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gnupg/agent/pksign.c

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/* pksign.c - public key signing (well, actually using a secret key)
* Copyright (C) 2001-2004, 2010 Free Software Foundation, Inc.
* Copyright (C) 2001-2004, 2010, 2013 Werner Koch
*
* 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/>.
*/
#include <config.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/stat.h>
#include "agent.h"
#include "../common/i18n.h"
static int
do_encode_md (const byte * md, size_t mdlen, int algo, gcry_sexp_t * r_hash,
int raw_value)
{
gcry_sexp_t hash;
int rc;
if (!raw_value)
{
const char *s;
char tmp[16+1];
int i;
s = gcry_md_algo_name (algo);
if (!s || strlen (s) >= 16)
{
hash = NULL;
rc = gpg_error (GPG_ERR_DIGEST_ALGO);
}
else
{
for (i=0; s[i]; i++)
tmp[i] = ascii_tolower (s[i]);
tmp[i] = '\0';
rc = gcry_sexp_build (&hash, NULL,
"(data (flags pkcs1) (hash %s %b))",
tmp, (int)mdlen, md);
}
}
else
{
gcry_mpi_t mpi;
rc = gcry_mpi_scan (&mpi, GCRYMPI_FMT_USG, md, mdlen, NULL);
if (!rc)
{
rc = gcry_sexp_build (&hash, NULL,
"(data (flags raw) (value %m))",
mpi);
gcry_mpi_release (mpi);
}
else
hash = NULL;
}
*r_hash = hash;
return rc;
}
/* 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, "private-key", 0);
if (!l1)
l1 = gcry_sexp_find_token (key, "protected-private-key", 0);
if (!l1)
l1 = gcry_sexp_find_token (key, "shadowed-private-key", 0);
if (!l1)
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;
}
/* Return an appropriate hash algorithm to be used with RFC-6979 for a
message digest of length MDLEN. Although a fallback of SHA-256 is
used the current implementation in Libgcrypt will reject a hash
algorithm which does not match the length of the message. */
static const char *
rfc6979_hash_algo_string (size_t mdlen)
{
switch (mdlen)
{
case 20: return "sha1";
case 28: return "sha224";
case 32: return "sha256";
case 48: return "sha384";
case 64: return "sha512";
default: return "sha256";
}
}
/* Encode a message digest for use with the EdDSA algorithm
(i.e. curve Ed25519). */
static gpg_error_t
do_encode_eddsa (const byte *md, size_t mdlen, gcry_sexp_t *r_hash)
{
gpg_error_t err;
gcry_sexp_t hash;
*r_hash = NULL;
err = gcry_sexp_build (&hash, NULL,
"(data(flags eddsa)(hash-algo sha512)(value %b))",
(int)mdlen, md);
if (!err)
*r_hash = hash;
return err;
}
/* Encode a message digest for use with an DSA algorithm. */
static gpg_error_t
do_encode_dsa (const byte *md, size_t mdlen, int pkalgo, gcry_sexp_t pkey,
gcry_sexp_t *r_hash)
{
gpg_error_t err;
gcry_sexp_t hash;
unsigned int qbits;
*r_hash = NULL;
if (pkalgo == GCRY_PK_ECDSA)
qbits = gcry_pk_get_nbits (pkey);
else if (pkalgo == GCRY_PK_DSA)
qbits = get_dsa_qbits (pkey);
else
return gpg_error (GPG_ERR_WRONG_PUBKEY_ALGO);
if (pkalgo == GCRY_PK_DSA && (qbits%8))
{
/* FIXME: We check the QBITS but print a message about the hash
length. */
log_error (_("DSA requires the hash length to be a"
" multiple of 8 bits\n"));
return gpg_error (GPG_ERR_INV_LENGTH);
}
/* 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_INV_LENGTH);
}
/* ECDSA 521 is special has it is larger than the largest hash
we have (SHA-512). Thus we change the size for further
processing to 512. */
if (pkalgo == GCRY_PK_ECDSA && qbits > 512)
qbits = 512;
/* Check if we're too short. Too long is safe as we'll
automatically left-truncate. */
if (mdlen < qbits/8)
{
log_error (_("a %zu bit hash is not valid for a %u bit %s key\n"),
mdlen*8,
gcry_pk_get_nbits (pkey),
gcry_pk_algo_name (pkalgo));
return gpg_error (GPG_ERR_INV_LENGTH);
}
/* Truncate. */
if (mdlen > qbits/8)
mdlen = qbits/8;
/* Create the S-expression. */
err = gcry_sexp_build (&hash, NULL,
"(data (flags rfc6979) (hash %s %b))",
rfc6979_hash_algo_string (mdlen),
(int)mdlen, md);
if (!err)
*r_hash = hash;
return err;
}
/* Special version of do_encode_md to take care of pkcs#1 padding.
