1#!/bin/bash 2# SPDX-License-Identifier: GPL-2.0 3# 4# Copyright © 2020, Microsoft Corporation. All rights reserved. 5# 6# Author: Mickaël Salaün <mic@linux.microsoft.com> 7# 8# Compute and print the To Be Signed (TBS) hash of a certificate. This is used 9# as description of keys in the blacklist keyring to identify certificates. 10# This output should be redirected, without newline, in a file (hash0.txt) and 11# signed to create a PKCS#7 file (hash0.p7s). Both of these files can then be 12# loaded in the kernel with. 13# 14# Exemple on a workstation: 15# ./print-cert-tbs-hash.sh certificate-to-invalidate.pem > hash0.txt 16# openssl smime -sign -in hash0.txt -inkey builtin-private-key.pem \ 17# -signer builtin-certificate.pem -certfile certificate-chain.pem \ 18# -noattr -binary -outform DER -out hash0.p7s 19# 20# Exemple on a managed system: 21# keyctl padd blacklist "$(< hash0.txt)" %:.blacklist < hash0.p7s 22 23set -u -e -o pipefail 24 25CERT="${1:-}" 26BASENAME="$(basename -- "${BASH_SOURCE[0]}")" 27 28if [ $# -ne 1 ] || [ ! -f "${CERT}" ]; then 29 echo "usage: ${BASENAME} <certificate>" >&2 30 exit 1 31fi 32 33# Checks that it is indeed a certificate (PEM or DER encoded) and exclude the 34# optional PEM text header. 35if ! PEM="$(openssl x509 -inform DER -in "${CERT}" 2>/dev/null || openssl x509 -in "${CERT}")"; then 36 echo "ERROR: Failed to parse certificate" >&2 37 exit 1 38fi 39 40# TBSCertificate starts at the second entry. 41# Cf. https://tools.ietf.org/html/rfc3280#section-4.1 42# 43# Exemple of first lines printed by openssl asn1parse: 44# 0:d=0 hl=4 l= 763 cons: SEQUENCE 45# 4:d=1 hl=4 l= 483 cons: SEQUENCE 46# 8:d=2 hl=2 l= 3 cons: cont [ 0 ] 47# 10:d=3 hl=2 l= 1 prim: INTEGER :02 48# 13:d=2 hl=2 l= 20 prim: INTEGER :3CEB2CB8818D968AC00EEFE195F0DF9665328B7B 49# 35:d=2 hl=2 l= 13 cons: SEQUENCE 50# 37:d=3 hl=2 l= 9 prim: OBJECT :sha256WithRSAEncryption 51RANGE_AND_DIGEST_RE=' 522s/^\s*\([0-9]\+\):d=\s*[0-9]\+\s\+hl=\s*[0-9]\+\s\+l=\s*\([0-9]\+\)\s\+cons:\s*SEQUENCE\s*$/\1 \2/p; 537s/^\s*[0-9]\+:d=\s*[0-9]\+\s\+hl=\s*[0-9]\+\s\+l=\s*[0-9]\+\s\+prim:\s*OBJECT\s*:\(.*\)$/\1/p; 54' 55 56RANGE_AND_DIGEST=($(echo "${PEM}" | \ 57 openssl asn1parse -in - | \ 58 sed -n -e "${RANGE_AND_DIGEST_RE}")) 59 60if [ "${#RANGE_AND_DIGEST[@]}" != 3 ]; then 61 echo "ERROR: Failed to parse TBSCertificate." >&2 62 exit 1 63fi 64 65OFFSET="${RANGE_AND_DIGEST[0]}" 66END="$(( OFFSET + RANGE_AND_DIGEST[1] ))" 67DIGEST="${RANGE_AND_DIGEST[2]}" 68 69# The signature hash algorithm is used by Linux to blacklist certificates. 70# Cf. crypto/asymmetric_keys/x509_cert_parser.c:x509_note_pkey_algo() 71DIGEST_MATCH="" 72while read -r DIGEST_ITEM; do 73 if [ -z "${DIGEST_ITEM}" ]; then 74 break 75 fi 76 if echo "${DIGEST}" | grep -qiF "${DIGEST_ITEM}"; then 77 DIGEST_MATCH="${DIGEST_ITEM}" 78 break 79 fi 80done < <(openssl list -digest-commands | tr ' ' '\n' | sort -ur) 81 82if [ -z "${DIGEST_MATCH}" ]; then 83 echo "ERROR: Unknown digest algorithm: ${DIGEST}" >&2 84 exit 1 85fi 86 87echo "${PEM}" | \ 88 openssl x509 -in - -outform DER | \ 89 dd "bs=1" "skip=${OFFSET}" "count=${END}" "status=none" | \ 90 openssl dgst "-${DIGEST_MATCH}" - | \ 91 awk '{printf "tbs:" $2}' 92