/* seskey.c -  make session keys etc.

 * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004,

 *               2006, 2009, 2010 Free Software Foundation, Inc.

 *

 * 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 <stdio.h>

#include <stdlib.h>

#include <string.h>

#include "gpg.h"

#include "../common/util.h"

#include "options.h"

#include "main.h"

#include "../common/i18n.h"

/* Generate a new session key in *DEK that is appropriate for the

 * algorithm DEK->ALGO (i.e., ensure that the key is not weak).

 *

 * This function overwrites DEK->KEYLEN, DEK->KEY.  The rest of the

 * fields are left as is.  */

void

make_session_key( DEK *dek )

{

    gcry_cipher_hd_t chd;

    int i, rc;

    dek->keylen = openpgp_cipher_get_algo_keylen (dek->algo);

    if (openpgp_cipher_open (&chd, dek->algo, GCRY_CIPHER_MODE_CFB,

           (GCRY_CIPHER_SECURE

            | (dek->algo >= 100 ?

         0 : GCRY_CIPHER_ENABLE_SYNC))) )

      BUG();

    gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM );

    for (i=0; i < 16; i++ )

      {

  rc = gcry_cipher_setkey (chd, dek->key, dek->keylen);

  if (!rc)

          {

      gcry_cipher_close (chd);

      return;

          }

        if (gpg_err_code (rc) != GPG_ERR_WEAK_KEY)

          BUG();

  log_info(_("weak key created - retrying\n") );

  /* Renew the session key until we get a non-weak key. */

  gcry_randomize (dek->key, dek->keylen, GCRY_STRONG_RANDOM);

      }

    log_fatal (_("cannot avoid weak key for symmetric cipher; "

                 "tried %d times!\n"), i);

}

/* Encode the session key stored in DEK as an MPI in preparation to

 * encrypt it with the public key algorithm OPENPGP_PK_ALGO with a key

 * whose length (the size of the public key) is NBITS.

 *

 * On success, returns an MPI, which the caller must free using

 * gcry_mpi_release().  */

gcry_mpi_t

encode_session_key (int openpgp_pk_algo, DEK *dek, unsigned int nbits)

{

  size_t nframe = (nbits+7) / 8;

  byte *p;

  byte *frame;

  int i,n;

  u16 csum;

  if (DBG_CRYPTO)

    log_debug ("encode_session_key: encoding %d byte DEK", dek->keylen);

  if (openpgp_pk_algo == PUBKEY_ALGO_KYBER)

    {

      /* Straightforward encoding w/o extra checksum as used by ECDH.  */

      nframe = dek->keylen;

      log_assert (nframe > 4);  /*(for the log_debug)*/

      frame = xmalloc_secure (nframe);

      memcpy (frame, dek->key, nframe);

      if (DBG_CRYPTO)

        log_debug ("encode_session_key: "

                   "[%d] %02x  %02x %02x ...  %02x %02x %02x\n",

                   (int) dek->keylen, frame[0], frame[1], frame[2],

                   frame[nframe-3], frame[nframe-2], frame[nframe-1]);

      return gcry_mpi_set_opaque (NULL, frame, 8*nframe);

    }

  csum = 0;

  for (p = dek->key, i=0; i < dek->keylen; i++)

    csum += *p++;

  /* Shortcut for ECDH.  It's padding is minimal to simply make the

     output be a multiple of 8 bytes.  */

  if (openpgp_pk_algo == PUBKEY_ALGO_ECDH)

    {

      /* Pad to 8 byte granularity; the padding byte is the number of

       * padded bytes.

