gnupg/scd/tlv.c

/* tlv.c - Tag-Length-Value Utilities
 *	Copyright (C) 2003, 2004 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 2 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, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
 */

#include <config.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>

#include <gpg-error.h>

#include "tlv.h"

static const unsigned char *
do_find_tlv (const unsigned char *buffer, size_t length,
             int tag, size_t *nbytes, int nestlevel)
{
  const unsigned char *s = buffer;
  size_t n = length;
  size_t len;
  int this_tag;
  int composite;
    
  for (;;)
    {
      buffer = s;
      if (n < 2)
        return NULL; /* Buffer definitely too short for tag and length. */
      if (!*s || *s == 0xff)
        { /* Skip optional filler between TLV objects. */
          s++;
          n--;
          continue;
        }
      composite = !!(*s & 0x20);
      if ((*s & 0x1f) == 0x1f)
        { /* more tag bytes to follow */
          s++;
          n--;
          if (n < 2)
            return NULL; /* buffer definitely too short for tag and length. */
          if ((*s & 0x1f) == 0x1f)
            return NULL; /* We support only up to 2 bytes. */
          this_tag = (s[-1] << 8) | (s[0] & 0x7f);
        }
      else
        this_tag = s[0];
      len = s[1];
      s += 2; n -= 2;
      if (len < 0x80)
        ;
      else if (len == 0x81)
        { /* One byte length follows. */
          if (!n)
            return NULL; /* we expected 1 more bytes with the length. */
          len = s[0];
          s++; n--;
        }
      else if (len == 0x82)
        { /* Two byte length follows. */
          if (n < 2)
            return NULL; /* We expected 2 more bytes with the length. */
          len = (s[0] << 8) | s[1];
          s += 2; n -= 2;
        }
      else
        return NULL; /* APDU limit is 65535, thus it does not make
                        sense to assume longer length fields. */

      if (composite && nestlevel < 100)
        { /* Dive into this composite DO after checking for a too deep
             nesting. */
          const unsigned char *tmp_s;
          size_t tmp_len;
          
          tmp_s = do_find_tlv (s, len, tag, &tmp_len, nestlevel+1);
          if (tmp_s)
            {
              *nbytes = tmp_len;
              return tmp_s;
            }
        }

      if (this_tag == tag)
        {
          *nbytes = len;
          return s;
        }
      if (len > n)
        return NULL; /* Buffer too short to skip to the next tag. */
      s += len; n -= len;
    }
}


/* Locate a TLV encoded data object in BUFFER of LENGTH and
   return a pointer to value as well as its length in NBYTES.  Return
   NULL if it was not found.  Note, that the function does not check
   whether the value fits into the provided buffer. */
const unsigned char *
find_tlv (const unsigned char *buffer, size_t length,
          int tag, size_t *nbytes)
{
  return do_find_tlv (buffer, length, tag, nbytes, 0);
}




/* ASN.1 BER parser: Parse BUFFER of length SIZE and return the tag
   and the length part from the TLV triplet.  Update BUFFER and SIZE
   on success. */
gpg_error_t
parse_ber_header (unsigned char const **buffer, size_t *size,
                  int *r_class, int *r_tag, 
                  int *r_constructed, int *r_ndef,
                  size_t *r_length, size_t *r_nhdr)
{
  int c;
  unsigned long tag;
  const unsigned char *buf = *buffer;
  size_t length = *size;

  *r_ndef = 0;
  *r_length = 0;
  *r_nhdr = 0;

  /* Get the tag. */
  if (!length)
    return gpg_error (GPG_ERR_EOF);
  c = *buf++; length--; ++*r_nhdr;

  *r_class = (c & 0xc0) >> 6;
  *r_constructed = !!(c & 0x20);
  tag = c & 0x1f;

  if (tag == 0x1f)
    {
      tag = 0;
      do
        {
          /* Simple check against overflow.  We limit our maximim tag
             value more than needed but that should not be a problem
             because I have nver encountered such large value.  We
             assume at least 32 bit integers. */
          if (tag > (1 << 24))
            return gpg_error (GPG_ERR_TOO_LARGE);
          tag <<= 7;
          if (!length)
            return gpg_error (GPG_ERR_EOF);
          c = *buf++; length--; ++*r_nhdr;
          tag |= c & 0x7f;

        }
      while (c & 0x80);
    }
  *r_tag = tag;

  /* Get the length. */
  if (!length)
    return gpg_error (GPG_ERR_EOF);
  c = *buf++; length--; ++*r_nhdr;

  if ( !(c & 0x80) )
    *r_length = c;
  else if (c == 0x80)
    *r_ndef = 1;
  else if (c == 0xff)
    return gpg_error (GPG_ERR_BAD_BER);
  else
    {
      unsigned long len = 0;
      int count = c & 0x7f;

      for (; count; count--)
        {
          /* Simple check against overflow.  We limit our maximim
             length more than needed but that should not be a problem
             because I have never encountered such large value and
             well they are managed in memory and thus we would run
             into memory problems anyway.  We assume at least 32 bit
             integers. */
          if (len > (1 << 24))
            return gpg_error (GPG_ERR_TOO_LARGE);
          len <<= 8;
          if (!length)
            return gpg_error (GPG_ERR_EOF);
          c = *buf++; length--; ++*r_nhdr;
          len |= c & 0xff;
        }
      *r_length = len;
    }
  
  /* Without this kludge some example certs can't be parsed. */
  if (*r_class == CLASS_UNIVERSAL && !*r_tag)
    *r_length = 0;
  
  *buffer = buf;
  *size = length;
  return 0;
}