library/asn1write.c - third_party/github/ARMmbed/mbedtls - Git at Google

 /*
  * ASN.1 buffer writing functionality
  *
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
  */

 #include "common.h"

 #if defined(MBEDTLS_ASN1_WRITE_C) || defined(MBEDTLS_X509_USE_C) || \
     defined(MBEDTLS_PSA_UTIL_HAVE_ECDSA)

 #include "mbedtls/asn1write.h"
 #include "mbedtls/error.h"

 #include <string.h>

 #include "mbedtls/platform.h"

 #if defined(MBEDTLS_ASN1_PARSE_C)
 #include "mbedtls/asn1.h"
 #endif

 int mbedtls_asn1_write_len(unsigned char **p, const unsigned char *start, size_t len)
 {
 #if SIZE_MAX > 0xFFFFFFFF
     if (len > 0xFFFFFFFF) {
         return MBEDTLS_ERR_ASN1_INVALID_LENGTH;
     }
 #endif

     int required = 1;

     if (len >= 0x80) {
         for (size_t l = len; l != 0; l >>= 8) {
             required++;
         }
     }

     if (required > (*p - start)) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
     }

     do {
         *--(*p) = MBEDTLS_BYTE_0(len);
         len >>= 8;
     } while (len);

     if (required > 1) {
         *--(*p) = (unsigned char) (0x80 + required - 1);
     }

     return required;
 }

 int mbedtls_asn1_write_tag(unsigned char **p, const unsigned char *start, unsigned char tag)
 {
     if (*p - start < 1) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
     }

     *--(*p) = tag;

     return 1;
 }
 #endif /* MBEDTLS_ASN1_WRITE_C || MBEDTLS_X509_USE_C || MBEDTLS_PSA_UTIL_HAVE_ECDSA */

 #if defined(MBEDTLS_ASN1_WRITE_C)
 static int mbedtls_asn1_write_len_and_tag(unsigned char **p,
                                           const unsigned char *start,
                                           size_t len,
                                           unsigned char tag)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_len(p, start, len));
     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_tag(p, start, tag));

     return (int) len;
 }

 int mbedtls_asn1_write_raw_buffer(unsigned char **p, const unsigned char *start,
                                   const unsigned char *buf, size_t size)
 {
     size_t len = 0;

     if (*p < start || (size_t) (*p - start) < size) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
     }

     len = size;
     (*p) -= len;
     memcpy(*p, buf, len);

     return (int) len;
 }

 #if defined(MBEDTLS_BIGNUM_C)
 int mbedtls_asn1_write_mpi(unsigned char **p, const unsigned char *start, const mbedtls_mpi *X)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;

     // Write the MPI
     //
     len = mbedtls_mpi_size(X);

     /* DER represents 0 with a sign bit (0=nonnegative) and 7 value bits, not
      * as 0 digits. We need to end up with 020100, not with 0200. */
     if (len == 0) {
         len = 1;
     }

     if (*p < start || (size_t) (*p - start) < len) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
     }

     (*p) -= len;
     MBEDTLS_MPI_CHK(mbedtls_mpi_write_binary(X, *p, len));

     // DER format assumes 2s complement for numbers, so the leftmost bit
     // should be 0 for positive numbers and 1 for negative numbers.
     //
     if (X->s == 1 && **p & 0x80) {
         if (*p - start < 1) {
             return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
         }

         *--(*p) = 0x00;
         len += 1;
     }

     ret = mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_INTEGER);

 cleanup:
     return ret;
 }
 #endif /* MBEDTLS_BIGNUM_C */

 int mbedtls_asn1_write_null(unsigned char **p, const unsigned char *start)
 {
     // Write NULL
     //
     return mbedtls_asn1_write_len_and_tag(p, start, 0, MBEDTLS_ASN1_NULL);
 }

 int mbedtls_asn1_write_oid(unsigned char **p, const unsigned char *start,
                            const char *oid, size_t oid_len)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;

