library/ssl_tls13_keys.c - mirror/mbed-tls - TrustedFirmware Git Browser

 /*
  *  TLS 1.3 key schedule
  *
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0
  *
  *  Licensed under the Apache License, Version 2.0 ( the "License" ); you may
  *  not use this file except in compliance with the License.
  *  You may obtain a copy of the License at
  *
  *  http://www.apache.org/licenses/LICENSE-2.0
  *
  *  Unless required by applicable law or agreed to in writing, software
  *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  *  See the License for the specific language governing permissions and
  *  limitations under the License.
  */

 #include "common.h"

 #if defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)

 #include "mbedtls/hkdf.h"
 #include "ssl_misc.h"
 #include "ssl_tls13_keys.h"

 #include <stdint.h>
 #include <string.h>

 #define MBEDTLS_SSL_TLS1_3_LABEL( name, string )       \
     .name = string,

 struct mbedtls_ssl_tls1_3_labels_struct const mbedtls_ssl_tls1_3_labels =
 {
     /* This seems to work in C, despite the string literal being one
      * character too long due to the 0-termination. */
     MBEDTLS_SSL_TLS1_3_LABEL_LIST
 };

 #undef MBEDTLS_SSL_TLS1_3_LABEL

 /*
  * This function creates a HkdfLabel structure used in the TLS 1.3 key schedule.
  *
  * The HkdfLabel is specified in RFC 8446 as follows:
  *
  * struct HkdfLabel {
  *   uint16 length;            // Length of expanded key material
  *   opaque label<7..255>;     // Always prefixed by "tls13 "
  *   opaque context<0..255>;   // Usually a communication transcript hash
  * };
  *
  * Parameters:
  * - desired_length: Length of expanded key material
  *                   Even though the standard allows expansion to up to
  *                   2**16 Bytes, TLS 1.3 never uses expansion to more than
  *                   255 Bytes, so we require `desired_length` to be at most
  *                   255. This allows us to save a few Bytes of code by
  *                   hardcoding the writing of the high bytes.
  * - (label, llen): label + label length, without "tls13 " prefix
  *                  The label length MUST be less than or equal to
  *                  MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN
  *                  It is the caller's responsibility to ensure this.
  *                  All (label, label length) pairs used in TLS 1.3
  *                  can be obtained via MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN().
  * - (ctx, clen): context + context length
  *                The context length MUST be less than or equal to
  *                MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN
  *                It is the caller's responsibility to ensure this.
  * - dst: Target buffer for HkdfLabel structure,
  *        This MUST be a writable buffer of size
  *        at least SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN Bytes.
  * - dlen: Pointer at which to store the actual length of
  *         the HkdfLabel structure on success.
  */

 static const char tls1_3_label_prefix[6] = "tls13 ";

 #define SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( label_len, context_len ) \
     (   2                  /* expansion length           */ \
       + 1                  /* label length               */ \
       + label_len                                           \
       + 1                  /* context length             */ \
       + context_len )

 #define SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN                      \
     SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN(                             \
                      sizeof(tls1_3_label_prefix) +                      \
                      MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN,     \
                      MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN )

 static void ssl_tls1_3_hkdf_encode_label(
                             size_t desired_length,
                             const unsigned char *label, size_t llen,
                             const unsigned char *ctx, size_t clen,
                             unsigned char *dst, size_t *dlen )
 {
     size_t total_label_len =
         sizeof(tls1_3_label_prefix) + llen;
     size_t total_hkdf_lbl_len =
         SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( total_label_len, clen );

     unsigned char *p = dst;

     /* Add the size of the expanded key material.
      * We're hardcoding the high byte to 0 here assuming that we never use
      * TLS 1.3 HKDF key expansion to more than 255 Bytes. */
 #if MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN > 255
 #error "The implementation of ssl_tls1_3_hkdf_encode_label() is not fit for the \
         value of MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN"
 #endif

     *p++ = 0;
     *p++ = (unsigned char)( ( desired_length >> 0 ) & 0xFF );

     /* Add label incl. prefix */
     *p++ = (unsigned char)( total_label_len & 0xFF );
     memcpy( p, tls1_3_label_prefix, sizeof(tls1_3_label_prefix) );
     p += sizeof(tls1_3_label_prefix);
     memcpy( p, label, llen );
     p += llen;

