der-iterator.c

/* der-iterator.c
 *
 * Parses DER encoded objects.
 */

/* nettle, low-level cryptographics library
 *
 * Copyright (C) 2005 Niels Möller
 *  
 * The nettle library 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.
 * 
 * The nettle library 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 the nettle library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

#if HAVE_CONFIG_H
# include "config.h"
#endif

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

#if HAVE_LIBGMP
#include "bignum.h"
#endif

#include "asn1.h"

#include "macros.h"

/* Basic DER syntax: (reference: A Layman's Guide to a Subset of ASN.1, BER, and DER,
   http://luca.ntop.org/Teaching/Appunti/asn1.html)

   The DER header contains a tag and a length. First, the tag. cls is
   the class number, c is one if the object is "constructed" and zero
   if it is primitive. The tag is represented either using a single
   byte,

     7 6   5   4 3 2 1 0
    _____________________
   |_cls_|_c_|_______tag_|   0 <= tag <= 30

   or multiple bytes

     7 6   5   4 3 2 1 0
    _____________________
   |_cls_|_c_|_1_1_1_1_1_|

   followed by the real tag number, in base 128, with all but the
   final byte having the most significant bit set. The tag must be
   represented with as few bytes as possible. High tag numbers are
   currently *not* supported.
   
   Next, the length, either a single byte with the most significant bit clear, or

     7 6 5 4 3 2 1 0
    _________________
   |_1_|___________k_|

   followed by k additional bytes that give the length, in network
   byte order. The length must be encoded using as few bytes as
   possible, and k = 0 is reserved for the "indefinite length form"
   which is not supported.

   After the length comes the contets. For primitive objects (c == 0),
   it's depends on the type. For constructed objects, it's a
   concatenation of the DER encodings of zero or more other objects.
*/

enum {
  TAG_MASK = 0x1f,
  CLASS_MASK = 0xc0,
  CONSTRUCTED_MASK = 0x20,
};

/* Initializes the iterator, but one has to call next to get to the
 * first element. */
static void
asn1_der_iterator_init(struct asn1_der_iterator *iterator,
		       unsigned length, const uint8_t *input)
{
  iterator->buffer_length = length;
  iterator->buffer = input;
  iterator->pos = 0;
  iterator->type = 0;
  iterator->length = 0;
  iterator->data = NULL;
}

#define LEFT(i) ((i)->buffer_length - (i)->pos)
#define NEXT(i) ((i)->buffer[(i)->pos++])

/* Gets type and length of the next object. */
enum asn1_iterator_result
asn1_der_iterator_next(struct asn1_der_iterator *i)
{
  uint8_t tag;
  
  if (!LEFT(i))
    return ASN1_ITERATOR_END;

  tag = NEXT(i);
  if (!LEFT(i))
    return ASN1_ITERATOR_ERROR;

  if ( (tag & TAG_MASK) == TAG_MASK)
    {
      /* FIXME: Long tags not supported */
      return ASN1_ITERATOR_ERROR;
    }

  i->length = NEXT(i);
  if (i->length & 0x80)
    {
      unsigned k = i->length & 0x7f;
      unsigned j;
      const uint8_t *data = i->buffer + i->pos;
      
      if (k == 0)
	/* Indefinite encoding. Not supported. */
	return ASN1_ITERATOR_ERROR;

      if (LEFT(i) < k)
	return ASN1_ITERATOR_ERROR;

      if (k > sizeof(unsigned))
	return ASN1_ITERATOR_ERROR;

      i->pos += k;
      i->length = data[0];
      if (i->length == 0
	  || (k == 1 && i->length < 0x80))
	return ASN1_ITERATOR_ERROR;

      for (j = 1; j < k; j++)
	i->length = (i->length << 8) | data[j];
    }
  if (LEFT(i) < i->length)
    return ASN1_ITERATOR_ERROR;

  i->data = i->buffer + i->pos;
  i->pos += i->length;

  i->type = tag & TAG_MASK;
  i->type |= (tag & CLASS_MASK) << (ASN1_CLASS_SHIFT - 6);
  if (tag & CONSTRUCTED_MASK)
    {
      i->type |= ASN1_TYPE_CONSTRUCTED;
      return ASN1_ITERATOR_CONSTRUCTED;
    }
  else
    return ASN1_ITERATOR_PRIMITIVE;
}

enum asn1_iterator_result
asn1_der_iterator_first(struct asn1_der_iterator *i,
			unsigned length, const uint8_t *input)
{
  asn1_der_iterator_init(i, length, input);
  return asn1_der_iterator_next(i);
}

enum asn1_iterator_result
asn1_der_decode_constructed(struct asn1_der_iterator *i,
			    struct asn1_der_iterator *contents)
{
  assert(i->type & ASN1_TYPE_CONSTRUCTED);
  return asn1_der_iterator_first(contents, i->length, i->data);
}

enum asn1_iterator_result
asn1_der_decode_constructed_last(struct asn1_der_iterator *i)
{
  if (LEFT(i) > 0)
    return ASN1_ITERATOR_ERROR;

  return asn1_der_decode_constructed(i, i);
}

/* Decoding a DER object which is wrapped in a bit string. */
enum asn1_iterator_result
asn1_der_decode_bitstring(struct asn1_der_iterator *i,
			  struct asn1_der_iterator *contents)
{
  assert(i->type == ASN1_BITSTRING);
  /* First byte is the number of padding bits, which must be zero. */
  if (i->length == 0  || i->data[0] != 0)
    return ASN1_ITERATOR_ERROR;

  return asn1_der_iterator_first(contents, i->length - 1, i->data + 1);
}

enum asn1_iterator_result
asn1_der_decode_bitstring_last(struct asn1_der_iterator *i)
{
  if (LEFT(i) > 0)
    return ASN1_ITERATOR_ERROR;

  return asn1_der_decode_bitstring(i, i);
}

int
asn1_der_get_uint32(struct asn1_der_iterator *i,
		    uint32_t *x)
{
  /* Big endian, two's complement, minimum number of octets (except 0,
     which is encoded as a single octet */
  uint32_t value = 0;
  unsigned length = i->length;
  unsigned k;

  if (!length || length > 5)
    return 0;

  if (i->data[length - 1] >= 0x80)
    /* Signed number */
    return 0;

  if (length > 1
      && i->data[length -1] == 0
      && i->data[length -2] < 0x80)
    /* Non-minimal number of digits */
    return 0;

  if (length == 5)
    {
      if (i->data[4])
	return 0;
      length--;
    }

  for (value = k = 0; k < length; k++)
    value = (value << 8) | i->data[k];

  *x = value;
  return 1;
}

#if HAVE_LIBGMP
int
asn1_der_get_bignum(struct asn1_der_iterator *i,
		    mpz_t x, unsigned max_bits)
{
  if (i->length > 1
      && ((i->data[0] == 0 && i->data[1] < 0x80)
	  || (i->data[0] == 0xff && i->data[1] >= 0x80)))
    /* Non-minimal number of digits */
    return 0;

  /* Allow some extra here, for leading sign octets. */
  if (max_bits && (8 * i->length > (16 + max_bits)))
    return 0;

  nettle_mpz_set_str_256_s(x, i->length, i->data);

  /* FIXME: How to interpret a max_bits for negative numbers? */
  if (max_bits && mpz_sizeinbase(x, 2) > max_bits)
    return 0;

  return 1;
}
#endif /* HAVE_LIBGMP */