/* decrypt-data.c - Decrypt an encrypted data packet

 * Copyright (C) 1998-2001, 2005-2006, 2009 Free Software Foundation, Inc.

 * Copyright (C) 1998-2001, 2005-2006, 2009, 2018 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/>.

 * SPDX-License-Identifier: GPL-3.0-or-later

 */

#include <config.h>

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include "gpg.h"

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

#include "packet.h"

#include "options.h"

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

#include "../common/status.h"

#include "../common/compliance.h"

static int aead_decode_filter (void *opaque, int control, iobuf_t a,

                               byte *buf, size_t *ret_len);

static int mdc_decode_filter ( void *opaque, int control, IOBUF a,

                               byte *buf, size_t *ret_len);

static int decode_filter ( void *opaque, int control, IOBUF a,

          byte *buf, size_t *ret_len);

/* Our context object.  */

struct decode_filter_context_s

{

  /* Redcounter (max value is 2).  We need it because we do not know

   * whether the iobuf or the outer control code frees this object

   * first.  */

  int  refcount;

  /* The cipher handle.  */

  gcry_cipher_hd_t cipher_hd;

  /* The hash handle for use in MDC mode.  */

  gcry_md_hd_t mdc_hash;

  /* The start IV for AEAD encryption.   */

  byte startiv[16];

  /* The holdback buffer and its used length.  For AEAD we need 32+1

   * bytes but we use 48 byte.  For MDC we need 22 bytes; here

   * holdbacklen will either 0 or 22.  */

  char holdback[48];

  unsigned int holdbacklen;

  /* Working on a partial length packet.  */

  unsigned int partial : 1;

  /* EOF indicator with these true values:

   *   1 = normal EOF

   *   2 = premature EOF (tag or hash incomplete)

   *   3 = premature EOF (general)       */

  unsigned int eof_seen : 2;

  /* Flag to convey an error from aead_checktag.  */

  unsigned int checktag_failed : 1;

  /* The actually used cipher algo for AEAD.  */

  byte cipher_algo;

  /* The AEAD algo.  */

  byte aead_algo;

  /* The encoded chunk byte for AEAD.  */

  byte chunkbyte;

  /* The decoded CHUNKBYTE.  */

  uint64_t chunksize;

  /* The chunk index for AEAD.  */

  uint64_t chunkindex;

  /* The number of bytes in the current chunk.  */

  uint64_t chunklen;

  /* The total count of decrypted plaintext octets.  */

  uint64_t total;

  /* Remaining bytes in the packet according to the packet header.

   * Not used if PARTIAL is true.  */

  size_t length;

};

typedef struct decode_filter_context_s *decode_filter_ctx_t;

/* Helper to release the decode context.  */

static void

release_dfx_context (decode_filter_ctx_t dfx)

{

  if (!dfx)

    return;

  log_assert (dfx->refcount);

  if ( !--dfx->refcount )

    {

      gcry_cipher_close (dfx->cipher_hd);

      dfx->cipher_hd = NULL;

      gcry_md_close (dfx->mdc_hash);

      dfx->mdc_hash = NULL;

      xfree (dfx);

    }

}

/* Set the nonce and the additional data for the current chunk.  This

 * also reset the decryption machinery so that the handle can be

 * used for a new chunk.  If FINAL is set the final AEAD chunk is

 * processed.  */

static gpg_error_t

aead_set_nonce_and_ad (decode_filter_ctx_t dfx, int final)

{

  gpg_error_t err;

  unsigned char ad[21];

  unsigned char nonce[16];

  int i;

  switch (dfx->aead_algo)

    {

    case AEAD_ALGO_OCB:

      memcpy (nonce, dfx->startiv, 15);

      i = 7;

      break;

    case AEAD_ALGO_EAX:

      memcpy (nonce, dfx->startiv, 16);

      i = 8;

      break;

    default:

      BUG ();