     /* Add context value */
     *p++ = (unsigned char)( clen & 0xFF );
     if( clen != 0 )
         memcpy( p, ctx, clen );

     /* Return total length to the caller.  */
     *dlen = total_hkdf_lbl_len;
 }

 int mbedtls_ssl_tls1_3_hkdf_expand_label(
                      mbedtls_md_type_t hash_alg,
                      const unsigned char *secret, size_t slen,
                      const unsigned char *label, size_t llen,
                      const unsigned char *ctx, size_t clen,
                      unsigned char *buf, size_t blen )
 {
     const mbedtls_md_info_t *md;
     unsigned char hkdf_label[ SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN ];
     size_t hkdf_label_len;

     if( llen > MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN )
     {
         /* Should never happen since this is an internal
          * function, and we know statically which labels
          * are allowed. */
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }

     if( clen > MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN )
     {
         /* Should not happen, as above. */
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }

     if( blen > MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN )
     {
         /* Should not happen, as above. */
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
     }

     md = mbedtls_md_info_from_type( hash_alg );
     if( md == NULL )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );

     ssl_tls1_3_hkdf_encode_label( blen,
                                   label, llen,
                                   ctx, clen,
                                   hkdf_label,
                                   &hkdf_label_len );

     return( mbedtls_hkdf_expand( md,
                                  secret, slen,
                                  hkdf_label, hkdf_label_len,
                                  buf, blen ) );
 }

 /*
  * The traffic keying material is generated from the following inputs:
  *
  *  - One secret value per sender.
  *  - A purpose value indicating the specific value being generated
  *  - The desired lengths of key and IV.
  *
  * The expansion itself is based on HKDF:
  *
  *   [sender]_write_key = HKDF-Expand-Label( Secret, "key", "", key_length )
  *   [sender]_write_iv  = HKDF-Expand-Label( Secret, "iv" , "", iv_length )
  *
  * [sender] denotes the sending side and the Secret value is provided
  * by the function caller. Note that we generate server and client side
  * keys in a single function call.
  */
 int mbedtls_ssl_tls1_3_make_traffic_keys(
                      mbedtls_md_type_t hash_alg,
                      const unsigned char *client_secret,
                      const unsigned char *server_secret,
                      size_t slen, size_t key_len, size_t iv_len,
                      mbedtls_ssl_key_set *keys )
 {
     int ret = 0;

     ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
                     client_secret, slen,
                     MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( key ),
                     NULL, 0,
                     keys->client_write_key, key_len );
     if( ret != 0 )
         return( ret );

     ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
                     server_secret, slen,
                     MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( key ),
                     NULL, 0,
                     keys->server_write_key, key_len );
     if( ret != 0 )
         return( ret );

     ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
                     client_secret, slen,
                     MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( iv ),
                     NULL, 0,
                     keys->client_write_iv, iv_len );
     if( ret != 0 )
         return( ret );

     ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
                     server_secret, slen,
                     MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( iv ),
                     NULL, 0,
                     keys->server_write_iv, iv_len );
     if( ret != 0 )
         return( ret );

     keys->key_len = key_len;
     keys->iv_len = iv_len;

     return( 0 );
 }

 int mbedtls_ssl_tls1_3_derive_secret(
                    mbedtls_md_type_t hash_alg,
                    const unsigned char *secret, size_t slen,
                    const unsigned char *label, size_t llen,
                    const unsigned char *ctx, size_t clen,
                    int ctx_hashed,
                    unsigned char *dstbuf, size_t buflen )
 {
     int ret;
     unsigned char hashed_context[ MBEDTLS_MD_MAX_SIZE ];

     const mbedtls_md_info_t *md;
     md = mbedtls_md_info_from_type( hash_alg );
     if( md == NULL )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );

     if( ctx_hashed == MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED )
     {
         ret = mbedtls_md( md, ctx, clen, hashed_context );
         if( ret != 0 )
             return( ret );
         clen = mbedtls_md_get_size( md );
     }
     else
     {
         if( clen > sizeof(hashed_context) )
         {
             /* This should never happen since this function is internal
              * and the code sets `ctx_hashed` correctly.
              * Let's double-check nonetheless to not run at the risk
              * of getting a stack overflow. */
             return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
         }

