"  # a string that describes the action that was attempted
    status=$2    # the exit code of the command
    if test $status -ne 0 ; then
        fatal_error "$action failed." $status
    fi
}


# Parse the command line.
keyring=
signature=
data=
for parameter in "$@" ; do
    case "${parameter}" in
        (--help)
            print_help
            exit
            ;;
        (--keyring=*)
            keyring="${parameter#*=}"
            ;;
        (--signature=*)
            signature="${parameter#*=}"
            ;;
        (--data=*)
            data="${parameter#*=}"
            ;;
        (*)
            fatal_error "Unknown parameter: \"${parameter}\"" 2
            ;;
    esac
done
require_parameter 'keyring' "${keyring}"
require_parameter 'signature' "${signature}"
require_parameter 'data file' "${data}"

# Make a temporary working directory.
workdir="$(mktemp --directory)"
check_status 'Making a temporary directory' $?
workring="${workdir}/keyring.gpg"

# Decode any ASCII armor on the keyring. This is harmless if the keyring isn't
# ASCII-armored.
gpg2 --homedir="${workdir}" --yes --output="${workring}" --dearmor "${keyring}"
check_status 'Decoding the keyring' $?

# Verify the signature using the decoded keyring.
gpgv2 --homedir="${workdir}" --keyring="${workring}" "${signature}" "${data}"
check_status 'Signature verification' $?

# (--homedir isn't actually necessary. --dearmor processes only the input file,
# and if --keyring is used and contains a slash, then gpgv2 uses only that
# keyring. Thus neither command will look for a default keyring, but --homedir
# makes extra double sure that no default keyring will be touched in case
# another version of GPG works differently.)

# Clean up. (This is not done in case of an error that may need inspection.)
rm --recursive --force ${workdir}