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