path: root/cipher/ecc-sm2.c

/* ecc-sm2.c  -  Elliptic Curve SM2 implementation
 * Copyright (C) 2020 Tianjia Zhang
 *
 * This file is part of Libgcrypt.
 *
 * Libgcrypt is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * Libgcrypt 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 Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program; if not, see <http://www.gnu.org/licenses/>.
 */

#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#include "g10lib.h"
#include "bithelp.h"
#include "mpi.h"
#include "cipher.h"
#include "context.h"
#include "ec-context.h"
#include "pubkey-internal.h"
#include "ecc-common.h"

#define MPI_NBYTES(m)   ((mpi_get_nbits(m) + 7) / 8)


/* Key derivation function from X9.63/SECG */
static gpg_err_code_t
kdf_x9_63 (int algo, const void *in, size_t inlen, void *out, size_t outlen)
{
  gpg_err_code_t rc;
  gcry_md_hd_t hd;
  int mdlen;
  u32 counter = 1;
  u32 counter_be;
  unsigned char *dgst;
  unsigned char *pout = out;
  size_t rlen = outlen;
  size_t len;

  rc = _gcry_md_open (&hd, algo, 0);
  if (rc)
    return rc;

  mdlen = _gcry_md_get_algo_dlen (algo);

  while (rlen > 0)
    {
      counter_be = be_bswap32 (counter);   /* cpu_to_be32 */
      counter++;

      _gcry_md_write (hd, in, inlen);
      _gcry_md_write (hd, &counter_be, sizeof(counter_be));

      dgst = _gcry_md_read (hd, algo);
      if (dgst == NULL)
        {
          rc = GPG_ERR_DIGEST_ALGO;
          break;
        }

      len = mdlen < rlen ? mdlen : rlen;  /* min(mdlen, rlen) */
      memcpy (pout, dgst, len);
      rlen -= len;
      pout += len;

      _gcry_md_reset (hd);
    }

  _gcry_md_close (hd);
  return rc;
}


/* _gcry_ecc_sm2_encrypt description:
 *   input:
 *     data[0] : octet string
 *   output: A new S-expression with the parameters:
 *     a: c1 : generated ephemeral public key (kG)
 *     b: c3 : Hash(x2 || IN || y2)
 *     c: c2 : cipher
 *
 * sm2_decrypt description:
 *   in contrast to encrypt
 */
gpg_err_code_t
_gcry_ecc_sm2_encrypt (gcry_sexp_t *r_ciph, gcry_mpi_t input, mpi_ec_t ec)
{
  gpg_err_code_t rc;
  const int algo = GCRY_MD_SM3;
  gcry_md_hd_t md = NULL;
  int mdlen;
  unsigned char *dgst;
  gcry_mpi_t k = NULL;
  mpi_point_struct kG, kP;
  gcry_mpi_t x1, y1;
  gcry_mpi_t x2, y2;
  gcry_mpi_t x2y2 = NULL;
  unsigned char *in = NULL;
  unsigned int inlen;
  unsigned char *raw;
  unsigned int rawlen;
  unsigned char *cipher = NULL;
  int i;

  point_init (&kG);
  point_init (&kP);
  x1 = mpi_new (0);
  y1 = mpi_new (0);
  x2 = mpi_new (0);
  y2 = mpi_new (0);

  in = _gcry_mpi_get_buffer (input, 0, &inlen, NULL);
  if (!in)
    {
      rc = gpg_err_code_from_syserror ();
      goto leave;
    }

  cipher = xtrymalloc (inlen);
  if (!cipher)
    {
      rc = gpg_err_code_from_syserror ();
      goto leave;
    }

  /* rand k in [1, n-1] */
  k = _gcry_dsa_gen_k (ec->n, GCRY_VERY_STRONG_RANDOM);

  /* [k]G = (x1, y1) */
  _gcry_mpi_ec_mul_point (&kG, k, ec->G, ec);
  if (_gcry_mpi_ec_get_affine (x1, y1, &kG, ec))
    {
      if (DBG_CIPHER)
        log_debug ("Bad check: kG can not be a Point at Infinity!\n");
      rc = GPG_ERR_INV_DATA;
      goto leave;
    }

  /* [k]P = (x2, y2) */
  _gcry_mpi_ec_mul_point (&kP, k, ec->Q, ec);
  if (_gcry_mpi_ec_get_affine (x2, y2, &kP, ec))
    {
      rc = GPG_ERR_INV_DATA;
      goto leave;
    }

  /* t = KDF(x2 || y2, klen) */
  x2y2 = _gcry_mpi_ec_ec2os (&kP, ec);
  raw = mpi_get_opaque (x2y2, &rawlen);
  rawlen = (rawlen + 7) / 8;

  /* skip the prefix '0x04' */
  raw += 1;
  rawlen -= 1;
  rc = kdf_x9_63 (algo, raw, rawlen, cipher, inlen);
  if (rc)
    goto leave;

  /* cipher = t xor in */
  for (i = 0; i < inlen; i++)
    cipher[i] ^= in[i];

  /* hash(x2 || IN || y2) */
  mdlen = _gcry_md_get_algo_dlen (algo);
  rc = _gcry_md_open (&md, algo, 0);
  if (rc)
    goto leave;
  _gcry_md_write (md, raw, MPI_NBYTES(x2));
  _gcry_md_write (md, in, inlen);
  _gcry_md_write (md, raw + MPI_NBYTES(x2), MPI_NBYTES(y2));
  dgst = _gcry_md_read (md, algo);
  if (dgst == NULL)
    {
      rc = GPG_ERR_DIGEST_ALGO;
      goto leave;
    }

  if (!rc)
    {
      gcry_mpi_t c1;
      gcry_mpi_t c3;
      gcry_mpi_t c2;

      c3 = mpi_new (0);
      c2 = mpi_new (0);

      c1 = _gcry_ecc_ec2os (x1, y1, ec->p);
      _gcry_mpi_set_opaque_copy (c3, dgst, mdlen * 8);
      _gcry_mpi_set_opaque_copy (c2, cipher, inlen * 8);

      rc = sexp_build (r_ciph, NULL,
                       "(enc-val(flags sm2)(sm2(a%M)(b%M)(c%M)))",
                       c1, c3, c2);

      mpi_free (c1);
      mpi_free (c3);
      mpi_free (c2);
    }

leave:
  _gcry_md_close (md);
  mpi_free (x2y2);
  mpi_free (k);

  point_free (&kG);
  point_free (&kP);
  mpi_free (x1);
  mpi_free (y1);
  mpi_free (x2);
  mpi_free (y2);

  xfree (cipher);
  xfree (in);

  return rc;
}


gpg_err_code_t
_gcry_ecc_sm2_decrypt (gcry_sexp_t *r_plain, gcry_sexp_t data_list, mpi_ec_t ec)
{
  gpg_err_code_t rc;
  gcry_mpi_t data_c1 = NULL;
  gcry_mpi_t data_c3 = NULL;
  gcry_mpi_t data_c2 = NULL;

  /*
   * Extract the data.
   */
  rc = sexp_extract_param (data_list, NULL, "/a/b/c",
                           &data_c1, &data_c3, &data_c2, NULL);
  if (rc)
    goto leave;
  if (DBG_CIPHER)
    {
      log_printmpi ("ecc_decrypt  d_c1", data_c1);
      log_printmpi ("ecc_decrypt  d_c3", data_c3);
      log_printmpi ("ecc_decrypt  d_c2", data_c2);
    }

  {
    const int algo = GCRY_MD_SM3;
    gcry_md_hd_t md = NULL;
    int mdlen;
    unsigned char *dgst;
    mpi_point_struct c1;
    mpi_point_struct kP;
    gcry_mpi_t x2, y2;
    gcry_mpi_t x2y2 = NULL;
    unsigned char *in = NULL;
    unsigned int inlen;
    unsigned char *plain = NULL;
    unsigned char *raw;
    unsigned int rawlen;
    unsigned char *c3 = NULL;
    unsigned int c3_len;
    int i;

    point_init (&c1);
    point_init (&kP);
    x2 = mpi_new (0);
    y2 = mpi_new (0);

    in = mpi_get_opaque (data_c2, &inlen);
    inlen = (inlen + 7) / 8;
    plain = xtrymalloc (inlen);
    if (!plain)
      {
        rc = gpg_err_code_from_syserror ();
        goto leave_main;
      }

    rc = _gcry_ecc_sec_decodepoint (data_c1, ec, &c1);
    if (rc)
      goto leave_main;

    if (!_gcry_mpi_ec_curve_point (&c1, ec))
      {
        rc = GPG_ERR_INV_DATA;
        goto leave_main;
      }