    }

  nonce[i++] ^= dfx->chunkindex >> 56;

  nonce[i++] ^= dfx->chunkindex >> 48;

  nonce[i++] ^= dfx->chunkindex >> 40;

  nonce[i++] ^= dfx->chunkindex >> 32;

  nonce[i++] ^= dfx->chunkindex >> 24;

  nonce[i++] ^= dfx->chunkindex >> 16;

  nonce[i++] ^= dfx->chunkindex >>  8;

  nonce[i++] ^= dfx->chunkindex;

  if (DBG_CRYPTO)

    log_printhex (nonce, i, "nonce:");

  err = gcry_cipher_setiv (dfx->cipher_hd, nonce, i);

  if (err)

    return err;

  ad[0] = (0xc0 | PKT_ENCRYPTED_AEAD);

  ad[1] = 1;

  ad[2] = dfx->cipher_algo;

  ad[3] = dfx->aead_algo;

  ad[4] = dfx->chunkbyte;

  ad[5] = dfx->chunkindex >> 56;

  ad[6] = dfx->chunkindex >> 48;

  ad[7] = dfx->chunkindex >> 40;

  ad[8] = dfx->chunkindex >> 32;

  ad[9] = dfx->chunkindex >> 24;

  ad[10]= dfx->chunkindex >> 16;

  ad[11]= dfx->chunkindex >>  8;

  ad[12]= dfx->chunkindex;

  if (final)

    {

      ad[13] = dfx->total >> 56;

      ad[14] = dfx->total >> 48;

      ad[15] = dfx->total >> 40;

      ad[16] = dfx->total >> 32;

      ad[17] = dfx->total >> 24;

      ad[18] = dfx->total >> 16;

      ad[19] = dfx->total >>  8;

      ad[20] = dfx->total;

    }

  if (DBG_CRYPTO)

    log_printhex (ad, final? 21 : 13, "authdata:");

  return gcry_cipher_authenticate (dfx->cipher_hd, ad, final? 21 : 13);

}

/* Helper to check the 16 byte tag in TAGBUF.  The FINAL flag is only

 * for debug messages.  */

static gpg_error_t

aead_checktag (decode_filter_ctx_t dfx, int final, const void *tagbuf)

{

  gpg_error_t err;

  if (DBG_FILTER)

    log_printhex (tagbuf, 16, "tag:");

  err = gcry_cipher_checktag (dfx->cipher_hd, tagbuf, 16);

  if (err)

    {

      log_error ("gcry_cipher_checktag%s failed: %s\n",

                 final? " (final)":"", gpg_strerror (err));

      write_status_error ("aead_checktag", err);

      dfx->checktag_failed = 1;

      return err;

    }

  if (DBG_FILTER)

    log_debug ("%stag is valid\n", final?"final ":"");

  return 0;

}

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

 * Decrypt the data, specified by ED with the key DEK.  On return

 * COMPLIANCE_ERROR is set to true iff the decryption can claim that

 * it was compliant in the current mode; otherwise this flag is set to

 * false.

 */

int

decrypt_data (ctrl_t ctrl, void *procctx, PKT_encrypted *ed, DEK *dek,

              int *compliance_error)

{

  decode_filter_ctx_t dfx;

  enum gcry_cipher_modes ciphermode;

  unsigned int startivlen;

  byte *p;

  int rc=0, c, i;

  byte temp[32];

  unsigned int blocksize;

  unsigned int nprefix;

  *compliance_error = 0;

  dfx = xtrycalloc (1, sizeof *dfx);

  if (!dfx)

    return gpg_error_from_syserror ();

  dfx->refcount = 1;

  if ( opt.verbose && !dek->algo_info_printed )

    {

      if (!openpgp_cipher_test_algo (dek->algo))

        log_info (_("%s encrypted data\n"),

                  openpgp_cipher_algo_mode_name (dek->algo, ed->aead_algo));

      else

        log_info (_("encrypted with unknown algorithm %d\n"), dek->algo );

      dek->algo_info_printed = 1;

    }

  if (ed->aead_algo)

    {

      rc = openpgp_aead_algo_info (ed->aead_algo, &ciphermode, &startivlen);

      if (rc)

        goto leave;

      log_assert (startivlen <= sizeof dfx->startiv);

    }

  else

    ciphermode = GCRY_CIPHER_MODE_CFB;