         memcpy( hashed_context, ctx, clen );
     }

     return( mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
                                                   secret, slen,
                                                   label, llen,
                                                   hashed_context, clen,
                                                   dstbuf, buflen ) );
 }

 int mbedtls_ssl_tls1_3_evolve_secret(
                    mbedtls_md_type_t hash_alg,
                    const unsigned char *secret_old,
                    const unsigned char *input, size_t input_len,
                    unsigned char *secret_new )
 {
     int ret = MBEDTLS_ERR_SSL_INTERNAL_ERROR;
     size_t hlen, ilen;
     unsigned char tmp_secret[ MBEDTLS_MD_MAX_SIZE ] = { 0 };
     unsigned char tmp_input [ MBEDTLS_MD_MAX_SIZE ] = { 0 };

     const mbedtls_md_info_t *md;
     md = mbedtls_md_info_from_type( hash_alg );
     if( md == NULL )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );

     hlen = mbedtls_md_get_size( md );

     /* For non-initial runs, call Derive-Secret( ., "derived", "")
      * on the old secret. */
     if( secret_old != NULL )
     {
         ret = mbedtls_ssl_tls1_3_derive_secret(
                    hash_alg,
                    secret_old, hlen,
                    MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( derived ),
                    NULL, 0, /* context */
                    MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED,
                    tmp_secret, hlen );
         if( ret != 0 )
             goto cleanup;
     }

     if( input != NULL )
     {
         memcpy( tmp_input, input, input_len );
         ilen = input_len;
     }
     else
     {
         ilen = hlen;
     }

     /* HKDF-Extract takes a salt and input key material.
      * The salt is the old secret, and the input key material
      * is the input secret (PSK / ECDHE). */
     ret = mbedtls_hkdf_extract( md,
                     tmp_secret, hlen,
                     tmp_input, ilen,
                     secret_new );
     if( ret != 0 )
         goto cleanup;

     ret = 0;

  cleanup:

     mbedtls_platform_zeroize( tmp_secret, sizeof(tmp_secret) );
     mbedtls_platform_zeroize( tmp_input,  sizeof(tmp_input)  );
     return( ret );
 }

 #endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */
	/*
	* TLS 1.3 key schedule
	*
	* Copyright The Mbed TLS Contributors
	* SPDX-License-Identifier: Apache-2.0
	*
	* Licensed under the Apache License, Version 2.0 ( the "License" ); you may
	* not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
	* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "common.h"

	#if defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)

	#include "mbedtls/hkdf.h"
	#include "ssl_misc.h"
	#include "ssl_tls13_keys.h"

	#include <stdint.h>
	#include <string.h>

	#define MBEDTLS_SSL_TLS1_3_LABEL( name, string ) \
	.name = string,

	struct mbedtls_ssl_tls1_3_labels_struct const mbedtls_ssl_tls1_3_labels =
	{
	/* This seems to work in C, despite the string literal being one
	* character too long due to the 0-termination. */
	MBEDTLS_SSL_TLS1_3_LABEL_LIST
	};

	#undef MBEDTLS_SSL_TLS1_3_LABEL

	/*
	* This function creates a HkdfLabel structure used in the TLS 1.3 key schedule.
	*
	* The HkdfLabel is specified in RFC 8446 as follows:
	*
	* struct HkdfLabel {
	* uint16 length; // Length of expanded key material
	* opaque label<7..255>; // Always prefixed by "tls13 "
	* opaque context<0..255>; // Usually a communication transcript hash
	* };
	*
	* Parameters:
	* - desired_length: Length of expanded key material
	* Even though the standard allows expansion to up to
	* 2**16 Bytes, TLS 1.3 never uses expansion to more than
	* 255 Bytes, so we require `desired_length` to be at most
	* 255. This allows us to save a few Bytes of code by
	* hardcoding the writing of the high bytes.
	* - (label, llen): label + label length, without "tls13 " prefix
	* The label length MUST be less than or equal to
	* MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN
	* It is the caller's responsibility to ensure this.
	* All (label, label length) pairs used in TLS 1.3
	* can be obtained via MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN().
	* - (ctx, clen): context + context length
	* The context length MUST be less than or equal to
	* MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN
	* It is the caller's responsibility to ensure this.
	* - dst: Target buffer for HkdfLabel structure,
	* This MUST be a writable buffer of size
	* at least SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN Bytes.
	* - dlen: Pointer at which to store the actual length of
	* the HkdfLabel structure on success.
	*/

	static const char tls1_3_label_prefix[6] = "tls13 ";

