gnupg/agent/pksign.c

/* pksign.c - public key signing (well, actually using a secret key)
 * Copyright (C) 2001, 2002, 2003, 2004, 2010 Free Software Foundation, Inc.
 * Copyright (C) 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 <http://www.gnu.org/licenses/>.
 */

#include <config.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <assert.h>
#include <unistd.h>
#include <sys/stat.h>

#include "agent.h"
#include "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)
	{
	  for (i=0; i < strlen (s); i++)
	    tmp[i] = 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);
	}

    }

  *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 dsaalgo, gcry_sexp_t pkey,
               gcry_sexp_t *r_hash)
{
  gpg_error_t err;
  gcry_sexp_t hash;
  unsigned int qbits;
  int pkalgo;

  *r_hash = NULL;

  pkalgo = map_pk_openpgp_to_gcry (dsaalgo);

  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);
    }

  /* Check if we're too short.  Too long is safe as we'll
   * automatically left-truncate.
   *
   * This check would require the use of SHA512 with ECDSA 512. I
   * think this is overkill to fail in this case.  Therefore, relax
   * the check, but only for ECDSA keys.  We may need to adjust it
   * later for general case.  (Note that the check is really a bug for
   * ECDSA 521 as the only hash that matches it is SHA 512, but 512 <
   * 521 ).
   */
  if (mdlen < ((pkalgo==GCRY_PK_ECDSA && qbits > 521) ? 512 : 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));
      /* FIXME: we need to check the requirements for ECDSA.  */
      if (mdlen < 20 || pkalgo == GCRY_PK_DSA)
        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 ;
  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;
  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.  */
int
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)
{
  gcry_sexp_t s_skey = NULL, s_sig = NULL;
  unsigned char *shadow_info = NULL;
  unsigned int rc = 0;		/* FIXME: gpg-error? */

  if (! ctrl->have_keygrip)
    return gpg_error (GPG_ERR_NO_SECKEY);

  rc = agent_key_from_file (ctrl, cache_nonce, desc_text, ctrl->keygrip,
                            &shadow_info, cache_mode, lookup_ttl,
                            &s_skey, NULL);
  if (rc)
    {
      log_error ("failed to read the secret key\n");
      goto leave;
    }

  if (!s_skey)
    {
      /* Divert operation to the smartcard */
      gcry_sexp_t s_pkey, l;
      const char *name;
      size_t len;
      unsigned char *buf = NULL;
      int is_RSA = 0;
      int is_ECDSA = 0;

      /* Check keytype by public key */
      rc = agent_public_key_from_file (ctrl, ctrl->keygrip, &s_pkey);
      if (rc)
        {
          log_error ("failed to read the public key\n");
          goto leave;
        }
      l = gcry_sexp_cadr (s_pkey);
      name = gcry_sexp_nth_data (l, 0, &len);
      if (len == 3 && !memcmp (name, "rsa", 3))
        is_RSA = 1;
      else if (len == 5 && !memcmp (name, "ecdsa", 5))
        is_ECDSA = 1;
      gcry_sexp_release (l);
      gcry_sexp_release (s_pkey);

      rc = divert_pksign (ctrl,
                          ctrl->digest.value,
                          ctrl->digest.valuelen,
                          ctrl->digest.algo,
                          shadow_info, &buf, &len);
      if (rc)
        {
          log_error ("smartcard signing failed: %s\n", gpg_strerror (rc));
          goto leave;
        }

      if (is_RSA)
        {
          if (*buf & 0x80)
            {
              len++;
              buf = xtryrealloc (buf, len);
              if (!buf)
                goto leave;

              memmove (buf + 1, buf, len - 1);
              *buf = 0;
            }

          rc = gcry_sexp_build (&s_sig, NULL, "(sig-val(rsa(s%b)))", len, buf);
        }
      else if (is_ECDSA)
        {
          unsigned char *r_buf_allocated = NULL;
          unsigned char *s_buf_allocated = NULL;
          unsigned char *r_buf, *s_buf;
          int r_buflen, s_buflen;

          r_buflen = s_buflen = len/2;