     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(p, start,
                                                             (const unsigned char *) oid, oid_len));
     return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_OID);
 }

 int mbedtls_asn1_write_algorithm_identifier(unsigned char **p, const unsigned char *start,
                                             const char *oid, size_t oid_len,
                                             size_t par_len)
 {
     return mbedtls_asn1_write_algorithm_identifier_ext(p, start, oid, oid_len, par_len, 1);
 }

 int mbedtls_asn1_write_algorithm_identifier_ext(unsigned char **p, const unsigned char *start,
                                                 const char *oid, size_t oid_len,
                                                 size_t par_len, int has_par)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;

     if (has_par) {
         if (par_len == 0) {
             MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_null(p, start));
         } else {
             len += par_len;
         }
     }

     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_oid(p, start, oid, oid_len));

     return mbedtls_asn1_write_len_and_tag(p, start, len,
                                           MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE);
 }

 int mbedtls_asn1_write_bool(unsigned char **p, const unsigned char *start, int boolean)
 {
     size_t len = 0;

     if (*p - start < 1) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
     }

     *--(*p) = (boolean) ? 255 : 0;
     len++;

     return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_BOOLEAN);
 }

 static int asn1_write_tagged_int(unsigned char **p, const unsigned char *start, int val, int tag)
 {
     size_t len = 0;

     do {
         if (*p - start < 1) {
             return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
         }
         len += 1;
         *--(*p) = val & 0xff;
         val >>= 8;
     } while (val > 0);

     if (**p & 0x80) {
         if (*p - start < 1) {
             return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
         }
         *--(*p) = 0x00;
         len += 1;
     }

     return mbedtls_asn1_write_len_and_tag(p, start, len, tag);
 }

 int mbedtls_asn1_write_int(unsigned char **p, const unsigned char *start, int val)
 {
     return asn1_write_tagged_int(p, start, val, MBEDTLS_ASN1_INTEGER);
 }

 int mbedtls_asn1_write_enum(unsigned char **p, const unsigned char *start, int val)
 {
     return asn1_write_tagged_int(p, start, val, MBEDTLS_ASN1_ENUMERATED);
 }

 int mbedtls_asn1_write_tagged_string(unsigned char **p, const unsigned char *start, int tag,
                                      const char *text, size_t text_len)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;

     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(p, start,
                                                             (const unsigned char *) text,
                                                             text_len));

     return mbedtls_asn1_write_len_and_tag(p, start, len, tag);
 }

 int mbedtls_asn1_write_utf8_string(unsigned char **p, const unsigned char *start,
                                    const char *text, size_t text_len)
 {
     return mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_UTF8_STRING, text, text_len);
 }

 int mbedtls_asn1_write_printable_string(unsigned char **p, const unsigned char *start,
                                         const char *text, size_t text_len)
 {
     return mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_PRINTABLE_STRING, text,
                                             text_len);
 }

 int mbedtls_asn1_write_ia5_string(unsigned char **p, const unsigned char *start,
                                   const char *text, size_t text_len)
 {
     return mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_IA5_STRING, text, text_len);
 }

 int mbedtls_asn1_write_named_bitstring(unsigned char **p,
                                        const unsigned char *start,
                                        const unsigned char *buf,
                                        size_t bits)
 {
     size_t unused_bits, byte_len;
     const unsigned char *cur_byte;
     unsigned char cur_byte_shifted;
     unsigned char bit;

     byte_len = (bits + 7) / 8;
     unused_bits = (byte_len * 8) - bits;

     /*
      * Named bitstrings require that trailing 0s are excluded in the encoding
      * of the bitstring. Trailing 0s are considered part of the 'unused' bits
      * when encoding this value in the first content octet
      */
     if (bits != 0) {
         cur_byte = buf + byte_len - 1;
         cur_byte_shifted = *cur_byte >> unused_bits;

         for (;;) {
             bit = cur_byte_shifted & 0x1;
             cur_byte_shifted >>= 1;

             if (bit != 0) {
                 break;
             }

             bits--;
             if (bits == 0) {
                 break;
             }

             if (bits % 8 == 0) {
                 cur_byte_shifted = *--cur_byte;
             }
         }
     }

     return mbedtls_asn1_write_bitstring(p, start, buf, bits);
 }

 int mbedtls_asn1_write_bitstring(unsigned char **p, const unsigned char *start,
                                  const unsigned char *buf, size_t bits)
 {
     size_t len = 0;
     size_t unused_bits, byte_len;

     byte_len = (bits + 7) / 8;
     unused_bits = (byte_len * 8) - bits;

     if (*p < start || (size_t) (*p - start) < byte_len + 1) {
         return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
     }

     len = byte_len + 1;