	#define SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( label_len, context_len ) \
	( 2 /* expansion length */ \
	+ 1 /* label length */ \
	+ label_len \
	+ 1 /* context length */ \
	+ context_len )

	#define SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN \
	SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( \
	sizeof(tls1_3_label_prefix) + \
	MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN, \
	MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN )

	static void ssl_tls1_3_hkdf_encode_label(
	size_t desired_length,
	const unsigned char *label, size_t llen,
	const unsigned char *ctx, size_t clen,
	unsigned char dst, size_t dlen )
	{
	size_t total_label_len =
	sizeof(tls1_3_label_prefix) + llen;
	size_t total_hkdf_lbl_len =
	SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN( total_label_len, clen );

	unsigned char *p = dst;

	/* Add the size of the expanded key material.
	* We're hardcoding the high byte to 0 here assuming that we never use
	* TLS 1.3 HKDF key expansion to more than 255 Bytes. */
	#if MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN > 255
	#error "The implementation of ssl_tls1_3_hkdf_encode_label() is not fit for the \
	value of MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN"
	#endif

	*p++ = 0;
	*p++ = (unsigned char)( ( desired_length >> 0 ) & 0xFF );

	/* Add label incl. prefix */
	*p++ = (unsigned char)( total_label_len & 0xFF );
	memcpy( p, tls1_3_label_prefix, sizeof(tls1_3_label_prefix) );
	p += sizeof(tls1_3_label_prefix);
	memcpy( p, label, llen );
	p += llen;

	/* Add context value */
	*p++ = (unsigned char)( clen & 0xFF );
	if( clen != 0 )
	memcpy( p, ctx, clen );

	/* Return total length to the caller. */
	*dlen = total_hkdf_lbl_len;
	}

	int mbedtls_ssl_tls1_3_hkdf_expand_label(
	mbedtls_md_type_t hash_alg,
	const unsigned char *secret, size_t slen,
	const unsigned char *label, size_t llen,
	const unsigned char *ctx, size_t clen,
	unsigned char *buf, size_t blen )
	{
	const mbedtls_md_info_t *md;
	unsigned char hkdf_label[ SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN ];
	size_t hkdf_label_len;

	if( llen > MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN )
	{
	/* Should never happen since this is an internal
	* function, and we know statically which labels
	* are allowed. */
	return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
	}

	if( clen > MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN )
	{
	/* Should not happen, as above. */
	return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
	}

	if( blen > MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN )
	{
	/* Should not happen, as above. */
	return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
	}

	md = mbedtls_md_info_from_type( hash_alg );
	if( md == NULL )
	return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );

	ssl_tls1_3_hkdf_encode_label( blen,
	label, llen,
	ctx, clen,
	hkdf_label,
	&hkdf_label_len );

	return( mbedtls_hkdf_expand( md,
	secret, slen,
	hkdf_label, hkdf_label_len,
	buf, blen ) );
	}

	/*
	* The traffic keying material is generated from the following inputs:
	*
	* - One secret value per sender.
	* - A purpose value indicating the specific value being generated
	* - The desired lengths of key and IV.
	*
	* The expansion itself is based on HKDF:
	*
	* [sender]_write_key = HKDF-Expand-Label( Secret, "key", "", key_length )
	* [sender]_write_iv = HKDF-Expand-Label( Secret, "iv" , "", iv_length )
	*
	* [sender] denotes the sending side and the Secret value is provided
	* by the function caller. Note that we generate server and client side
	* keys in a single function call.
	*/
	int mbedtls_ssl_tls1_3_make_traffic_keys(
	mbedtls_md_type_t hash_alg,
	const unsigned char *client_secret,
	const unsigned char *server_secret,
	size_t slen, size_t key_len, size_t iv_len,
	mbedtls_ssl_key_set *keys )
	{
	int ret = 0;

	ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
	client_secret, slen,
	MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( key ),
	NULL, 0,
	keys->client_write_key, key_len );
	if( ret != 0 )
	return( ret );

	ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
	server_secret, slen,
	MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( key ),
	NULL, 0,
	keys->server_write_key, key_len );
	if( ret != 0 )
	return( ret );

	ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
	client_secret, slen,
	MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( iv ),
	NULL, 0,
	keys->client_write_iv, iv_len );
	if( ret != 0 )
	return( ret );

	ret = mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
	server_secret, slen,
	MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( iv ),
	NULL, 0,
	keys->server_write_iv, iv_len );
	if( ret != 0 )
	return( ret );

	keys->key_len = key_len;
	keys->iv_len = iv_len;

	return( 0 );
	}

	int mbedtls_ssl_tls1_3_derive_secret(
	mbedtls_md_type_t hash_alg,
	const unsigned char *secret, size_t slen,
	const unsigned char *label, size_t llen,
	const unsigned char *ctx, size_t clen,
	int ctx_hashed,
	unsigned char *dstbuf, size_t buflen )
	{
	int ret;
	unsigned char hashed_context[ MBEDTLS_MD_MAX_SIZE ];

	const mbedtls_md_info_t *md;
	md = mbedtls_md_info_from_type( hash_alg );
	if( md == NULL )
	return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );

	if( ctx_hashed == MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED )
	{
	ret = mbedtls_md( md, ctx, clen, hashed_context );
	if( ret != 0 )
	return( ret );
	clen = mbedtls_md_get_size( md );
	}
	else
	{
	if( clen > sizeof(hashed_context) )
	{
	/* This should never happen since this function is internal
	* and the code sets `ctx_hashed` correctly.
	* Let's double-check nonetheless to not run at the risk
	* of getting a stack overflow. */
	return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
	}

	memcpy( hashed_context, ctx, clen );
	}

	return( mbedtls_ssl_tls1_3_hkdf_expand_label( hash_alg,
	secret, slen,
	label, llen,
	hashed_context, clen,
	dstbuf, buflen ) );
	}

	int mbedtls_ssl_tls1_3_evolve_secret(
	mbedtls_md_type_t hash_alg,
	const unsigned char *secret_old,
	const unsigned char *input, size_t input_len,
	unsigned char *secret_new )
	{
	int ret = MBEDTLS_ERR_SSL_INTERNAL_ERROR;
	size_t hlen, ilen;
	unsigned char tmp_secret[ MBEDTLS_MD_MAX_SIZE ] = { 0 };
	unsigned char tmp_input [ MBEDTLS_MD_MAX_SIZE ] = { 0 };

	const mbedtls_md_info_t *md;
	md = mbedtls_md_info_from_type( hash_alg );
	if( md == NULL )
	return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );

	hlen = mbedtls_md_get_size( md );

	/* For non-initial runs, call Derive-Secret( ., "derived", "")
	* on the old secret. */
	if( secret_old != NULL )
	{
	ret = mbedtls_ssl_tls1_3_derive_secret(
	hash_alg,
	secret_old, hlen,
	MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( derived ),
	NULL, 0, /* context */
	MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED,
	tmp_secret, hlen );
	if( ret != 0 )
	goto cleanup;
	}

	if( input != NULL )
	{
	memcpy( tmp_input, input, input_len );
	ilen = input_len;
	}
	else
	{
	ilen = hlen;
	}

	/* HKDF-Extract takes a salt and input key material.
	* The salt is the old secret, and the input key material
	* is the input secret (PSK / ECDHE). */
	ret = mbedtls_hkdf_extract( md,
	tmp_secret, hlen,
	tmp_input, ilen,
	secret_new );
	if( ret != 0 )
	goto cleanup;

	ret = 0;

	cleanup:

	mbedtls_platform_zeroize( tmp_secret, sizeof(tmp_secret) );
	mbedtls_platform_zeroize( tmp_input, sizeof(tmp_input) );
	return( ret );
	}

	#endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */