/* certcheck.c - check one certificate
* Copyright (C) 2001, 2003, 2004 Free Software Foundation, Inc.
* Copyright (C) 2001-2019 Werner Koch
* Copyright (C) 2015-2020 g10 Code GmbH
*
* This file is part of GnuPG.
*
* GnuPG is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3 of the License, or
* (at your option) any later version.
*
* GnuPG is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, see <https://www.gnu.org/licenses/>.
* SPDX-License-Identifier: GPL-3.0-or-later
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <time.h>
#include <assert.h>
#include "gpgsm.h"
#include <gcrypt.h>
#include <ksba.h>
#include "keydb.h"
#include "../common/i18n.h"
/* Return the number of bits of the Q parameter from the DSA key
KEY. */
static unsigned int
get_dsa_qbits (gcry_sexp_t key)
{
gcry_sexp_t l1, l2;
gcry_mpi_t q;
unsigned int nbits;
l1 = gcry_sexp_find_token (key, "public-key", 0);
if (!l1)
return 0; /* Does not contain a key object. */
l2 = gcry_sexp_cadr (l1);
gcry_sexp_release (l1);
l1 = gcry_sexp_find_token (l2, "q", 1);
gcry_sexp_release (l2);
if (!l1)
return 0; /* Invalid object. */
q = gcry_sexp_nth_mpi (l1, 1, GCRYMPI_FMT_USG);
gcry_sexp_release (l1);
if (!q)
return 0; /* Missing value. */
nbits = gcry_mpi_get_nbits (q);
gcry_mpi_release (q);
return nbits;
}
static int
do_encode_md (gcry_md_hd_t md, int algo, int pkalgo, unsigned int nbits,
gcry_sexp_t pkey, gcry_mpi_t *r_val)
{
int n;
size_t nframe;
unsigned char *frame;
if (pkalgo == GCRY_PK_DSA || pkalgo == GCRY_PK_ECDSA)
{
unsigned int qbits;
if ( pkalgo == GCRY_PK_ECDSA )
qbits = gcry_pk_get_nbits (pkey);
else
qbits = get_dsa_qbits (pkey);
if ( (qbits%8) )
{
log_error(_("DSA requires the hash length to be a"
" multiple of 8 bits\n"));
return gpg_error (GPG_ERR_INTERNAL);
}
/* Don't allow any Q smaller than 160 bits. We don't want
someone to issue signatures from a key with a 16-bit Q or
something like that, which would look correct but allow
trivial forgeries. Yes, I know this rules out using MD5 with
DSA. ;) */
if (qbits < 160)
{
log_error (_("%s key uses an unsafe (%u bit) hash\n"),
gcry_pk_algo_name (pkalgo), qbits);
return gpg_error (GPG_ERR_INTERNAL);
}
/* Check if we're too short. Too long is safe as we'll
automatically left-truncate. */
nframe = gcry_md_get_algo_dlen (algo);
if (nframe < qbits/8)
{
log_error (_("a %u bit hash is not valid for a %u bit %s key\n"),
(unsigned int)nframe*8,
gcry_pk_get_nbits (pkey),
gcry_pk_algo_name (pkalgo));
/* FIXME: we need to check the requirements for ECDSA. */
if (nframe < 20 || pkalgo == GCRY_PK_DSA )
return gpg_error (GPG_ERR_INTERNAL);
}
frame = xtrymalloc (nframe);
if (!frame)
return out_of_core ();
memcpy (frame, gcry_md_read (md, algo), nframe);
n = nframe;
/* Truncate. */
if (n > qbits/8)
n = qbits/8;
}
else
{
int i;
unsigned char asn[100];
size_t asnlen;
size_t len;
nframe = (nbits+7) / 8;
asnlen = DIM(asn);
if (!algo || gcry_md_test_algo (algo))
return gpg_error (GPG_ERR_DIGEST_ALGO);
if (gcry_md_algo_info (algo, GCRYCTL_GET_ASNOID, asn, &asnlen))
{
log_error ("no object identifier for algo %d\n", algo);
return gpg_error (GPG_ERR_INTERNAL);
}
len = gcry_md_get_algo_dlen (algo);
if ( len + asnlen + 4 > nframe )
{
log_error ("can't encode a %d bit MD into a %d bits frame\n",
(int)(len*8), (int)nbits);
return gpg_error (GPG_ERR_INTERNAL);
}
/* We encode the MD in this way:
*
* 0 A PAD(n bytes) 0 ASN(asnlen bytes) MD(len bytes)
*
* PAD consists of FF bytes.
