library/ecdh.c - security/mbedtls - Rivoreo Source Code Repositories

 /*
  *  Elliptic curve Diffie-Hellman
  *
  *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
  *  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.
  *
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */

 /*
  * References:
  *
  * SEC1 http://www.secg.org/index.php?action=secg,docs_secg
  * RFC 4492
  */

 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else
 #include MBEDTLS_CONFIG_FILE
 #endif

 #if defined(MBEDTLS_ECDH_C)

 #include "mbedtls/ecdh.h"

 #include <string.h>

 #if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
 /*
  * Generate public key (restartable version)
  *
  * Note: this internal function relies on its caller preserving the value of
  * the output parameter 'd' accross continuation calls. This would not be
  * acceptable for a public function but is OK here as we control call sites.
  */
 static int ecdh_gen_public_restartable( mbedtls_ecp_group *grp,
                     mbedtls_mpi *d, mbedtls_ecp_point *Q,
                     int (*f_rng)(void *, unsigned char *, size_t),
                     void *p_rng,
                     mbedtls_ecp_restart_ctx *rs_ctx )
 {
     int ret;

     /* If multiplication is in progress, we already generated a privkey */
 #if defined(MBEDTLS_ECP_RESTARTABLE)
     if( rs_ctx == NULL || rs_ctx->rsm == NULL )
 #endif
         MBEDTLS_MPI_CHK( mbedtls_ecp_gen_privkey( grp, d, f_rng, p_rng ) );

     MBEDTLS_MPI_CHK( mbedtls_ecp_mul_restartable( grp, Q, d, &grp->G,
                                                   f_rng, p_rng, rs_ctx ) );

 cleanup:
     return( ret );
 }

 /*
  * Generate public key
  */
 int mbedtls_ecdh_gen_public( mbedtls_ecp_group *grp, mbedtls_mpi *d, mbedtls_ecp_point *Q,
                      int (*f_rng)(void *, unsigned char *, size_t),
                      void *p_rng )
 {
     return( ecdh_gen_public_restartable( grp, d, Q, f_rng, p_rng, NULL ) );
 }
 #endif /* MBEDTLS_ECDH_GEN_PUBLIC_ALT */

 #if !defined(MBEDTLS_ECDH_COMPUTE_SHARED_ALT)
 /*
  * Compute shared secret (SEC1 3.3.1)
  */
 static int ecdh_compute_shared_restartable( mbedtls_ecp_group *grp,
                          mbedtls_mpi *z,
                          const mbedtls_ecp_point *Q, const mbedtls_mpi *d,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng,
                          mbedtls_ecp_restart_ctx *rs_ctx )
 {
     int ret;
     mbedtls_ecp_point P;

     mbedtls_ecp_point_init( &P );

     MBEDTLS_MPI_CHK( mbedtls_ecp_mul_restartable( grp, &P, d, Q,
                                                   f_rng, p_rng, rs_ctx ) );

     if( mbedtls_ecp_is_zero( &P ) )
     {
         ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
         goto cleanup;
     }

     MBEDTLS_MPI_CHK( mbedtls_mpi_copy( z, &P.X ) );

 cleanup:
     mbedtls_ecp_point_free( &P );

     return( ret );
 }
 #endif /* MBEDTLS_ECDH_COMPUTE_SHARED_ALT */

 /*
  * Compute shared secret (SEC1 3.3.1)
  */
 int mbedtls_ecdh_compute_shared( mbedtls_ecp_group *grp, mbedtls_mpi *z,
                          const mbedtls_ecp_point *Q, const mbedtls_mpi *d,
                          int (*f_rng)(void *, unsigned char *, size_t),
                          void *p_rng )
 {
     return( ecdh_compute_shared_restartable( grp, z, Q, d,
                                              f_rng, p_rng, NULL ) );
 }

 /*
  * Initialize context
  */
 void mbedtls_ecdh_init( mbedtls_ecdh_context *ctx )
 {
     mbedtls_ecp_group_init( &ctx->grp );
     mbedtls_mpi_init( &ctx->d  );
     mbedtls_ecp_point_init( &ctx->Q   );
     mbedtls_ecp_point_init( &ctx->Qp  );
     mbedtls_mpi_init( &ctx->z  );
     ctx->point_format = MBEDTLS_ECP_PF_UNCOMPRESSED;
     mbedtls_ecp_point_init( &ctx->Vi  );
     mbedtls_ecp_point_init( &ctx->Vf  );
     mbedtls_mpi_init( &ctx->_d );

 #if defined(MBEDTLS_ECP_RESTARTABLE)
     ctx->restart_enabled = 0;
     mbedtls_ecp_restart_init( &ctx->rs );
 #endif
 }

 /*
  * Free context
  */
 void mbedtls_ecdh_free( mbedtls_ecdh_context *ctx )
 {
     if( ctx == NULL )
         return;

     mbedtls_ecp_group_free( &ctx->grp );
     mbedtls_mpi_free( &ctx->d  );
     mbedtls_ecp_point_free( &ctx->Q   );
     mbedtls_ecp_point_free( &ctx->Qp  );
     mbedtls_mpi_free( &ctx->z  );
     mbedtls_ecp_point_free( &ctx->Vi  );
     mbedtls_ecp_point_free( &ctx->Vf  );
     mbedtls_mpi_free( &ctx->_d );

 #if defined(MBEDTLS_ECP_RESTARTABLE)
     mbedtls_ecp_restart_free( &ctx->rs );
 #endif
 }

 #if defined(MBEDTLS_ECP_RESTARTABLE)
 /*
  * Enable restartable operations for context
  */
 void mbedtls_ecdh_enable_restart( mbedtls_ecdh_context *ctx )
 {
     ctx->restart_enabled = 1;
 }
 #endif

 /*
  * Setup and write the ServerKeyExhange parameters (RFC 4492)
  *      struct {
  *          ECParameters    curve_params;
  *          ECPoint         public;
  *      } ServerECDHParams;
  */
 int mbedtls_ecdh_make_params( mbedtls_ecdh_context *ctx, size_t *olen,
                       unsigned char *buf, size_t blen,
                       int (*f_rng)(void *, unsigned char *, size_t),
                       void *p_rng )
 {
     int ret;
     size_t grp_len, pt_len;
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;

     if( ctx == NULL || ctx->grp.pbits == 0 )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

 #if defined(MBEDTLS_ECP_RESTARTABLE)
     if( ctx->restart_enabled )
         rs_ctx = &ctx->rs;
 #endif

     if( ( ret = ecdh_gen_public_restartable( &ctx->grp, &ctx->d, &ctx->Q,
                                              f_rng, p_rng, rs_ctx ) ) != 0 )
         return( ret );

     if( ( ret = mbedtls_ecp_tls_write_group( &ctx->grp, &grp_len, buf, blen ) )
                 != 0 )
         return( ret );

     buf += grp_len;
     blen -= grp_len;

     if( ( ret = mbedtls_ecp_tls_write_point( &ctx->grp, &ctx->Q, ctx->point_format,
                                              &pt_len, buf, blen ) ) != 0 )
         return( ret );

     *olen = grp_len + pt_len;
     return( 0 );
 }

 /*
  * Read the ServerKeyExhange parameters (RFC 4492)
  *      struct {
  *          ECParameters    curve_params;
  *          ECPoint         public;
  *      } ServerECDHParams;
  */
 int mbedtls_ecdh_read_params( mbedtls_ecdh_context *ctx,
                       const unsigned char **buf, const unsigned char *end )
 {
     int ret;

     if( ( ret = mbedtls_ecp_tls_read_group( &ctx->grp, buf, end - *buf ) ) != 0 )
         return( ret );

     if( ( ret = mbedtls_ecp_tls_read_point( &ctx->grp, &ctx->Qp, buf, end - *buf ) )
                 != 0 )
         return( ret );

     return( 0 );
 }

 /*
  * Get parameters from a keypair
  */
 int mbedtls_ecdh_get_params( mbedtls_ecdh_context *ctx, const mbedtls_ecp_keypair *key,
                      mbedtls_ecdh_side side )
 {
     int ret;

     if( ( ret = mbedtls_ecp_group_copy( &ctx->grp, &key->grp ) ) != 0 )
         return( ret );

     /* If it's not our key, just import the public part as Qp */
     if( side == MBEDTLS_ECDH_THEIRS )
         return( mbedtls_ecp_copy( &ctx->Qp, &key->Q ) );

     /* Our key: import public (as Q) and private parts */
     if( side != MBEDTLS_ECDH_OURS )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

     if( ( ret = mbedtls_ecp_copy( &ctx->Q, &key->Q ) ) != 0 ||
         ( ret = mbedtls_mpi_copy( &ctx->d, &key->d ) ) != 0 )
         return( ret );

     return( 0 );
 }

 /*
  * Setup and export the client public value
  */
 int mbedtls_ecdh_make_public( mbedtls_ecdh_context *ctx, size_t *olen,
                       unsigned char *buf, size_t blen,
                       int (*f_rng)(void *, unsigned char *, size_t),
                       void *p_rng )
 {
     int ret;
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;

     if( ctx == NULL || ctx->grp.pbits == 0 )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