          if (*buf & 0x80)
            {
              r_buflen++;
              r_buf_allocated = xtrymalloc (r_buflen);
              if (!r_buf_allocated)
                goto leave;

              r_buf = r_buf_allocated;
              memcpy (r_buf + 1, buf, len/2);
              *r_buf = 0;
            }
          else
            r_buf = buf;

          if (*(buf + len/2) & 0x80)
            {
              s_buflen++;
              s_buf_allocated = xtrymalloc (s_buflen);
              if (!s_buf_allocated)
                {
                  xfree (r_buf_allocated);
                  goto leave;
                }

              s_buf = s_buf_allocated;
              memcpy (s_buf + 1, buf + len/2, len/2);
              *s_buf = 0;
            }
          else
            s_buf = buf + len/2;

          rc = gcry_sexp_build (&s_sig, NULL, "(sig-val(ecdsa(r%b)(s%b)))",
                                r_buflen, r_buf,
                                s_buflen, s_buf);
          xfree (r_buf_allocated);
          xfree (s_buf_allocated);
        }
      else
        rc = gpg_error (GPG_ERR_NOT_IMPLEMENTED);

      xfree (buf);
      if (rc)
	{
	  log_error ("failed to convert sigbuf returned by divert_pksign "
		     "into S-Exp: %s", gpg_strerror (rc));
	  goto leave;
	}
    }
  else
    {
      /* No smartcard, but a private key */
      gcry_sexp_t s_hash = NULL;
      int dsaalgo;

      /* Put the hash into a sexp */
      if (agent_is_eddsa_key (s_skey))
        rc = do_encode_eddsa (ctrl->digest.value,
                              ctrl->digest.valuelen,
                              &s_hash);
      else if (ctrl->digest.algo == MD_USER_TLS_MD5SHA1)
        rc = do_encode_raw_pkcs1 (ctrl->digest.value,
                                  ctrl->digest.valuelen,
                                  gcry_pk_get_nbits (s_skey),
                                  &s_hash);
      else if ( (dsaalgo = agent_is_dsa_key (s_skey)) )
        rc = do_encode_dsa (ctrl->digest.value,
                            ctrl->digest.valuelen,
                            dsaalgo, s_skey,
                            &s_hash);
      else
        rc = do_encode_md (ctrl->digest.value,
                           ctrl->digest.valuelen,
                           ctrl->digest.algo,
                           &s_hash,
                           ctrl->digest.raw_value);
      if (rc)
        goto leave;

      if (DBG_CRYPTO)
        {
          gcry_log_debugsxp ("skey", s_skey);
          gcry_log_debugsxp ("hash", s_hash);
        }

      /* sign */
      rc = gcry_pk_sign (&s_sig, s_hash, s_skey);
      gcry_sexp_release (s_hash);
      if (rc)
        {
          log_error ("signing failed: %s\n", gpg_strerror (rc));
          goto leave;
        }

      if (DBG_CRYPTO)
        gcry_log_debugsxp ("rslt", s_sig);
    }

 leave:

  *signature_sexp = s_sig;

  gcry_sexp_release (s_skey);
  xfree (shadow_info);

  return rc;
}

/* 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.  */
int
agent_pksign (ctrl_t ctrl, const char *cache_nonce, const char *desc_text,
              membuf_t *outbuf, cache_mode_t cache_mode)
{
  gcry_sexp_t s_sig = NULL;
  char *buf = NULL;
  size_t len = 0;
  int rc = 0;

  rc = agent_pksign_do (ctrl, cache_nonce, desc_text, &s_sig, cache_mode, NULL);
  if (rc)
    goto leave;

  len = gcry_sexp_sprint (s_sig, GCRYSEXP_FMT_CANON, NULL, 0);
  assert (len);
  buf = xmalloc (len);
  len = gcry_sexp_sprint (s_sig, GCRYSEXP_FMT_CANON, buf, len);
  assert (len);

  put_membuf (outbuf, buf, len);

 leave:
  gcry_sexp_release (s_sig);
  xfree (buf);

  return rc;
}