For TLS-MD5SHA1 we need to do the padding ourself as Libgrypt does
not know about this special scheme. Fixme: We should have a
pkcs1-only-padding flag for Libgcrypt. */
static int
do_encode_raw_pkcs1 (const byte *md, size_t mdlen, unsigned int nbits,
gcry_sexp_t *r_hash)
{
int rc;
gcry_sexp_t hash;
unsigned char *frame;
size_t i, n, nframe;
nframe = (nbits+7) / 8;
if ( !mdlen || mdlen + 8 + 4 > nframe )
{
/* Can't encode this hash into a frame of size NFRAME. */
return gpg_error (GPG_ERR_TOO_SHORT);
}
frame = xtrymalloc (nframe);
if (!frame)
return gpg_error_from_syserror ();
/* Assemble the pkcs#1 block type 1. */
n = 0;
frame[n++] = 0;
frame[n++] = 1; /* Block type. */
i = nframe - mdlen - 3 ;
log_assert (i >= 8); /* At least 8 bytes of padding. */
memset (frame+n, 0xff, i );
n += i;
frame[n++] = 0;
memcpy (frame+n, md, mdlen );
n += mdlen;
log_assert (n == nframe);
/* Create the S-expression. */
rc = gcry_sexp_build (&hash, NULL,
"(data (flags raw) (value %b))",
(int)nframe, frame);
xfree (frame);
*r_hash = hash;
return rc;
}
/* SIGN whatever information we have accumulated in CTRL and return
* the signature S-expression. LOOKUP is an optional function to
* provide a way for lower layers to ask for the caching TTL. If a
* CACHE_NONCE is given that cache item is first tried to get a
* passphrase. If OVERRIDEDATA is not NULL, OVERRIDEDATALEN bytes
* from this buffer are used instead of the data in CTRL. The
* override feature is required to allow the use of Ed25519 with ssh
* because Ed25519 does the hashing itself. */
gpg_error_t
agent_pksign_do (ctrl_t ctrl, const char *cache_nonce,
const char *desc_text,
gcry_sexp_t *signature_sexp,
cache_mode_t cache_mode, lookup_ttl_t lookup_ttl,
const void *overridedata, size_t overridedatalen)
{
gpg_error_t err = 0;
gcry_sexp_t s_skey = NULL;
gcry_sexp_t s_sig = NULL;
gcry_sexp_t s_hash = NULL;
gcry_sexp_t s_pkey = NULL;
unsigned char *shadow_info = NULL;
int no_shadow_info = 0;
const unsigned char *data;
int datalen;
int check_signature = 0;
if (overridedata)
{
data = overridedata;
datalen = overridedatalen;
}
else
{
data = ctrl->digest.value;
datalen = ctrl->digest.valuelen;
}
if (!ctrl->have_keygrip)
return gpg_error (GPG_ERR_NO_SECKEY);
err = agent_key_from_file (ctrl, cache_nonce, desc_text, ctrl->keygrip,
&shadow_info, cache_mode, lookup_ttl,
&s_skey, NULL);
if (gpg_err_code (err) == GPG_ERR_NO_SECKEY)
no_shadow_info = 1;
else if (err)
{
log_error ("failed to read the secret key\n");
goto leave;
}
if (shadow_info || no_shadow_info)
{
/* Divert operation to the smartcard. With NO_SHADOW_INFO set
* we don't have the keystub but we want to see whether the key