    /* [d]C1 = (x2, y2), C1 = [k]G */
    _gcry_mpi_ec_mul_point (&kP, ec->d, &c1, ec);
    if (_gcry_mpi_ec_get_affine (x2, y2, &kP, ec))
      {
        rc = GPG_ERR_INV_DATA;
        goto leave_main;
      }

    /* t = KDF(x2 || y2, inlen) */
    x2y2 = _gcry_mpi_ec_ec2os (&kP, ec);
    raw = mpi_get_opaque (x2y2, &rawlen);
    rawlen = (rawlen + 7) / 8;
    /* skip the prefix '0x04' */
    raw += 1;
    rawlen -= 1;
    rc = kdf_x9_63 (algo, raw, rawlen, plain, inlen);
    if (rc)
      goto leave_main;

    /* plain = C2 xor t */
    for (i = 0; i < inlen; i++)
      plain[i] ^= in[i];

    /* Hash(x2 || IN || y2) == C3 */
    mdlen = _gcry_md_get_algo_dlen (algo);
    rc = _gcry_md_open (&md, algo, 0);
    if (rc)
      goto leave_main;
    _gcry_md_write (md, raw, MPI_NBYTES(x2));
    _gcry_md_write (md, plain, inlen);
    _gcry_md_write (md, raw + MPI_NBYTES(x2), MPI_NBYTES(y2));
    dgst = _gcry_md_read (md, algo);
    if (dgst == NULL)
      {
        memset (plain, 0, inlen);
        rc = GPG_ERR_DIGEST_ALGO;
        goto leave_main;
      }
    c3 = mpi_get_opaque (data_c3, &c3_len);
    c3_len = (c3_len + 7) / 8;
    if (c3_len != mdlen || memcmp (dgst, c3, c3_len) != 0)
      {
        memset (plain, 0, inlen);
        rc = GPG_ERR_INV_DATA;
        goto leave_main;
      }

    if (!rc)
      {
        gcry_mpi_t r;

        r = mpi_new (inlen * 8);
        _gcry_mpi_set_buffer (r, plain, inlen, 0);

        rc = sexp_build (r_plain, NULL, "(value %m)", r);

        mpi_free (r);
      }

  leave_main:
    _gcry_md_close (md);
    mpi_free (x2y2);
    xfree (plain);

    point_free (&c1);
    point_free (&kP);
    mpi_free (x2);
    mpi_free (y2);
  }

 leave:
  _gcry_mpi_release (data_c1);
  _gcry_mpi_release (data_c3);
  _gcry_mpi_release (data_c2);

  return rc;
}


/* Compute an SM2 signature.
 * Return the signature struct (r,s) from the message hash.  The caller
 * must have allocated R and S.
 */
gpg_err_code_t
_gcry_ecc_sm2_sign (gcry_mpi_t input, mpi_ec_t ec,
                    gcry_mpi_t r, gcry_mpi_t s,
                    int flags, int hashalgo)
{
  gpg_err_code_t rc = 0;
  int extraloops = 0;
  gcry_mpi_t hash;
  const void *abuf;
  unsigned int abits, qbits;
  gcry_mpi_t tmp = NULL;
  gcry_mpi_t k = NULL;
  gcry_mpi_t rk = NULL;
  mpi_point_struct kG;
  gcry_mpi_t x1;

  if (DBG_CIPHER)
    log_mpidump ("sm2 sign hash  ", input);

  qbits = mpi_get_nbits (ec->n);

  /* Convert the INPUT into an MPI if needed.  */
  rc = _gcry_dsa_normalize_hash (input, &hash, qbits);
  if (rc)
    return rc;

  point_init (&kG);
  x1 = mpi_new (0);
  rk = mpi_new (0);
  tmp = mpi_new (0);

  for (;;)
    {
      /* rand k in [1, n-1] */
      if ((flags & PUBKEY_FLAG_RFC6979) && hashalgo)
        {
          /* Use Pornin's method for deterministic DSA.  If this
             flag is set, it is expected that HASH is an opaque
             MPI with the to be signed hash.  That hash is also
             used as h1 from 3.2.a.  */
          if (!mpi_is_opaque (input))
            {
              rc = GPG_ERR_CONFLICT;
              goto leave;
            }

          abuf = mpi_get_opaque (input, &abits);
          rc = _gcry_dsa_gen_rfc6979_k (&k, ec->n, ec->d,
                                        abuf, (abits+7)/8,
                                        hashalgo, extraloops);
          if (rc)
            goto leave;
          extraloops++;
        }
      else
        k = _gcry_dsa_gen_k (ec->n, GCRY_VERY_STRONG_RANDOM);