*/
frame = xtrymalloc (nframe);
if (!frame)
return out_of_core ();
n = 0;
frame[n++] = 0;
frame[n++] = 1; /* block type */
i = nframe - len - asnlen -3 ;
assert ( i > 1 );
memset ( frame+n, 0xff, i ); n += i;
frame[n++] = 0;
memcpy ( frame+n, asn, asnlen ); n += asnlen;
memcpy ( frame+n, gcry_md_read(md, algo), len ); n += len;
assert ( n == nframe );
}
if (DBG_CRYPTO)
{
int j;
log_debug ("encoded hash:");
for (j=0; j < nframe; j++)
log_printf (" %02X", frame[j]);
log_printf ("\n");
}
gcry_mpi_scan (r_val, GCRYMPI_FMT_USG, frame, n, &nframe);
xfree (frame);
return 0;
}
/* Return the public key algorithm id from the S-expression PKEY.
FIXME: libgcrypt should provide such a function. Note that this
implementation uses the names as used by libksba. */
static int
pk_algo_from_sexp (gcry_sexp_t pkey)
{
gcry_sexp_t l1, l2;
const char *name;
size_t n;
int algo;
l1 = gcry_sexp_find_token (pkey, "public-key", 0);
if (!l1)
return 0; /* Not found. */
l2 = gcry_sexp_cadr (l1);
gcry_sexp_release (l1);
name = gcry_sexp_nth_data (l2, 0, &n);
if (!name)
algo = 0; /* Not found. */
else if (n==3 && !memcmp (name, "rsa", 3))
algo = GCRY_PK_RSA;
else if (n==3 && !memcmp (name, "dsa", 3))
algo = GCRY_PK_DSA;
/* Because this function is called only for verification we can
assume that ECC actually means ECDSA. */
else if (n==3 && !memcmp (name, "ecc", 3))
algo = GCRY_PK_ECDSA;
else if (n==13 && !memcmp (name, "ambiguous-rsa", 13))
algo = GCRY_PK_RSA;
else
algo = 0;
gcry_sexp_release (l2);
return algo;
}
/* Return the hash algorithm's algo id from its name given in the
* non-null termnated string in (buffer,buflen). Returns 0 on failure
* or if the algo is not known. */
static int
hash_algo_from_buffer (const void *buffer, size_t buflen)
{
char *string;
int algo;
string = xtrymalloc (buflen + 1);
if (!string)
{
log_error (_("out of core\n"));
return 0;
}
memcpy (string, buffer, buflen);
string[buflen] = 0;
algo = gcry_md_map_name (string);
if (!algo)
log_error ("unknown digest algorithm '%s' used in certificate\n", string);
xfree (string);
return algo;
}
/* Return an unsigned integer from the non-null termnated string
* (buffer,buflen). Returns 0 on failure. */
static unsigned int
uint_from_buffer (const void *buffer, size_t buflen)
{
char *string;
unsigned int val;
string = xtrymalloc (buflen + 1);
if (!string)
{
log_error (_("out of core\n"));
return 0;
}
memcpy (string, buffer, buflen);
string[buflen] = 0;
val = strtoul (string, NULL, 10);
xfree (string);
return val;
}
/* Extract the hash algorithm and the salt length from the sigval. */
static gpg_error_t
extract_pss_params (gcry_sexp_t s_sig, int *r_algo, unsigned int *r_saltlen)
{
gpg_error_t err;
gcry_buffer_t ioarray[2] = { {0}, {0} };
err = gcry_sexp_extract_param (s_sig, "sig-val",
"&'hash-algo''salt-length'",
ioarray+0, ioarray+1, NULL);
if (err)
{
log_error ("extracting params from PSS failed: %s\n", gpg_strerror (err));
return err;
}
*r_algo = hash_algo_from_buffer (ioarray[0].data, ioarray[0].len);
*r_saltlen = uint_from_buffer (ioarray[1].data, ioarray[1].len);
xfree (ioarray[0].data);
xfree (ioarray[1].data);
if (*r_saltlen < 20)
{
log_error ("length of PSS salt too short\n");
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
if (!*r_algo)
{
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
/* PSS has no hash function firewall like PKCS#1 and thus offers
* a path for hash algorithm replacement. To avoid this it makes
* sense to restrict the allowed hash algorithms and also allow only
* matching salt lengths. According to Peter Gutmann:
* "Beware of bugs in the above signature scheme;
* I have only proved it secure, not implemented it"
* - Apologies to Donald Knuth.