     /* Write the bitstring. Ensure the unused bits are zeroed */
     if (byte_len > 0) {
         byte_len--;
         *--(*p) = buf[byte_len] & ~((0x1 << unused_bits) - 1);
         (*p) -= byte_len;
         memcpy(*p, buf, byte_len);
     }

     /* Write unused bits */
     *--(*p) = (unsigned char) unused_bits;

     return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_BIT_STRING);
 }

 int mbedtls_asn1_write_octet_string(unsigned char **p, const unsigned char *start,
                                     const unsigned char *buf, size_t size)
 {
     int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     size_t len = 0;

     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(p, start, buf, size));

     return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_OCTET_STRING);
 }


 #if !defined(MBEDTLS_ASN1_PARSE_C)
 /* This is a copy of the ASN.1 parsing function mbedtls_asn1_find_named_data(),
  * which is replicated to avoid a dependency ASN1_WRITE_C on ASN1_PARSE_C. */
 static mbedtls_asn1_named_data *asn1_find_named_data(
     mbedtls_asn1_named_data *list,
     const char *oid, size_t len)
 {
     while (list != NULL) {
         if (list->oid.len == len &&
             memcmp(list->oid.p, oid, len) == 0) {
             break;
         }

         list = list->next;
     }

     return list;
 }
 #else
 #define asn1_find_named_data(list, oid, len) \
     ((mbedtls_asn1_named_data *) mbedtls_asn1_find_named_data(list, oid, len))
 #endif

 mbedtls_asn1_named_data *mbedtls_asn1_store_named_data(
     mbedtls_asn1_named_data **head,
     const char *oid, size_t oid_len,
     const unsigned char *val,
     size_t val_len)
 {
     mbedtls_asn1_named_data *cur;

     if ((cur = asn1_find_named_data(*head, oid, oid_len)) == NULL) {
         // Add new entry if not present yet based on OID
         //
         cur = (mbedtls_asn1_named_data *) mbedtls_calloc(1,
                                                          sizeof(mbedtls_asn1_named_data));
         if (cur == NULL) {
             return NULL;
         }

         cur->oid.len = oid_len;
         cur->oid.p = mbedtls_calloc(1, oid_len);
         if (cur->oid.p == NULL) {
             mbedtls_free(cur);
             return NULL;
         }

         memcpy(cur->oid.p, oid, oid_len);

         cur->val.len = val_len;
         if (val_len != 0) {
             cur->val.p = mbedtls_calloc(1, val_len);
             if (cur->val.p == NULL) {
                 mbedtls_free(cur->oid.p);
                 mbedtls_free(cur);
                 return NULL;
             }
         }

         cur->next = *head;
         *head = cur;
     } else if (val_len == 0) {
         mbedtls_free(cur->val.p);
         cur->val.p = NULL;
     } else if (cur->val.len != val_len) {
         /*
          * Enlarge existing value buffer if needed
          * Preserve old data until the allocation succeeded, to leave list in
          * a consistent state in case allocation fails.
          */
         void *p = mbedtls_calloc(1, val_len);
         if (p == NULL) {
             return NULL;
         }

         mbedtls_free(cur->val.p);
         cur->val.p = p;
         cur->val.len = val_len;
     }

     if (val != NULL && val_len != 0) {
         memcpy(cur->val.p, val, val_len);
     }

     return cur;
 }
 #endif /* MBEDTLS_ASN1_WRITE_C */
	/*
	* ASN.1 buffer writing functionality
	*
	* Copyright The Mbed TLS Contributors
	* SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
	*/