      _gcry_dsa_modify_k (k, ec->n, qbits);

      /* [k]G = (x1, y1) */
      _gcry_mpi_ec_mul_point (&kG, k, ec->G, ec);
      if (_gcry_mpi_ec_get_affine (x1, NULL, &kG, ec))
        {
          rc = GPG_ERR_INV_DATA;
          goto leave;
        }

      /* r = (e + x1) % n */
      mpi_addm (r, hash, x1, ec->n);

      /* r != 0 && r + k != n */
      if (mpi_cmp_ui (r, 0) == 0)
        continue;
      mpi_add (rk, r, k);
      if (mpi_cmp (rk, ec->n) == 0)
        continue;

      /* s = ((d + 1)^-1 * (k - rd)) % n */
      mpi_addm (s, ec->d, GCRYMPI_CONST_ONE, ec->n);
      mpi_invm (s, s, ec->n);
      mpi_mulm (tmp, r, ec->d, ec->n);
      mpi_subm (tmp, k, tmp, ec->n);
      mpi_mulm (s, s, tmp, ec->n);

      /* s != 0 */
      if (mpi_cmp_ui (s, 0) == 0)
        continue;

      break;    /* Okay */
    }

  if (DBG_CIPHER)
    {
      log_mpidump ("sm2 sign result r ", r);
      log_mpidump ("sm2 sign result s ", s);
    }

leave:
  point_free (&kG);
  mpi_free (k);
  mpi_free (x1);
  mpi_free (rk);
  mpi_free (tmp);

  if (hash != input)
    mpi_free (hash);

  return rc;
}


/* Verify an SM2 signature.
 * Check if R and S verifies INPUT.
 */
gpg_err_code_t
_gcry_ecc_sm2_verify (gcry_mpi_t input, mpi_ec_t ec,
                      gcry_mpi_t r, gcry_mpi_t s)
{
  gpg_err_code_t err = 0;
  gcry_mpi_t hash = NULL;
  gcry_mpi_t t = NULL;
  mpi_point_struct sG, tP;
  gcry_mpi_t x1, y1;
  unsigned int nbits;

  if (!_gcry_mpi_ec_curve_point (ec->Q, ec))
    return GPG_ERR_BROKEN_PUBKEY;

  /* r, s within [1, n-1] */
  if (mpi_cmp_ui (r, 1) < 0 || mpi_cmp (r, ec->n) > 0)
    return GPG_ERR_BAD_SIGNATURE;
  if (mpi_cmp_ui (s, 1) < 0 || mpi_cmp (s, ec->n) > 0)
    return GPG_ERR_BAD_SIGNATURE;

  nbits = mpi_get_nbits (ec->n);
  err = _gcry_dsa_normalize_hash (input, &hash, nbits);
  if (err)
    return err;

  point_init (&sG);
  point_init (&tP);
  x1 = mpi_new (0);
  y1 = mpi_new (0);
  t = mpi_new (0);

  /* t = (r + s) % n, t != 0 */
  mpi_addm (t, r, s, ec->n);
  if (mpi_cmp_ui (t, 0) == 0)
    {
      err = GPG_ERR_BAD_SIGNATURE;
      goto leave;
    }

  /* sG + tP = (x1, y1) */
  _gcry_mpi_ec_mul_point (&sG, s, ec->G, ec);
  _gcry_mpi_ec_mul_point (&tP, t, ec->Q, ec);
  _gcry_mpi_ec_add_points (&sG, &sG, &tP, ec);
  if (_gcry_mpi_ec_get_affine (x1, y1, &sG, ec))
    {
      err = GPG_ERR_INV_DATA;
      goto leave;
    }

  /* R = (e + x1) % n */
  mpi_addm (t, hash, x1, ec->n);

  /* check R == r */
  if (mpi_cmp (t, r))
    {
      if (DBG_CIPHER)
        {
          log_mpidump ("     R", t);
          log_mpidump ("     r", r);
          log_mpidump ("     s", s);
        }
      err = GPG_ERR_BAD_SIGNATURE;
      goto leave;
    }
  if (DBG_CIPHER)
    log_debug ("sm2 verify: Accepted\n");

 leave:
  point_free (&sG);
  point_free (&tP);
  mpi_free (x1);
  mpi_free (y1);
  mpi_free (t);
  if (hash != input)
    mpi_free (hash);

  return err;
}