  /* Check compliance.  */

  if (!gnupg_cipher_is_allowed (opt.compliance, 0, dek->algo, ciphermode))

    {

      gpgrt_log (opt.show_only_session_key? GPGRT_LOGLVL_INFO

                 /*                     */: GPGRT_LOGLVL_ERROR,

                 _("cipher algorithm '%s' may not be used in %s mode\n"),

     openpgp_cipher_algo_mode_name (dek->algo,ed->aead_algo),

     gnupg_compliance_option_string (opt.compliance));

      *compliance_error = 1;

      if (opt.flags.require_compliance && !opt.show_only_session_key)

        {

          /* We fail early in this case because it does not make sense

           * to first decrypt everything.  */

          rc = gpg_error (GPG_ERR_CIPHER_ALGO);

          goto leave;

        }

    }

  write_status_printf (STATUS_DECRYPTION_INFO, "%d %d %d %d",

                       ed->mdc_method, dek->algo, ed->aead_algo,

                       *compliance_error);

  if (opt.show_session_key)

    {

      char numbuf[30];

      char *hexbuf;

      if (ed->aead_algo)

        snprintf (numbuf, sizeof numbuf, "%d.%u:", dek->algo, ed->aead_algo);

      else

        snprintf (numbuf, sizeof numbuf, "%d:", dek->algo);

      hexbuf = bin2hex (dek->key, dek->keylen, NULL);

      if (!hexbuf)

        {

          rc = gpg_error_from_syserror ();

          goto leave;

        }

      log_info ("session key: '%s%s'\n", numbuf, hexbuf);

      write_status_strings (STATUS_SESSION_KEY, numbuf, hexbuf, NULL);

      xfree (hexbuf);

    }

  if (opt.show_session_hash)

    {

      char hashbuf[32];  /* For SHA-256.  */

      char *rad64buf;

      gcry_md_hash_buffer (GCRY_MD_SHA256, hashbuf, dek->key, dek->keylen);

      rad64buf = make_radix64_string (hashbuf, sizeof hashbuf);

      write_status_text (STATUS_SESSION_HASH, rad64buf);

      xfree (rad64buf);

    }

  if (opt.show_only_session_key)

    {

      rc = 0;

      goto leave;

    }

  rc = openpgp_cipher_test_algo (dek->algo);

  if (rc)

    goto leave;

  blocksize = openpgp_cipher_get_algo_blklen (dek->algo);

  if ( !blocksize || blocksize > 16 )

    log_fatal ("unsupported blocksize %u\n", blocksize );

  if (ed->aead_algo)

    {

      if (blocksize != 16)

        {

          rc = gpg_error (GPG_ERR_CIPHER_ALGO);

          goto leave;

        }

      if (ed->chunkbyte > 56)

        {

          log_error ("invalid AEAD chunkbyte %u\n", ed->chunkbyte);

          rc = gpg_error (GPG_ERR_INV_PACKET);

          goto leave;

        }

      /* Read the Start-IV. */

      if (ed->len)

        {

          for (i=0; i < startivlen && ed->len; i++, ed->len--)

            {

              if ((c=iobuf_get (ed->buf)) == -1)

                break;

              dfx->startiv[i] = c;

            }

        }

      else

        {

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

            if ( (c=iobuf_get (ed->buf)) == -1 )

              break;

            else

              dfx->startiv[i] = c;

        }

      if (i != startivlen)

        {

          log_error ("Start-IV in AEAD packet too short (%d/%u)\n",

                     i, startivlen);

          rc = gpg_error (GPG_ERR_TOO_SHORT);

          goto leave;

        }

      dfx->cipher_algo = ed->cipher_algo;

      dfx->aead_algo = ed->aead_algo;

      dfx->chunkbyte = ed->chunkbyte;

      dfx->chunksize = (uint64_t)1 << (dfx->chunkbyte + 6);

      if (dek->algo != dfx->cipher_algo)

        log_info ("Note: different cipher algorithms used (%s/%s)\n",

                  openpgp_cipher_algo_name (dek->algo),

                  openpgp_cipher_algo_name (dfx->cipher_algo));

      rc = openpgp_cipher_open (&dfx->cipher_hd,

                                dfx->cipher_algo,

                                ciphermode,

                                GCRY_CIPHER_SECURE);

      if (rc)

        goto leave; /* Should never happen.  */

      if (DBG_CRYPTO)

        log_printhex (dek->key, dek->keylen, "thekey:");

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

      if (gpg_err_code (rc) == GPG_ERR_WEAK_KEY)

        {

          log_info (_("WARNING: message was encrypted with"

                      " a weak key in the symmetric cipher.\n"));

          rc = 0;

        }

      else if (rc)

        {

          log_error("key setup failed: %s\n", gpg_strerror (rc));

          goto leave;

        }

      if (!ed->buf)

        {

          log_error(_("problem handling encrypted packet\n"));

          goto leave;

        }

    }

  else /* CFB encryption.  */

    {

      nprefix = blocksize;

      if ( ed->len && ed->len < (nprefix+2) )

        {

          /* An invalid message.  We can't check that during parsing

           * because we may not know the used cipher then.  */

          rc = gpg_error (GPG_ERR_INV_PACKET);

          goto leave;

        }

      if ( ed->mdc_method )

        {

          if (gcry_md_open (&dfx->mdc_hash, ed->mdc_method, 0 ))

            BUG ();

          if ( DBG_HASHING )

            gcry_md_debug (dfx->mdc_hash, "checkmdc");

        }

      rc = openpgp_cipher_open (&dfx->cipher_hd, dek->algo,

                                GCRY_CIPHER_MODE_CFB,

                                (GCRY_CIPHER_SECURE

                                 | ((ed->mdc_method || dek->algo >= 100)?

                                    0 : GCRY_CIPHER_ENABLE_SYNC)));

      if (rc)

        {

          /* We should never get an error here cause we already checked

           * that the algorithm is available.  */

          BUG();

        }

      /* log_hexdump( "thekey", dek->key, dek->keylen );*/

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

      if ( gpg_err_code (rc) == GPG_ERR_WEAK_KEY )

        {

          log_info (_("WARNING: message was encrypted with"

                      " a weak key in the symmetric cipher.\n"));

          rc=0;

        }

      else if (rc)

        {

          log_error ("key setup failed: %s\n", gpg_strerror (rc) );

          goto leave;

        }

      if (!ed->buf)

        {

          log_error (_("problem handling encrypted packet\n"));

          rc = gpg_error (GPG_ERR_INV_PACKET);

          goto leave;

        }

      gcry_cipher_setiv (dfx->cipher_hd, NULL, 0);

      if ( ed->len )

        {

          for (i=0; i < (nprefix+2) && ed->len; i++, ed->len-- )

            {

              if ( (c=iobuf_get(ed->buf)) == -1 )

                break;

              else

                temp[i] = c;

            }

        }

      else

        {

          for (i=0; i < (nprefix+2); i++ )

            if ( (c=iobuf_get(ed->buf)) == -1 )

              break;

            else

              temp[i] = c;

        }

      gcry_cipher_decrypt (dfx->cipher_hd, temp, nprefix+2, NULL, 0);

      gcry_cipher_sync (dfx->cipher_hd);

      p = temp;

      /* log_hexdump( "prefix", temp, nprefix+2 ); */

      if (dek->symmetric

          && (p[nprefix-2] != p[nprefix] || p[nprefix-1] != p[nprefix+1]) )

        {

          rc = gpg_error (GPG_ERR_BAD_KEY);

          goto leave;

        }

      if ( dfx->mdc_hash )

        gcry_md_write (dfx->mdc_hash, temp, nprefix+2);

    }

  dfx->refcount++;

  dfx->partial = !!ed->is_partial;

  dfx->length = ed->len;

  dfx->checktag_failed = 0;

  if (ed->aead_algo)

    iobuf_push_filter ( ed->buf, aead_decode_filter, dfx );

  else if (ed->mdc_method)

    iobuf_push_filter ( ed->buf, mdc_decode_filter, dfx );

  else

    iobuf_push_filter ( ed->buf, decode_filter, dfx );

  if (opt.unwrap_encryption)

    {

      char *filename = NULL;

      estream_t fp;

      rc = get_output_file ("", 0, ed->buf, &filename, &fp);

      if (! rc)

        {

          iobuf_t output = iobuf_esopen (fp, "w", 0, 0);

          armor_filter_context_t *afx = NULL;

    es_setbuf (fp, NULL);

          if (opt.armor)

            {

              afx = new_armor_context ();

              push_armor_filter (afx, output);

            }

          iobuf_copy (output, ed->buf);

          if ((rc = iobuf_error (ed->buf)))

            log_error (_("error reading '%s': %s\n"),

                       filename, gpg_strerror (rc));

          else if ((rc = iobuf_error (output)))

            log_error (_("error writing '%s': %s\n"),

                       filename, gpg_strerror (rc));

          iobuf_close (output);

          release_armor_context (afx);

        }

      xfree (filename);

    }

  else

    proc_packets (ctrl, procctx, ed->buf );

  ed->buf = NULL;

  if (dfx->eof_seen > 1 )

    rc = gpg_error (GPG_ERR_INV_PACKET);

  else if (dfx->checktag_failed)

    {

      rc = gpg_error (GPG_ERR_BAD_SIGNATURE);

    }

  else if ( ed->mdc_method )

    {

      /* We used to let parse-packet.c handle the MDC packet but this

         turned out to be a problem with compressed packets: With old

         style packets there is no length information available and

         the decompressor uses an implicit end.  However we can't know

         this implicit end beforehand (:-) and thus may feed the

         decompressor with more bytes than actually needed.  It would

         be possible to unread the extra bytes but due to our weird

         iobuf system any unread is non reliable due to filters

         already popped off.  The easy and sane solution is to care

         about the MDC packet only here and never pass it to the

         packet parser.  Fortunatley the OpenPGP spec requires a

         strict format for the MDC packet so that we know that 22

         bytes are appended.  */

      int datalen = gcry_md_get_algo_dlen (ed->mdc_method);

      log_assert (dfx->cipher_hd);

      log_assert (dfx->mdc_hash);

      gcry_cipher_decrypt (dfx->cipher_hd, dfx->holdback, 22, NULL, 0);

      gcry_md_write (dfx->mdc_hash, dfx->holdback, 2);

      gcry_md_final (dfx->mdc_hash);

      if (   dfx->holdback[0] != '\xd3'

          || dfx->holdback[1] != '\x14'

          || datalen != 20

          || memcmp (gcry_md_read (dfx->mdc_hash, 0), dfx->holdback+2, datalen))

        rc = gpg_error (GPG_ERR_BAD_SIGNATURE);

      /* log_printhex("MDC message:", dfx->holdback, 22); */

      /* log_printhex("MDC calc:", gcry_md_read (dfx->mdc_hash,0), datalen); */

    }

 leave:

  release_dfx_context (dfx);

  return rc;

}

/* Fill BUFFER with up to NBYTES-OFFSET from STREAM utilizing

 * information from the context DFX.  Returns the new offset which is

 * the number of bytes read plus the original offset.  On EOF the

 * respective flag in DFX is set. */

static size_t

fill_buffer (decode_filter_ctx_t dfx, iobuf_t stream,

             byte *buffer, size_t nbytes, size_t offset)

{

  size_t nread = offset;

  size_t curr;

  int ret;

  if (dfx->partial)

    {

      while (nread < nbytes)

        {

          curr = nbytes - nread;

          ret = iobuf_read (stream, &buffer[nread], curr);

          if (ret == -1)

            {

              dfx->eof_seen = 1; /* Normal EOF. */

              break;

            }

          nread += ret;

        }

    }

  else

    {

      while (nread < nbytes && dfx->length)

        {

          curr = nbytes - nread;

          if (curr > dfx->length)

            curr = dfx->length;

          ret = iobuf_read (stream, &buffer[nread], curr);

          if (ret == -1)

            {

              dfx->eof_seen = 3; /* Premature EOF. */

              break;

            }

          nread += ret;

          dfx->length -= ret;

        }

      if (!dfx->length)

        dfx->eof_seen = 1; /* Normal EOF.  */

    }

  return nread;

}

/* The core of the AEAD decryption.  This is the underflow function of

 * the aead_decode_filter.  */

static gpg_error_t

aead_underflow (decode_filter_ctx_t dfx, iobuf_t a, byte *buf, size_t *ret_len)

{

  const size_t size = *ret_len; /* The allocated size of BUF.  */

  gpg_error_t err = 0;

  size_t totallen = 0; /* The number of bytes to return on success or EOF.  */

  size_t off = 0;      /* The offset into the buffer.  */

  size_t len;          /* The current number of bytes in BUF+OFF.  */

  log_assert (size > 48); /* Our code requires at least this size.  */

  /* Copy the rest from the last call of this function into BUF.  */

  len = dfx->holdbacklen;

  dfx->holdbacklen = 0;

  memcpy (buf, dfx->holdback, len);

  if (DBG_FILTER)

    log_debug ("aead_underflow: size=%zu len=%zu%s%s\n", size, len,

               dfx->partial? " partial":"", dfx->eof_seen? " eof":"");

  /* Read and fill up BUF.  We need to watch out for an EOF so that we

   * can detect the last chunk which is commonly shorter than the

   * chunksize.  After the last data byte from the last chunk 32 more

   * bytes are expected for the last chunk's tag and the following

   * final chunk's tag.  To detect the EOF we need to try reading at least

   * one further byte; however we try to read 16 extra bytes to avoid

   * single byte reads in some lower layers.  The outcome is that we

   * have up to 48 extra extra octets which we will later put into the

   * holdback buffer for the next invocation (which handles the EOF

   * case).  */

  len = fill_buffer (dfx, a, buf, size, len);

  if (len < 32)

    {

      /* Not enough data for the last two tags.  */

      err = gpg_error (GPG_ERR_TRUNCATED);

      goto leave;

    }

  if (dfx->eof_seen)

    {

      /* If have seen an EOF we copy only the last two auth tags into

       * the holdback buffer.  */

      dfx->holdbacklen = 32;

      memcpy (dfx->holdback, buf+len-32, 32);

      len -= 32;

    }

  else

    {

      /* If have not seen an EOF we copy the entire extra 48 bytes

       * into the holdback buffer for processing at the next call of

       * this function.  */

      dfx->holdbacklen = len > 48? 48 : len;

      memcpy (dfx->holdback, buf+len-dfx->holdbacklen, dfx->holdbacklen);

      len -= dfx->holdbacklen;

    }

  /* log_printhex (dfx->holdback, dfx->holdbacklen, "holdback:"); */

  /* Decrypt the buffer.  This first requires a loop to handle the

   * case when a chunk ends within the buffer.  */

  if (DBG_FILTER)

    log_debug ("decrypt: chunklen=%llu total=%llu size=%zu len=%zu%s\n",

               (unsigned long long)dfx->chunklen,

               (unsigned long long)dfx->total,

               size, len,

               dfx->eof_seen? " eof":"");

  while (len && dfx->chunklen + len >= dfx->chunksize)

    {

      size_t n = dfx->chunksize - dfx->chunklen;

      byte tagbuf[16];

      if (DBG_FILTER)

        log_debug ("chunksize will be reached: n=%zu\n", n);

      if (!dfx->chunklen)

        {

          /* First data for this chunk - prepare.  */

          err = aead_set_nonce_and_ad (dfx, 0);

          if (err)

            goto leave;

        }

      /* log_printhex (buf, n, "ciph:"); */

      gcry_cipher_final (dfx->cipher_hd);

      err = gcry_cipher_decrypt (dfx->cipher_hd, buf+off, n, NULL, 0);

      if (err)

        {

          log_error ("gcry_cipher_decrypt failed (1): %s\n",

                     gpg_strerror (err));

          goto leave;

        }

      /* log_printhex (buf, n, "plai:"); */

      totallen += n;

      dfx->chunklen += n;

      dfx->total += n;

      off += n;

      len -= n;

      if (DBG_FILTER)

        log_debug ("ndecrypted: %zu (nchunk=%llu) bytes left: %zu at off=%zu\n",

                   totallen, (unsigned long long)dfx->chunklen, len, off);