 #if defined(MBEDTLS_ECP_RESTARTABLE)
     if( ctx->restart_enabled )
         rs_ctx = &ctx->rs;
 #endif

     if( ( ret = ecdh_gen_public_restartable( &ctx->grp, &ctx->d, &ctx->Q,
                     f_rng, p_rng, rs_ctx ) ) != 0 )
         return( ret );

     return mbedtls_ecp_tls_write_point( &ctx->grp, &ctx->Q, ctx->point_format,
                                 olen, buf, blen );
 }

 /*
  * Parse and import the client's public value
  */
 int mbedtls_ecdh_read_public( mbedtls_ecdh_context *ctx,
                       const unsigned char *buf, size_t blen )
 {
     int ret;
     const unsigned char *p = buf;

     if( ctx == NULL )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

     if( ( ret = mbedtls_ecp_tls_read_point( &ctx->grp, &ctx->Qp, &p, blen ) ) != 0 )
         return( ret );

     if( (size_t)( p - buf ) != blen )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

     return( 0 );
 }

 /*
  * Derive and export the shared secret
  */
 int mbedtls_ecdh_calc_secret( mbedtls_ecdh_context *ctx, size_t *olen,
                       unsigned char *buf, size_t blen,
                       int (*f_rng)(void *, unsigned char *, size_t),
                       void *p_rng )
 {
     int ret;
     mbedtls_ecp_restart_ctx *rs_ctx = NULL;

     if( ctx == NULL || ctx->grp.pbits == 0 )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

 #if defined(MBEDTLS_ECP_RESTARTABLE)
     if( ctx->restart_enabled )
         rs_ctx = &ctx->rs;
 #endif

     if( ( ret = ecdh_compute_shared_restartable( &ctx->grp,
                     &ctx->z, &ctx->Qp, &ctx->d, f_rng, p_rng, rs_ctx ) ) != 0 )
     {
         return( ret );
     }

     if( mbedtls_mpi_size( &ctx->z ) > blen )
         return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

     *olen = ctx->grp.pbits / 8 + ( ( ctx->grp.pbits % 8 ) != 0 );
     return mbedtls_mpi_write_binary( &ctx->z, buf, *olen );
 }

 #endif /* MBEDTLS_ECDH_C */
	/*
	* Elliptic curve Diffie-Hellman
	*
	* Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
	* 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.
	*
	* This file is part of mbed TLS (https://tls.mbed.org)
	*/

	/*
	* References:
	*
	* SEC1 http://www.secg.org/index.php?action=secg,docs_secg
	* RFC 4492
	*/

	#if !defined(MBEDTLS_CONFIG_FILE)
	#include "mbedtls/config.h"
	#else
	#include MBEDTLS_CONFIG_FILE
	#endif

	#if defined(MBEDTLS_ECDH_C)

	#include "mbedtls/ecdh.h"

	#include <string.h>

	#if !defined(MBEDTLS_ECDH_GEN_PUBLIC_ALT)
	/*
	* Generate public key (restartable version)
	*
	* Note: this internal function relies on its caller preserving the value of
	* the output parameter 'd' accross continuation calls. This would not be
	* acceptable for a public function but is OK here as we control call sites.
	*/
	static int ecdh_gen_public_restartable( mbedtls_ecp_group *grp,
	mbedtls_mpi d, mbedtls_ecp_point Q,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng,
	mbedtls_ecp_restart_ctx *rs_ctx )
	{
	int ret;

	/* If multiplication is in progress, we already generated a privkey */
	#if defined(MBEDTLS_ECP_RESTARTABLE)
	if( rs_ctx == NULL \|\| rs_ctx->rsm == NULL )
	#endif
	MBEDTLS_MPI_CHK( mbedtls_ecp_gen_privkey( grp, d, f_rng, p_rng ) );

	MBEDTLS_MPI_CHK( mbedtls_ecp_mul_restartable( grp, Q, d, &grp->G,
	f_rng, p_rng, rs_ctx ) );

	cleanup:
	return( ret );
	}

	/*
	* Generate public key
	*/
	int mbedtls_ecdh_gen_public( mbedtls_ecp_group grp, mbedtls_mpi d, mbedtls_ecp_point *Q,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng )
	{
	return( ecdh_gen_public_restartable( grp, d, Q, f_rng, p_rng, NULL ) );
	}
	#endif /* MBEDTLS_ECDH_GEN_PUBLIC_ALT */

	#if !defined(MBEDTLS_ECDH_COMPUTE_SHARED_ALT)
	/*
	* Compute shared secret (SEC1 3.3.1)
	*/
	static int ecdh_compute_shared_restartable( mbedtls_ecp_group *grp,
	mbedtls_mpi *z,
	const mbedtls_ecp_point Q, const mbedtls_mpi d,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng,
	mbedtls_ecp_restart_ctx *rs_ctx )
	{
	int ret;
	mbedtls_ecp_point P;

	mbedtls_ecp_point_init( &P );

	MBEDTLS_MPI_CHK( mbedtls_ecp_mul_restartable( grp, &P, d, Q,
	f_rng, p_rng, rs_ctx ) );

	if( mbedtls_ecp_is_zero( &P ) )
	{
	ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
	goto cleanup;
	}

	MBEDTLS_MPI_CHK( mbedtls_mpi_copy( z, &P.X ) );

	cleanup:
	mbedtls_ecp_point_free( &P );

	return( ret );
	}
	#endif /* MBEDTLS_ECDH_COMPUTE_SHARED_ALT */

	/*
	* Compute shared secret (SEC1 3.3.1)
	*/
	int mbedtls_ecdh_compute_shared( mbedtls_ecp_group grp, mbedtls_mpi z,
	const mbedtls_ecp_point Q, const mbedtls_mpi d,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng )
	{
	return( ecdh_compute_shared_restartable( grp, z, Q, d,
	f_rng, p_rng, NULL ) );
	}

	/*
	* Initialize context
	*/
	void mbedtls_ecdh_init( mbedtls_ecdh_context *ctx )
	{
	mbedtls_ecp_group_init( &ctx->grp );
	mbedtls_mpi_init( &ctx->d );
	mbedtls_ecp_point_init( &ctx->Q );
	mbedtls_ecp_point_init( &ctx->Qp );
	mbedtls_mpi_init( &ctx->z );
	ctx->point_format = MBEDTLS_ECP_PF_UNCOMPRESSED;
	mbedtls_ecp_point_init( &ctx->Vi );
	mbedtls_ecp_point_init( &ctx->Vf );
	mbedtls_mpi_init( &ctx->_d );

	#if defined(MBEDTLS_ECP_RESTARTABLE)
	ctx->restart_enabled = 0;
	mbedtls_ecp_restart_init( &ctx->rs );
	#endif
	}

	/*
	* Free context
	*/
	void mbedtls_ecdh_free( mbedtls_ecdh_context *ctx )
	{
	if( ctx == NULL )
	return;

	mbedtls_ecp_group_free( &ctx->grp );
	mbedtls_mpi_free( &ctx->d );
	mbedtls_ecp_point_free( &ctx->Q );
	mbedtls_ecp_point_free( &ctx->Qp );
	mbedtls_mpi_free( &ctx->z );
	mbedtls_ecp_point_free( &ctx->Vi );
	mbedtls_ecp_point_free( &ctx->Vf );
	mbedtls_mpi_free( &ctx->_d );

	#if defined(MBEDTLS_ECP_RESTARTABLE)
	mbedtls_ecp_restart_free( &ctx->rs );
	#endif
	}

	#if defined(MBEDTLS_ECP_RESTARTABLE)
	/*
	* Enable restartable operations for context
	*/
	void mbedtls_ecdh_enable_restart( mbedtls_ecdh_context *ctx )
	{
	ctx->restart_enabled = 1;
	}
	#endif

	/*
	* Setup and write the ServerKeyExhange parameters (RFC 4492)
	* struct {
	* ECParameters curve_params;
	* ECPoint public;
	* } ServerECDHParams;
	*/
	int mbedtls_ecdh_make_params( mbedtls_ecdh_context ctx, size_t olen,
	unsigned char *buf, size_t blen,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng )
	{
	int ret;
	size_t grp_len, pt_len;
	mbedtls_ecp_restart_ctx *rs_ctx = NULL;

	if( ctx == NULL \|\| ctx->grp.pbits == 0 )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	#if defined(MBEDTLS_ECP_RESTARTABLE)
	if( ctx->restart_enabled )
	rs_ctx = &ctx->rs;
	#endif

	if( ( ret = ecdh_gen_public_restartable( &ctx->grp, &ctx->d, &ctx->Q,
	f_rng, p_rng, rs_ctx ) ) != 0 )
	return( ret );

	if( ( ret = mbedtls_ecp_tls_write_group( &ctx->grp, &grp_len, buf, blen ) )
	!= 0 )
	return( ret );

	buf += grp_len;
	blen -= grp_len;

	if( ( ret = mbedtls_ecp_tls_write_point( &ctx->grp, &ctx->Q, ctx->point_format,
	&pt_len, buf, blen ) ) != 0 )
	return( ret );