* is on the active card. */
size_t len;
unsigned char *buf = NULL;
int key_type;
int is_RSA = 0;
int is_ECDSA = 0;
int is_EdDSA = 0;
if (no_shadow_info)
{
/* Try to get the public key from the card or fail with the
* original NO_SECKEY error. We also write a stub file (we
* are here only because no stub exists). */
char *serialno;
unsigned char *pkbuf = NULL;
size_t pkbuflen;
char hexgrip[2*KEYGRIP_LEN+1];
char *keyref;
if (agent_card_serialno (ctrl, &serialno, NULL))
{
/* No card available or error reading the card. */
err = gpg_error (GPG_ERR_NO_SECKEY);
goto leave;
}
bin2hex (ctrl->keygrip, KEYGRIP_LEN, hexgrip);
if (agent_card_readkey (ctrl, hexgrip, &pkbuf, &keyref))
{
/* No such key on the card. */
xfree (serialno);
err = gpg_error (GPG_ERR_NO_SECKEY);
goto leave;
}
pkbuflen = gcry_sexp_canon_len (pkbuf, 0, NULL, NULL);
err = gcry_sexp_sscan (&s_pkey, NULL, (char*)pkbuf, pkbuflen);
if (err)
{
xfree (serialno);
xfree (pkbuf);
xfree (keyref);
log_error ("%s: corrupted key returned by scdaemon\n", __func__);
goto leave;
}
if (keyref)
agent_write_shadow_key (ctrl->keygrip, serialno, keyref, pkbuf, 0);
xfree (serialno);
xfree (pkbuf);
xfree (keyref);
}
else
{
/* Get the public key from the stub file. */
err = agent_public_key_from_file (ctrl, ctrl->keygrip, &s_pkey);
if (err)
{
log_error ("failed to read the public key\n");
goto leave;
}
}
if (agent_is_eddsa_key (s_pkey))
is_EdDSA = 1;
else
{
key_type = agent_is_dsa_key (s_pkey);
if (key_type == 0)
is_RSA = 1;
else if (key_type == GCRY_PK_ECDSA)
is_ECDSA = 1;
}
{
char *desc2 = NULL;
if (desc_text)
agent_modify_description (desc_text, NULL, s_pkey, &desc2);
err = divert_pksign (ctrl, desc2? desc2 : desc_text,
ctrl->keygrip,
data, datalen,
ctrl->digest.algo,
shadow_info, &buf, &len);
xfree (desc2);
}
if (err)
{
log_error ("smartcard signing failed: %s\n", gpg_strerror (err));
goto leave;
}
if (is_RSA)
{
unsigned char *p = buf;
check_signature = 1;
/*
* Smartcard returns fixed-size data, which is good for
* PKCS1. If variable-size unsigned MPI is needed, remove
* zeros.
*/
if (ctrl->digest.algo == MD_USER_TLS_MD5SHA1
|| ctrl->digest.raw_value)
{
int i;
for (i = 0; i < len - 1; i++)
if (p[i])
break;
p += i;
len -= i;
}
err = gcry_sexp_build (&s_sig, NULL, "(sig-val(rsa(s%b)))",
(int)len, p);
}
else if (is_EdDSA)
{
err = gcry_sexp_build (&s_sig, NULL, "(sig-val(eddsa(r%b)(s%b)))",
(int)len/2, buf, (int)len/2, buf + len/2);
}
else if (is_ECDSA)
{
unsigned char *r_buf, *s_buf;
int r_buflen, s_buflen;
int i;
r_buflen = s_buflen = len/2;
/*
* Smartcard returns fixed-size data. For ECDSA signature,
* variable-size unsigned MPI is assumed, thus, remove
* zeros.
*/
r_buf = buf;
for (i = 0; i < r_buflen - 1; i++)
if (r_buf[i])
break;
r_buf += i;
r_buflen -= i;
s_buf = buf + len/2;
for (i = 0; i < s_buflen - 1; i++)
if (s_buf[i])
break;
s_buf += i;
s_buflen -= i;
err = gcry_sexp_build (&s_sig, NULL, "(sig-val(ecdsa(r%b)(s%b)))",
r_buflen, r_buf,
s_buflen, s_buf);
}
else
err = gpg_error (GPG_ERR_NOT_IMPLEMENTED);
xfree (buf);
if (err)
{
log_error ("failed to convert sigbuf returned by divert_pksign "
"into S-Exp: %s", gpg_strerror (err));
goto leave;
}
}
else
{
/* No smartcard, but a private key (in S_SKEY). */
int dsaalgo = 0;
/* Put the hash into a sexp */
if (agent_is_eddsa_key (s_skey))
err = do_encode_eddsa (data, datalen,
&s_hash);
else if (ctrl->digest.algo == MD_USER_TLS_MD5SHA1)
err = do_encode_raw_pkcs1 (data, datalen,
gcry_pk_get_nbits (s_skey),
&s_hash);
else if ( (dsaalgo = agent_is_dsa_key (s_skey)) )
err = do_encode_dsa (data, datalen,
dsaalgo, s_skey,
&s_hash);
else
err = do_encode_md (data, datalen,
ctrl->digest.algo,
&s_hash,
ctrl->digest.raw_value);
if (err)
goto leave;
if (dsaalgo == 0 && GCRYPT_VERSION_NUMBER < 0x010700)
{
/* It's RSA and Libgcrypt < 1.7 */
check_signature = 1;
}
if (DBG_CRYPTO)
{
gcry_log_debugsxp ("skey", s_skey);
gcry_log_debugsxp ("hash", s_hash);
}
/* sign */
err = gcry_pk_sign (&s_sig, s_hash, s_skey);
if (err)
{
log_error ("signing failed: %s\n", gpg_strerror (err));
goto leave;
}
if (DBG_CRYPTO)
gcry_log_debugsxp ("rslt", s_sig);
}
/* Check that the signature verification worked and nothing is
* fooling us e.g. by a bug in the signature create code or by
* deliberately introduced faults. Because Libgcrypt 1.7 does this
* for RSA internally there is no need to do it here again. We do
* this always for card based RSA keys, though. */
if (check_signature)
{
gcry_sexp_t sexp_key = s_pkey? s_pkey: s_skey;
if (s_hash == NULL)
{
if (ctrl->digest.algo == MD_USER_TLS_MD5SHA1)
err = do_encode_raw_pkcs1 (data, datalen,
gcry_pk_get_nbits (sexp_key), &s_hash);
else
err = do_encode_md (data, datalen, ctrl->digest.algo, &s_hash,
ctrl->digest.raw_value);
}
if (!err)
err = gcry_pk_verify (s_sig, s_hash, sexp_key);
if (err)
{
log_error (_("checking created signature failed: %s\n"),
gpg_strerror (err));
gcry_sexp_release (s_sig);
s_sig = NULL;
}
}
leave:
*signature_sexp = s_sig;
gcry_sexp_release (s_pkey);
gcry_sexp_release (s_skey);
gcry_sexp_release (s_hash);
xfree (shadow_info);
return err;
}
/* SIGN whatever information we have accumulated in CTRL and write it
* back to OUTFP. If a CACHE_NONCE is given that cache item is first
* tried to get a passphrase. */
gpg_error_t
agent_pksign (ctrl_t ctrl, const char *cache_nonce, const char *desc_text,
membuf_t *outbuf, cache_mode_t cache_mode)
{
gpg_error_t err;
gcry_sexp_t s_sig = NULL;
char *buf = NULL;
size_t len = 0;
err = agent_pksign_do (ctrl, cache_nonce, desc_text, &s_sig, cache_mode,
NULL, NULL, 0);
if (err)
goto leave;
len = gcry_sexp_sprint (s_sig, GCRYSEXP_FMT_CANON, NULL, 0);
log_assert (len);
buf = xtrymalloc (len);
if (!buf)
{
err = gpg_error_from_syserror ();
goto leave;
}
len = gcry_sexp_sprint (s_sig, GCRYSEXP_FMT_CANON, buf, len);
log_assert (len);
put_membuf (outbuf, buf, len);
leave:
gcry_sexp_release (s_sig);
xfree (buf);
return err;
}
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