      /* Check the tag.  */

      if (len < 16)

        {

          /* The tag is not entirely in the buffer.  Read the rest of

           * the tag from the holdback buffer.  Then shift the holdback

           * buffer and fill it up again.  */

          memcpy (tagbuf, buf+off, len);

          memcpy (tagbuf + len, dfx->holdback, 16 - len);

          dfx->holdbacklen -= 16-len;

          memmove (dfx->holdback, dfx->holdback + (16-len), dfx->holdbacklen);

          if (dfx->eof_seen)

            {

              /* We should have the last chunk's tag in TAGBUF and the

               * final tag in HOLDBACKBUF.  */

              if (len || dfx->holdbacklen != 16)

                {

                  /* Not enough data for the last two tags.  */

                  err = gpg_error (GPG_ERR_TRUNCATED);

                  goto leave;

                }

            }

          else

            {

              len = 0;

              dfx->holdbacklen = fill_buffer (dfx, a, dfx->holdback, 48,

                                              dfx->holdbacklen);

              if (dfx->holdbacklen < 32)

                {

                  /* Not enough data for the last two tags.  */

                  err = gpg_error (GPG_ERR_TRUNCATED);

                  goto leave;

                }

            }

        }

      else /* We already have the full tag.  */

        {

          memcpy (tagbuf, buf+off, 16);

          /* Remove that tag from the output.  */

          memmove (buf + off, buf + off + 16, len - 16);

          len -= 16;

        }

      err = aead_checktag (dfx, 0, tagbuf);

      if (err)

        goto leave;

      dfx->chunklen = 0;

      dfx->chunkindex++;

      continue;

    }

  /* The bulk decryption of our buffer.  */

  if (len)

    {

      if (!dfx->chunklen)

        {

          /* First data for this chunk - prepare.  */

          err = aead_set_nonce_and_ad (dfx, 0);

          if (err)

            goto leave;

        }

      if (dfx->eof_seen)

        {

          /* This is the last block of the last chunk.  Its length may

           * not be a multiple of the block length.  */

          gcry_cipher_final (dfx->cipher_hd);

        }

      err = gcry_cipher_decrypt (dfx->cipher_hd, buf + off, len, NULL, 0);

      if (err)

        {

          log_error ("gcry_cipher_decrypt failed (2): %s\n",

                     gpg_strerror (err));

          goto leave;

        }

      totallen += len;

      dfx->chunklen += len;

      dfx->total += len;

      if (DBG_FILTER)

        log_debug ("ndecrypted: %zu (nchunk=%llu)\n",

                   totallen, (unsigned long long)dfx->chunklen);

    }

  if (dfx->eof_seen)

    {

      if (dfx->chunklen)

        {

          if (DBG_FILTER)

            log_debug ("eof seen: holdback has the last and final tag\n");

          log_assert (dfx->holdbacklen >= 32);

          err = aead_checktag (dfx, 0, dfx->holdback);

          if (err)

            goto leave;

          dfx->chunklen = 0;

          dfx->chunkindex++;

          off = 16;

        }

      else

        {

          if (DBG_FILTER)

            log_debug ("eof seen: holdback has the final tag\n");

          log_assert (dfx->holdbacklen >= 16);

          off = 0;

        }

      /* Check the final chunk.  */

      err = aead_set_nonce_and_ad (dfx, 1);

      if (err)

        goto leave;

      gcry_cipher_final (dfx->cipher_hd);

      /* Decrypt an empty string (using HOLDBACK as a dummy).  */

      err = gcry_cipher_decrypt (dfx->cipher_hd, dfx->holdback, 0, NULL, 0);

      if (err)

        {

          log_error ("gcry_cipher_decrypt failed (final): %s\n",

                     gpg_strerror (err));

          goto leave;

        }

      err = aead_checktag (dfx, 1, dfx->holdback+off);

      if (err)

        goto leave;

      err = gpg_error (GPG_ERR_EOF);

    }

 leave:

  if (DBG_FILTER)