support/download/check-hash

#!/usr/bin/env bash
set -e

# Helper to check a file matches its known hash
# Call it with:
#   $1: the path of the file containing all the expected hashes
#   $2: the full path to the temporary file that was downloaded, and
#       that is to be checked
#   $3: the final basename of the file, to which it will be ultimately
#       saved as, to be able to match it to the corresponding hashes
#       in the .hash file
#
# Exit codes:
#   0:  the hash file exists and the file to check matches all its hashes,
#       or the hash file does not exist
#   1:  unknown command-line option
#   2:  the hash file exists and the file to check does not match at least
#       one of its hashes
#   3:  the hash file exists and there was no hash to check the file against
#   4:  the hash file exists and at least one hash type is unknown

while getopts :q OPT; do
    case "${OPT}" in
    q)  exec >/dev/null;;
    \?) exit 1;;
    esac
done
shift $((OPTIND-1))

h_file="${1}"
file="${2}"
base="${3}"

# Bail early if no hash to check
if [ -z "${h_file}" ]; then
    exit 0
fi
# Does the hash-file exist?
if [ ! -f "${h_file}" ]; then
    printf "WARNING: no hash file for %s\n" "${base}" >&2
    exit 0
fi

# Check one hash for a file
# $1: algo hash
# $2: known hash
# $3: file (full path)
check_one_hash() {
    _h="${1}"
    _known="${2}"
    _file="${3}"

    # Note: md5 is supported, but undocumented on purpose.
    # Note: sha3 is not supported, since there is currently no implementation
    #       (the NIST has yet to publish the parameters).
    # Note: 'none' means there is explicitly no hash for that file.
    case "${_h}" in
        none)
            return 0
            ;;
        md5|sha1)                       ;;
        sha224|sha256|sha384|sha512)    ;;
        *) # Unknown hash, exit with error
            printf "ERROR: unknown hash '%s' for '%s'\n"  \
                   "${_h}" "${base}" >&2
            exit 4
            ;;
    esac

    # Do the hashes match?
    _hash=$( ${_h}sum "${_file}" |cut -d ' ' -f 1 )
    if [ "${_hash}" = "${_known}" ]; then
        printf "%s: OK (%s: %s)\n" "${base}" "${_h}" "${_hash}"
        return 0
    fi

    printf "ERROR: %s has wrong %s hash:\n" "${base}" "${_h}" >&2
    printf "ERROR: expected: %s\n" "${_known}" >&2
    printf "ERROR: got     : %s\n" "${_hash}" >&2
    printf "ERROR: Incomplete download, or man-in-the-middle (MITM) attack\n" >&2

    exit 2
}

# Do we know one or more hashes for that file?
nb_checks=0
while read t h f; do
    case "${t}" in
        ''|'#'*)
            # Skip comments and empty lines
            continue
            ;;
        *)
            if [ "${f}" = "${base}" ]; then
                check_one_hash "${t}" "${h}" "${file}"
                : $((nb_checks++))
            fi
            ;;
    esac
done <"${h_file}"

if [ ${nb_checks} -eq 0 ]; then
    case " ${BR_NO_CHECK_HASH_FOR} " in
    *" ${base} "*)
        # File explicitly has no hash
        exit 0
        ;;
    esac
    printf "ERROR: No hash found for %s\n" "${base}" >&2
    exit 3
fi