	#include "common.h"

	#if defined(MBEDTLS_ASN1_WRITE_C) \|\| defined(MBEDTLS_X509_USE_C) \|\| \
	defined(MBEDTLS_PSA_UTIL_HAVE_ECDSA)

	#include "mbedtls/asn1write.h"
	#include "mbedtls/error.h"

	#include <string.h>

	#include "mbedtls/platform.h"

	#if defined(MBEDTLS_ASN1_PARSE_C)
	#include "mbedtls/asn1.h"
	#endif

	int mbedtls_asn1_write_len(unsigned char *p, const unsigned char start, size_t len)
	{
	#if SIZE_MAX > 0xFFFFFFFF
	if (len > 0xFFFFFFFF) {
	return MBEDTLS_ERR_ASN1_INVALID_LENGTH;
	}
	#endif

	int required = 1;

	if (len >= 0x80) {
	for (size_t l = len; l != 0; l >>= 8) {
	required++;
	}
	}

	if (required > (*p - start)) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	do {
	--(p) = MBEDTLS_BYTE_0(len);
	len >>= 8;
	} while (len);

	if (required > 1) {
	--(p) = (unsigned char) (0x80 + required - 1);
	}

	return required;
	}

	int mbedtls_asn1_write_tag(unsigned char *p, const unsigned char start, unsigned char tag)
	{
	if (*p - start < 1) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	--(p) = tag;

	return 1;
	}
	#endif /* MBEDTLS_ASN1_WRITE_C \|\| MBEDTLS_X509_USE_C \|\| MBEDTLS_PSA_UTIL_HAVE_ECDSA */

	#if defined(MBEDTLS_ASN1_WRITE_C)
	static int mbedtls_asn1_write_len_and_tag(unsigned char **p,
	const unsigned char *start,
	size_t len,
	unsigned char tag)
	{
	int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_len(p, start, len));
	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_tag(p, start, tag));

	return (int) len;
	}

	int mbedtls_asn1_write_raw_buffer(unsigned char *p, const unsigned char start,
	const unsigned char *buf, size_t size)
	{
	size_t len = 0;

	if (p < start \|\| (size_t) (p - start) < size) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	len = size;
	(*p) -= len;
	memcpy(*p, buf, len);

	return (int) len;
	}

	#if defined(MBEDTLS_BIGNUM_C)
	int mbedtls_asn1_write_mpi(unsigned char *p, const unsigned char start, const mbedtls_mpi *X)
	{
	int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
	size_t len = 0;

	// Write the MPI
	//
	len = mbedtls_mpi_size(X);

	/* DER represents 0 with a sign bit (0=nonnegative) and 7 value bits, not
	* as 0 digits. We need to end up with 020100, not with 0200. */
	if (len == 0) {
	len = 1;
	}

	if (p < start \|\| (size_t) (p - start) < len) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	(*p) -= len;
	MBEDTLS_MPI_CHK(mbedtls_mpi_write_binary(X, *p, len));

	// DER format assumes 2s complement for numbers, so the leftmost bit
	// should be 0 for positive numbers and 1 for negative numbers.
	//
	if (X->s == 1 && **p & 0x80) {
	if (*p - start < 1) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	--(p) = 0x00;
	len += 1;
	}

	ret = mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_INTEGER);

	cleanup:
	return ret;
	}
	#endif /* MBEDTLS_BIGNUM_C */

	int mbedtls_asn1_write_null(unsigned char *p, const unsigned char start)
	{
	// Write NULL
	//
	return mbedtls_asn1_write_len_and_tag(p, start, 0, MBEDTLS_ASN1_NULL);
	}

	int mbedtls_asn1_write_oid(unsigned char *p, const unsigned char start,
	const char *oid, size_t oid_len)
	{
	int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
	size_t len = 0;