* https://www.metzdowd.com/pipermail/cryptography/2019-November/035449.html
*
* Given the set of supported algorithms currently available in
* Libgcrypt and the extra hash checks we have in some compliance
* modes, it would be hard to trick gpgsm to verify a forged
* signature. However, if eventually someone adds the xor256 hash
* algorithm (1.3.6.1.4.1.3029.3.2) to Libgcrypt we would be doomed.
*/
switch (*r_algo)
{
case GCRY_MD_SHA1:
case GCRY_MD_SHA256:
case GCRY_MD_SHA384:
case GCRY_MD_SHA512:
case GCRY_MD_SHA3_256:
case GCRY_MD_SHA3_384:
case GCRY_MD_SHA3_512:
break;
default:
log_error ("PSS hash algorithm '%s' rejected\n",
gcry_md_algo_name (*r_algo));
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
if (gcry_md_get_algo_dlen (*r_algo) != *r_saltlen)
{
log_error ("PSS hash algorithm '%s' rejected due to salt length %u\n",
gcry_md_algo_name (*r_algo), *r_saltlen);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
return 0;
}
/* Check the signature on CERT using the ISSUER-CERT. This function
does only test the cryptographic signature and nothing else. It is
assumed that the ISSUER_CERT is valid. */
int
gpgsm_check_cert_sig (ksba_cert_t issuer_cert, ksba_cert_t cert)
{
const char *algoid;
gcry_md_hd_t md;
int rc, algo;
ksba_sexp_t p;
size_t n;
gcry_sexp_t s_sig, s_data, s_pkey;
int use_pss = 0;
unsigned int saltlen;
algo = gcry_md_map_name ( (algoid=ksba_cert_get_digest_algo (cert)));
if (!algo && algoid && !strcmp (algoid, "1.2.840.113549.1.1.10"))
use_pss = 1;
else if (!algo)
{
log_error ("unknown digest algorithm '%s' used certificate\n",
algoid? algoid:"?");
if (algoid
&& ( !strcmp (algoid, "1.2.840.113549.1.1.2")
||!strcmp (algoid, "1.2.840.113549.2.2")))
log_info (_("(this is the MD2 algorithm)\n"));
return gpg_error (GPG_ERR_GENERAL);
}
/* The the signature from the certificate. */
p = ksba_cert_get_sig_val (cert);
n = gcry_sexp_canon_len (p, 0, NULL, NULL);
if (!n)
{
log_error ("libksba did not return a proper S-Exp\n");
ksba_free (p);
return gpg_error (GPG_ERR_BUG);
}
rc = gcry_sexp_sscan ( &s_sig, NULL, (char*)p, n);
ksba_free (p);
if (rc)
{
log_error ("gcry_sexp_scan failed: %s\n", gpg_strerror (rc));
return rc;
}
if (DBG_CRYPTO)
gcry_log_debugsxp ("sigval", s_sig);
if (use_pss)
{
rc = extract_pss_params (s_sig, &algo, &saltlen);
if (rc)
{
gcry_sexp_release (s_sig);
return rc;
}
}
/* Hash the to-be-signed parts of the certificate. */
rc = gcry_md_open (&md, algo, 0);
if (rc)
{
log_error ("md_open failed: %s\n", gpg_strerror (rc));
return rc;
}
if (DBG_HASHING)
gcry_md_debug (md, "hash.cert");
rc = ksba_cert_hash (cert, 1, HASH_FNC, md);
if (rc)
{
log_error ("ksba_cert_hash failed: %s\n", gpg_strerror (rc));
gcry_md_close (md);
return rc;
}
gcry_md_final (md);
/* Get the public key from the certificate. */
p = ksba_cert_get_public_key (issuer_cert);
n = gcry_sexp_canon_len (p, 0, NULL, NULL);
if (!n)
{
log_error ("libksba did not return a proper S-Exp\n");
gcry_md_close (md);
ksba_free (p);
gcry_sexp_release (s_sig);
return gpg_error (GPG_ERR_BUG);
}
rc = gcry_sexp_sscan ( &s_pkey, NULL, (char*)p, n);
ksba_free (p);
if (rc)
{
log_error ("gcry_sexp_scan failed: %s\n", gpg_strerror (rc));
gcry_md_close (md);
gcry_sexp_release (s_sig);
return rc;
}
if (DBG_CRYPTO)
gcry_log_debugsxp ("pubkey:", s_pkey);
if (use_pss)
{
rc = gcry_sexp_build (&s_data, NULL,