       *

       * A  DEK(k bytes)  CSUM(2 bytes) 0x 0x 0x 0x ... 0x

       *                                +---- x times ---+

       */

      nframe = (( 1 + dek->keylen + 2 /* The value so far is always odd. */

                  + 7 ) & (~7));

      /* alg+key+csum fit and the size is congruent to 8.  */

      log_assert (!(nframe%8) && nframe > 1 + dek->keylen + 2 );

      frame = xmalloc_secure (nframe);

      n = 0;

      frame[n++] = dek->algo;

      memcpy (frame+n, dek->key, dek->keylen);

      n += dek->keylen;

      frame[n++] = csum >> 8;

      frame[n++] = csum;

      i = nframe - n;         /* Number of padded bytes.  */

      memset (frame+n, i, i); /* Use it as the value of each padded byte.  */

      log_assert (n+i == nframe);

      if (DBG_CRYPTO)

        log_debug ("encode_session_key: "

                   "[%d] %02x  %02x %02x ...  %02x %02x %02x\n",

                   (int) nframe, frame[0], frame[1], frame[2],

                   frame[nframe-3], frame[nframe-2], frame[nframe-1]);

      return gcry_mpi_set_opaque (NULL, frame, 8*nframe);

    }

  /* The current limitation is that we can only use a session key

   * whose length is a multiple of BITS_PER_MPI_LIMB

   * I think we can live with that.

   */

  if (dek->keylen + 7 > nframe || !nframe)

    log_bug ("can't encode a %d bit key in a %d bits frame\n",

             dek->keylen*8, nbits );

  /* We encode the session key according to PKCS#1 v1.5 (see section

   * 13.1.1 of RFC 4880):

   *

   *     0  2  RND(i bytes)  0  A  DEK(k bytes)  CSUM(2 bytes)

   *

   * (But how can we store the leading 0 - the external representation

   *  of MPIs doesn't allow leading zeroes =:-)

   *

   * RND are (at least 1) non-zero random bytes.

   * A   is the cipher algorithm

   * DEK is the encryption key (session key) length k depends on the

   *     cipher algorithm (20 is used with blowfish160).

   * CSUM is the 16 bit checksum over the DEK

   */

  frame = xmalloc_secure( nframe );

  n = 0;

  frame[n++] = 0;

  frame[n++] = 2;

  /* The number of random bytes are the number of otherwise unused

     bytes.  See diagram above.  */

  i = nframe - 6 - dek->keylen;

  log_assert( i > 0 );

  p = gcry_random_bytes_secure (i, GCRY_STRONG_RANDOM);

  /* Replace zero bytes by new values.  */

  for (;;)

    {

      int j, k;

      byte *pp;

      /* Count the zero bytes. */

      for (j=k=0; j < i; j++ )

        if (!p[j])

          k++;

      if (!k)

        break; /* Okay: no zero bytes. */

      k += k/128 + 3; /* Better get some more. */

      pp = gcry_random_bytes_secure (k, GCRY_STRONG_RANDOM);

      for (j=0; j < i && k ;)

        {

          if (!p[j])

            p[j] = pp[--k];

          if (p[j])

            j++;

        }

      xfree (pp);

    }

  memcpy (frame+n, p, i);

  xfree (p);

  n += i;

  frame[n++] = 0;

  frame[n++] = dek->algo;

  memcpy (frame+n, dek->key, dek->keylen );

  n += dek->keylen;

  frame[n++] = csum >>8;

  frame[n++] = csum;

  log_assert (n == nframe);

  return gcry_mpi_set_opaque (NULL, frame, 8*n);

}

static gcry_mpi_t

do_encode_md( gcry_md_hd_t md, int algo, size_t len, unsigned nbits,

        const byte *asn, size_t asnlen )

{

    size_t nframe = (nbits+7) / 8;

    byte *frame;

    int i,n;

    gcry_mpi_t a;

    if (len + asnlen + 4  > nframe)

      {

        log_error ("can't encode a %d bit MD into a %d bits frame, algo=%d\n",

                   (int)(len*8), (int)nbits, algo);

        return NULL;

      }

    /* We encode the MD in this way:

     *

     *     0  1 PAD(n bytes)   0  ASN(asnlen bytes)  MD(len bytes)

     *

     * PAD consists of FF bytes.

