coap_asn1.c

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/* coap_asn1.c -- ASN.1 handling functions
*
* Copyright (C) 2020-2023 Jon Shallow <supjps-libcoap@jpshallow.com>
*
 * SPDX-License-Identifier: BSD-2-Clause
 *
* This file is part of the CoAP library libcoap. Please see
* README for terms of use.
*/
 
#include "coap3/coap_internal.h"
 
size_t
asn1_len(const uint8_t **ptr) {
  size_t len = 0;
 
  if ((**ptr) & 0x80) {
    size_t octets = (**ptr) & 0x7f;
    (*ptr)++;
    while (octets) {
      len = (len << 8) + (**ptr);
      (*ptr)++;
      octets--;
    }
  } else {
    len = (**ptr) & 0x7f;
    (*ptr)++;
  }
  return len;
}
 
coap_asn1_tag_t
asn1_tag_c(const uint8_t **ptr, int *constructed, int *cls) {
  coap_asn1_tag_t tag = 0;
  uint8_t byte;
 
  byte = (**ptr);
  *constructed = (byte & 0x20) ? 1 : 0;
  *cls = byte >> 6;
  tag = byte & 0x1F;
  (*ptr)++;
  if (tag < 0x1F)
    return tag;
 
  /* Tag can be one byte or more based on B8 */
  byte = (**ptr);
  while (byte & 0x80) {
    tag = (tag << 7) + (byte & 0x7F);
    (*ptr)++;
    byte = (**ptr);
  }
  /* Do the final one */
  tag = (tag << 7) + (byte & 0x7F);
  (*ptr)++;
  return tag;
}
 
/* caller must free off returned coap_binary_t* */
coap_binary_t *
get_asn1_tag(coap_asn1_tag_t ltag, const uint8_t *ptr, size_t tlen,
             asn1_validate validate) {
  int constructed;
  int class;
  const uint8_t *acp = ptr;
  uint8_t tag = asn1_tag_c(&acp, &constructed, &class);
  size_t len = asn1_len(&acp);
  coap_binary_t *tag_data;
 
  while (tlen > 0 && len <= tlen) {
    if (class == 2 && constructed == 1) {
      /* Skip over element description */
      tag = asn1_tag_c(&acp, &constructed, &class);
      len = asn1_len(&acp);
    }
    if (tag == ltag) {
      if (!validate || validate(acp, len)) {
        tag_data = coap_new_binary(len);
        if (tag_data == NULL)
          return NULL;
        tag_data->length = len;
        memcpy(tag_data->s, acp, len);
        return tag_data;
      }
    }
    if (tag == 0x10 && constructed == 1) {
      /* SEQUENCE or SEQUENCE OF */
      tag_data = get_asn1_tag(ltag, acp, len, validate);
      if (tag_data)
        return tag_data;
    }
    acp += len;
    tlen -= len;
    tag = asn1_tag_c(&acp, &constructed, &class);
    len = asn1_len(&acp);
  }
  return NULL;
}