	*olen = grp_len + pt_len;
	return( 0 );
	}

	/*
	* Read the ServerKeyExhange parameters (RFC 4492)
	* struct {
	* ECParameters curve_params;
	* ECPoint public;
	* } ServerECDHParams;
	*/
	int mbedtls_ecdh_read_params( mbedtls_ecdh_context *ctx,
	const unsigned char *buf, const unsigned char end )
	{
	int ret;

	if( ( ret = mbedtls_ecp_tls_read_group( &ctx->grp, buf, end - *buf ) ) != 0 )
	return( ret );

	if( ( ret = mbedtls_ecp_tls_read_point( &ctx->grp, &ctx->Qp, buf, end - *buf ) )
	!= 0 )
	return( ret );

	return( 0 );
	}

	/*
	* Get parameters from a keypair
	*/
	int mbedtls_ecdh_get_params( mbedtls_ecdh_context ctx, const mbedtls_ecp_keypair key,
	mbedtls_ecdh_side side )
	{
	int ret;

	if( ( ret = mbedtls_ecp_group_copy( &ctx->grp, &key->grp ) ) != 0 )
	return( ret );

	/* If it's not our key, just import the public part as Qp */
	if( side == MBEDTLS_ECDH_THEIRS )
	return( mbedtls_ecp_copy( &ctx->Qp, &key->Q ) );

	/* Our key: import public (as Q) and private parts */
	if( side != MBEDTLS_ECDH_OURS )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	if( ( ret = mbedtls_ecp_copy( &ctx->Q, &key->Q ) ) != 0 \|\|
	( ret = mbedtls_mpi_copy( &ctx->d, &key->d ) ) != 0 )
	return( ret );

	return( 0 );
	}

	/*
	* Setup and export the client public value
	*/
	int mbedtls_ecdh_make_public( mbedtls_ecdh_context ctx, size_t olen,
	unsigned char *buf, size_t blen,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng )
	{
	int ret;
	mbedtls_ecp_restart_ctx *rs_ctx = NULL;

	if( ctx == NULL \|\| ctx->grp.pbits == 0 )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	#if defined(MBEDTLS_ECP_RESTARTABLE)
	if( ctx->restart_enabled )
	rs_ctx = &ctx->rs;
	#endif

	if( ( ret = ecdh_gen_public_restartable( &ctx->grp, &ctx->d, &ctx->Q,
	f_rng, p_rng, rs_ctx ) ) != 0 )
	return( ret );

	return mbedtls_ecp_tls_write_point( &ctx->grp, &ctx->Q, ctx->point_format,
	olen, buf, blen );
	}

	/*
	* Parse and import the client's public value
	*/
	int mbedtls_ecdh_read_public( mbedtls_ecdh_context *ctx,
	const unsigned char *buf, size_t blen )
	{
	int ret;
	const unsigned char *p = buf;

	if( ctx == NULL )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	if( ( ret = mbedtls_ecp_tls_read_point( &ctx->grp, &ctx->Qp, &p, blen ) ) != 0 )
	return( ret );

	if( (size_t)( p - buf ) != blen )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	return( 0 );
	}

	/*
	* Derive and export the shared secret
	*/
	int mbedtls_ecdh_calc_secret( mbedtls_ecdh_context ctx, size_t olen,
	unsigned char *buf, size_t blen,
	int (f_rng)(void , unsigned char *, size_t),
	void *p_rng )
	{
	int ret;
	mbedtls_ecp_restart_ctx *rs_ctx = NULL;

	if( ctx == NULL \|\| ctx->grp.pbits == 0 )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	#if defined(MBEDTLS_ECP_RESTARTABLE)
	if( ctx->restart_enabled )
	rs_ctx = &ctx->rs;
	#endif

	if( ( ret = ecdh_compute_shared_restartable( &ctx->grp,
	&ctx->z, &ctx->Qp, &ctx->d, f_rng, p_rng, rs_ctx ) ) != 0 )
	{
	return( ret );
	}

	if( mbedtls_mpi_size( &ctx->z ) > blen )
	return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

	*olen = ctx->grp.pbits / 8 + ( ( ctx->grp.pbits % 8 ) != 0 );
	return mbedtls_mpi_write_binary( &ctx->z, buf, *olen );
	}

	#endif /* MBEDTLS_ECDH_C */