	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(p, start,
	(const unsigned char *) oid, oid_len));
	return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_OID);
	}

	int mbedtls_asn1_write_algorithm_identifier(unsigned char *p, const unsigned char start,
	const char *oid, size_t oid_len,
	size_t par_len)
	{
	return mbedtls_asn1_write_algorithm_identifier_ext(p, start, oid, oid_len, par_len, 1);
	}

	int mbedtls_asn1_write_algorithm_identifier_ext(unsigned char *p, const unsigned char start,
	const char *oid, size_t oid_len,
	size_t par_len, int has_par)
	{
	int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
	size_t len = 0;

	if (has_par) {
	if (par_len == 0) {
	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_null(p, start));
	} else {
	len += par_len;
	}
	}

	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_oid(p, start, oid, oid_len));

	return mbedtls_asn1_write_len_and_tag(p, start, len,
	MBEDTLS_ASN1_CONSTRUCTED \| MBEDTLS_ASN1_SEQUENCE);
	}

	int mbedtls_asn1_write_bool(unsigned char *p, const unsigned char start, int boolean)
	{
	size_t len = 0;

	if (*p - start < 1) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	--(p) = (boolean) ? 255 : 0;
	len++;

	return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_BOOLEAN);
	}

	static int asn1_write_tagged_int(unsigned char *p, const unsigned char start, int val, int tag)
	{
	size_t len = 0;

	do {
	if (*p - start < 1) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}
	len += 1;
	--(p) = val & 0xff;
	val >>= 8;
	} while (val > 0);

	if (**p & 0x80) {
	if (*p - start < 1) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}
	--(p) = 0x00;
	len += 1;
	}

	return mbedtls_asn1_write_len_and_tag(p, start, len, tag);
	}

	int mbedtls_asn1_write_int(unsigned char *p, const unsigned char start, int val)
	{
	return asn1_write_tagged_int(p, start, val, MBEDTLS_ASN1_INTEGER);
	}

	int mbedtls_asn1_write_enum(unsigned char *p, const unsigned char start, int val)
	{
	return asn1_write_tagged_int(p, start, val, MBEDTLS_ASN1_ENUMERATED);
	}

	int mbedtls_asn1_write_tagged_string(unsigned char *p, const unsigned char start, int tag,
	const char *text, size_t text_len)
	{
	int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
	size_t len = 0;

	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(p, start,
	(const unsigned char *) text,
	text_len));

	return mbedtls_asn1_write_len_and_tag(p, start, len, tag);
	}

	int mbedtls_asn1_write_utf8_string(unsigned char *p, const unsigned char start,
	const char *text, size_t text_len)
	{
	return mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_UTF8_STRING, text, text_len);
	}

	int mbedtls_asn1_write_printable_string(unsigned char *p, const unsigned char start,
	const char *text, size_t text_len)
	{
	return mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_PRINTABLE_STRING, text,
	text_len);
	}

	int mbedtls_asn1_write_ia5_string(unsigned char *p, const unsigned char start,
	const char *text, size_t text_len)
	{
	return mbedtls_asn1_write_tagged_string(p, start, MBEDTLS_ASN1_IA5_STRING, text, text_len);
	}

	int mbedtls_asn1_write_named_bitstring(unsigned char **p,
	const unsigned char *start,
	const unsigned char *buf,
	size_t bits)
	{
	size_t unused_bits, byte_len;
	const unsigned char *cur_byte;
	unsigned char cur_byte_shifted;
	unsigned char bit;

	byte_len = (bits + 7) / 8;
	unused_bits = (byte_len * 8) - bits;

	/*
	* Named bitstrings require that trailing 0s are excluded in the encoding
	* of the bitstring. Trailing 0s are considered part of the 'unused' bits
	* when encoding this value in the first content octet
	*/
	if (bits != 0) {
	cur_byte = buf + byte_len - 1;
	cur_byte_shifted = *cur_byte >> unused_bits;

	for (;;) {
	bit = cur_byte_shifted & 0x1;
	cur_byte_shifted >>= 1;

	if (bit != 0) {
	break;
	}

	bits--;
	if (bits == 0) {
	break;
	}

	if (bits % 8 == 0) {
	cur_byte_shifted = *--cur_byte;
	}
	}
	}

	return mbedtls_asn1_write_bitstring(p, start, buf, bits);
	}

	int mbedtls_asn1_write_bitstring(unsigned char *p, const unsigned char start,
	const unsigned char *buf, size_t bits)
	{
	size_t len = 0;
	size_t unused_bits, byte_len;

	byte_len = (bits + 7) / 8;
	unused_bits = (byte_len * 8) - bits;

	if (p < start \|\| (size_t) (p - start) < byte_len + 1) {
	return MBEDTLS_ERR_ASN1_BUF_TOO_SMALL;
	}

	len = byte_len + 1;