"(data (flags pss)"
"(hash %s %b)"
"(salt-length %u))",
hash_algo_to_string (algo),
(int)gcry_md_get_algo_dlen (algo),
gcry_md_read (md, algo),
saltlen);
if (rc)
BUG ();
}
else
{
/* RSA or DSA: Prepare the hash for verification. */
gcry_mpi_t frame;
rc = do_encode_md (md, algo, pk_algo_from_sexp (s_pkey),
gcry_pk_get_nbits (s_pkey), s_pkey, &frame);
if (rc)
{
gcry_md_close (md);
gcry_sexp_release (s_sig);
gcry_sexp_release (s_pkey);
return rc;
}
if ( gcry_sexp_build (&s_data, NULL, "%m", frame) )
BUG ();
gcry_mpi_release (frame);
}
if (DBG_CRYPTO)
gcry_log_debugsxp ("data:", s_data);
/* Verify. */
rc = gcry_pk_verify (s_sig, s_data, s_pkey);
if (DBG_X509)
log_debug ("gcry_pk_verify: %s\n", gpg_strerror (rc));
gcry_md_close (md);
gcry_sexp_release (s_sig);
gcry_sexp_release (s_data);
gcry_sexp_release (s_pkey);
return rc;
}
int
gpgsm_check_cms_signature (ksba_cert_t cert, gcry_sexp_t s_sig,
gcry_md_hd_t md, int mdalgo,
unsigned int pkalgoflags, int *r_pkalgo)
{
int rc;
ksba_sexp_t p;
gcry_sexp_t s_hash, s_pkey;
size_t n;
int pkalgo;
int use_pss;
unsigned int saltlen = 0;
if (r_pkalgo)
*r_pkalgo = 0;
/* Check whether rsaPSS is needed. This information is indicated in
* the SIG-VAL and already provided to us by the caller so that we
* do not need to parse this out. */
use_pss = !!(pkalgoflags & PK_ALGO_FLAG_RSAPSS);
if (use_pss)
{
int algo;
rc = extract_pss_params (s_sig, &algo, &saltlen);
if (rc)
{
gcry_sexp_release (s_sig);
return rc;
}
if (algo != mdalgo)
{
log_error ("PSS hash algo mismatch (%d/%d)\n", mdalgo, algo);
gcry_sexp_release (s_sig);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
}
p = ksba_cert_get_public_key (cert);
n = gcry_sexp_canon_len (p, 0, NULL, NULL);
if (!n)
{
log_error ("libksba did not return a proper S-Exp\n");
ksba_free (p);
return gpg_error (GPG_ERR_BUG);
}
if (DBG_CRYPTO)
log_printhex (p, n, "public key: ");
rc = gcry_sexp_sscan ( &s_pkey, NULL, (char*)p, n);
ksba_free (p);
if (rc)
{
log_error ("gcry_sexp_scan failed: %s\n", gpg_strerror (rc));
return rc;
}
pkalgo = pk_algo_from_sexp (s_pkey);
if (r_pkalgo)
*r_pkalgo = pkalgo;
if (use_pss)
{
rc = gcry_sexp_build (&s_hash, NULL,
"(data (flags pss)"
"(hash %s %b)"
"(salt-length %u))",
hash_algo_to_string (mdalgo),
(int)gcry_md_get_algo_dlen (mdalgo),
gcry_md_read (md, mdalgo),
saltlen);
if (rc)
BUG ();
}
else
{
/* RSA or DSA: Prepare the hash for verification. */
gcry_mpi_t frame;
rc = do_encode_md (md, mdalgo, pkalgo,
gcry_pk_get_nbits (s_pkey), s_pkey, &frame);
if (rc)
{
gcry_sexp_release (s_pkey);
return rc;
}
/* put hash into the S-Exp s_hash */
if ( gcry_sexp_build (&s_hash, NULL, "%m", frame) )
BUG ();
gcry_mpi_release (frame);
}
rc = gcry_pk_verify (s_sig, s_hash, s_pkey);
if (DBG_X509)
log_debug ("gcry_pk_verify: %s\n", gpg_strerror (rc));
gcry_sexp_release (s_hash);
gcry_sexp_release (s_pkey);
return rc;
}
int
gpgsm_create_cms_signature (ctrl_t ctrl, ksba_cert_t cert,
gcry_md_hd_t md, int mdalgo,
unsigned char **r_sigval)
{
int rc;
char *grip, *desc;
size_t siglen;
grip = gpgsm_get_keygrip_hexstring (cert);
if (!grip)
return gpg_error (GPG_ERR_BAD_CERT);
desc = gpgsm_format_keydesc (cert);
rc = gpgsm_agent_pksign (ctrl, grip, desc, gcry_md_read(md, mdalgo),
gcry_md_get_algo_dlen (mdalgo), mdalgo,
r_sigval, &siglen);
xfree (desc);
xfree (grip);
return rc;
}