     */

    frame = gcry_md_is_secure (md)? xmalloc_secure (nframe) : xmalloc (nframe);

    n = 0;

    frame[n++] = 0;

    frame[n++] = 1; /* block type */

    i = nframe - len - asnlen -3 ;

    log_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;

    log_assert( n == nframe );

    if (gcry_mpi_scan( &a, GCRYMPI_FMT_USG, frame, n, &nframe ))

  BUG();

    xfree(frame);

    /* Note that PGP before version 2.3 encoded the MD as:

     *

     *   0   1   MD(16 bytes)   0   PAD(n bytes)   1

     *

     * The MD is always 16 bytes here because it's always MD5.  We do

     * not support pre-v2.3 signatures, but I'm including this comment

     * so the information is easily found in the future.

     */

    return a;

}

/****************

 * Encode a message digest into an MPI.

 * If it's for a DSA signature, make sure that the hash is large

 * enough to fill up q.  If the hash is too big, take the leftmost

 * bits.

 */

gcry_mpi_t

encode_md_value (PKT_public_key *pk, gcry_md_hd_t md, int hash_algo)

{

  gcry_mpi_t frame;

  size_t mdlen;

  log_assert (hash_algo);

  log_assert (pk);

  if (pk->pubkey_algo == PUBKEY_ALGO_EDDSA)

    {

      /* EdDSA signs data of arbitrary length.  Thus no special

         treatment is required.  */

      frame = gcry_mpi_set_opaque_copy (NULL, gcry_md_read (md, hash_algo),

                                        8*gcry_md_get_algo_dlen (hash_algo));

    }

  else if (pk->pubkey_algo == PUBKEY_ALGO_DSA

           || pk->pubkey_algo == PUBKEY_ALGO_ECDSA)

    {

      /* It's a DSA signature, so find out the size of q.  */

      size_t qbits = gcry_mpi_get_nbits (pk->pkey[1]);

      /* pkey[1] is Q for ECDSA, which is an uncompressed point,

         i.e.  04 <x> <y>  */

      if (pk->pubkey_algo == PUBKEY_ALGO_ECDSA)

        qbits = ecdsa_qbits_from_Q (qbits);

      /* Make sure it is a multiple of 8 bits. */

      if ((qbits%8))

  {

    log_error(_("DSA requires the hash length to be a"

          " multiple of 8 bits\n"));

    return NULL;

  }

      /* Don't allow any q smaller than 160 bits.  This might need a

   revisit as the DSA2 design firms up, but for now, 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 %s uses an unsafe (%zu bit) hash\n"),

                     openpgp_pk_algo_name (pk->pubkey_algo),

                     keystr_from_pk (pk), qbits);

    return NULL;

  }

      /* 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 (pk->pubkey_algo == PUBKEY_ALGO_ECDSA && qbits > 512)

        qbits = 512;

      /* Check if we're too short.  Too long is safe as we'll

   automatically left-truncate.  */

      mdlen = gcry_md_get_algo_dlen (hash_algo);

      if (mdlen < qbits/8)

  {

    log_error (_("%s key %s requires a %zu bit or larger hash "

                       "(hash is %s)\n"),

                     openpgp_pk_algo_name (pk->pubkey_algo),

                     keystr_from_pk (pk), qbits,

                     gcry_md_algo_name (hash_algo));

    return NULL;

  }

     /* Note that we do the truncation by passing QBITS/8 as length to

        mpi_scan.  */

      if (gcry_mpi_scan (&frame, GCRYMPI_FMT_USG,

                         gcry_md_read (md, hash_algo), qbits/8, NULL))

        BUG();

    }

  else

    {

      gpg_error_t rc;

      byte *asn;

      size_t asnlen;

      rc = gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, NULL, &asnlen);

      if (rc)

        log_fatal ("can't get OID of digest algorithm %d: %s\n",

                   hash_algo, gpg_strerror (rc));

      asn = xtrymalloc (asnlen);

      if (!asn)

        return NULL;

      if ( gcry_md_algo_info (hash_algo, GCRYCTL_GET_ASNOID, asn, &asnlen) )

        BUG();

      frame = do_encode_md (md, hash_algo, gcry_md_get_algo_dlen (hash_algo),

                            gcry_mpi_get_nbits (pk->pkey[0]), asn, asnlen);

      xfree (asn);

    }

  return frame;

}