	/* Write the bitstring. Ensure the unused bits are zeroed */
	if (byte_len > 0) {
	byte_len--;
	--(p) = buf[byte_len] & ~((0x1 << unused_bits) - 1);
	(*p) -= byte_len;
	memcpy(*p, buf, byte_len);
	}

	/* Write unused bits */
	--(p) = (unsigned char) unused_bits;

	return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_BIT_STRING);
	}

	int mbedtls_asn1_write_octet_string(unsigned char *p, const unsigned char start,
	const unsigned char *buf, size_t size)
	{
	int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
	size_t len = 0;

	MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(p, start, buf, size));

	return mbedtls_asn1_write_len_and_tag(p, start, len, MBEDTLS_ASN1_OCTET_STRING);
	}


	#if !defined(MBEDTLS_ASN1_PARSE_C)
	/* This is a copy of the ASN.1 parsing function mbedtls_asn1_find_named_data(),
	* which is replicated to avoid a dependency ASN1_WRITE_C on ASN1_PARSE_C. */
	static mbedtls_asn1_named_data *asn1_find_named_data(
	mbedtls_asn1_named_data *list,
	const char *oid, size_t len)
	{
	while (list != NULL) {
	if (list->oid.len == len &&
	memcmp(list->oid.p, oid, len) == 0) {
	break;
	}

	list = list->next;
	}

	return list;
	}
	#else
	#define asn1_find_named_data(list, oid, len) \
	((mbedtls_asn1_named_data *) mbedtls_asn1_find_named_data(list, oid, len))
	#endif

	mbedtls_asn1_named_data *mbedtls_asn1_store_named_data(
	mbedtls_asn1_named_data **head,
	const char *oid, size_t oid_len,
	const unsigned char *val,
	size_t val_len)
	{
	mbedtls_asn1_named_data *cur;

	if ((cur = asn1_find_named_data(*head, oid, oid_len)) == NULL) {
	// Add new entry if not present yet based on OID
	//
	cur = (mbedtls_asn1_named_data *) mbedtls_calloc(1,
	sizeof(mbedtls_asn1_named_data));
	if (cur == NULL) {
	return NULL;
	}

	cur->oid.len = oid_len;
	cur->oid.p = mbedtls_calloc(1, oid_len);
	if (cur->oid.p == NULL) {
	mbedtls_free(cur);
	return NULL;
	}

	memcpy(cur->oid.p, oid, oid_len);

	cur->val.len = val_len;
	if (val_len != 0) {
	cur->val.p = mbedtls_calloc(1, val_len);
	if (cur->val.p == NULL) {
	mbedtls_free(cur->oid.p);
	mbedtls_free(cur);
	return NULL;
	}
	}

	cur->next = *head;
	*head = cur;
	} else if (val_len == 0) {
	mbedtls_free(cur->val.p);
	cur->val.p = NULL;
	} else if (cur->val.len != val_len) {
	/*
	* Enlarge existing value buffer if needed
	* Preserve old data until the allocation succeeded, to leave list in
	* a consistent state in case allocation fails.
	*/
	void *p = mbedtls_calloc(1, val_len);
	if (p == NULL) {
	return NULL;
	}

	mbedtls_free(cur->val.p);
	cur->val.p = p;
	cur->val.len = val_len;
	}

	if (val != NULL && val_len != 0) {
	memcpy(cur->val.p, val, val_len);
	}

	return cur;
	}
	#endif /* MBEDTLS_ASN1_WRITE_C */