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mbedtls/library/ssl_tls13_keys.c
Hanno Becker cd817b0630 Remove misleading comment in TLS 1.3 key schedule code 
The implementation documentation of

```
   mbedtls_ssl_tls1_3_derive_early_secrets()
```

mentioned the PSK binder key, which is misleading because the
function doesn't actually calculate it.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
2021-05-31 19:40:45 +01:00

702 lines
24 KiB
C
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/*
* 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 "mbedtls/debug.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 );
}
int mbedtls_ssl_tls1_3_derive_early_secrets(
mbedtls_md_type_t md_type,
unsigned char const *early_secret,
unsigned char const *transcript, size_t transcript_len,
mbedtls_ssl_tls1_3_early_secrets *derived )
{
int ret;
mbedtls_md_info_t const * const md_info = mbedtls_md_info_from_type( md_type );
size_t const md_size = mbedtls_md_get_size( md_info );
/* We should never call this function with an unknown hash,
* but add an assertion anyway. */
if( md_info == 0 )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
/*
* 0
* |
* v
* PSK -> HKDF-Extract = Early Secret
* |
* +-----> Derive-Secret(., "c e traffic", ClientHello)
* | = client_early_traffic_secret
* |
* +-----> Derive-Secret(., "e exp master", ClientHello)
* | = early_exporter_master_secret
* v
*/
/* Create client_early_traffic_secret */
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
early_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( c_e_traffic ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->client_early_traffic_secret,
md_size );
if( ret != 0 )
return( ret );
/* Create early exporter */
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
early_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( e_exp_master ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->early_exporter_master_secret,
md_size );
if( ret != 0 )
return( ret );
return( 0 );
}
int mbedtls_ssl_tls1_3_derive_handshake_secrets(
mbedtls_md_type_t md_type,
unsigned char const *handshake_secret,
unsigned char const *transcript, size_t transcript_len,
mbedtls_ssl_tls1_3_handshake_secrets *derived )
{
int ret;
mbedtls_md_info_t const * const md_info = mbedtls_md_info_from_type( md_type );
size_t const md_size = mbedtls_md_get_size( md_info );
/* We should never call this function with an unknown hash,
* but add an assertion anyway. */
if( md_info == 0 )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
/*
*
* Handshake Secret
* |
* +-----> Derive-Secret( ., "c hs traffic",
* | ClientHello...ServerHello )
* | = client_handshake_traffic_secret
* |
* +-----> Derive-Secret( ., "s hs traffic",
* | ClientHello...ServerHello )
* | = server_handshake_traffic_secret
*
*/
/*
* Compute client_handshake_traffic_secret with
* Derive-Secret( ., "c hs traffic", ClientHello...ServerHello )
*/
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
handshake_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( c_hs_traffic ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->client_handshake_traffic_secret,
md_size );
if( ret != 0 )
return( ret );
/*
* Compute server_handshake_traffic_secret with
* Derive-Secret( ., "s hs traffic", ClientHello...ServerHello )
*/
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
handshake_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( s_hs_traffic ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->server_handshake_traffic_secret,
md_size );
if( ret != 0 )
return( ret );
return( 0 );
}
int mbedtls_ssl_tls1_3_derive_application_secrets(
mbedtls_md_type_t md_type,
unsigned char const *application_secret,
unsigned char const *transcript, size_t transcript_len,
mbedtls_ssl_tls1_3_application_secrets *derived )
{
int ret;
mbedtls_md_info_t const * const md_info = mbedtls_md_info_from_type( md_type );
size_t const md_size = mbedtls_md_get_size( md_info );
/* We should never call this function with an unknown hash,
* but add an assertion anyway. */
if( md_info == 0 )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
/* Generate {client,server}_application_traffic_secret_0
*
* Master Secret
* |
* +-----> Derive-Secret( ., "c ap traffic",
* | ClientHello...server Finished )
* | = client_application_traffic_secret_0
* |
* +-----> Derive-Secret( ., "s ap traffic",
* | ClientHello...Server Finished )
* | = server_application_traffic_secret_0
* |
* +-----> Derive-Secret( ., "exp master",
* | ClientHello...server Finished)
* | = exporter_master_secret
*
*/
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
application_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( c_ap_traffic ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->client_application_traffic_secret_N,
md_size );
if( ret != 0 )
return( ret );
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
application_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( s_ap_traffic ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->server_application_traffic_secret_N,
md_size );
if( ret != 0 )
return( ret );
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
application_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( exp_master ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->exporter_master_secret,
md_size );
if( ret != 0 )
return( ret );
return( 0 );
}
/* Generate resumption_master_secret for use with the ticket exchange.
*
* This is not integrated with mbedtls_ssl_tls1_3_derive_application_secrets()
* because it uses the transcript hash up to and including ClientFinished. */
int mbedtls_ssl_tls1_3_derive_resumption_master_secret(
mbedtls_md_type_t md_type,
unsigned char const *application_secret,
unsigned char const *transcript, size_t transcript_len,
mbedtls_ssl_tls1_3_application_secrets *derived )
{
int ret;
mbedtls_md_info_t const * const md_info = mbedtls_md_info_from_type( md_type );
size_t const md_size = mbedtls_md_get_size( md_info );
/* We should never call this function with an unknown hash,
* but add an assertion anyway. */
if( md_info == 0 )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
application_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( res_master ),
transcript, transcript_len,
MBEDTLS_SSL_TLS1_3_CONTEXT_HASHED,
derived->resumption_master_secret,
md_size );
if( ret != 0 )
return( ret );
return( 0 );
}
static int ssl_tls1_3_calc_finished_core( mbedtls_md_type_t md_type,
unsigned char const *base_key,
unsigned char const *transcript,
unsigned char *dst )
{
const mbedtls_md_info_t* const md_info = mbedtls_md_info_from_type( md_type );
size_t const md_size = mbedtls_md_get_size( md_info );
unsigned char finished_key[MBEDTLS_MD_MAX_SIZE];
int ret;
/* We should never call this function with an unknown hash,
* but add an assertion anyway. */
if( md_info == 0 )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
/* TLS 1.3 Finished message
*
* struct {
* opaque verify_data[Hash.length];
* } Finished;
*
* verify_data =
* HMAC( finished_key,
* Hash( Handshake Context +
* Certificate* +
* CertificateVerify* )
* )
*
* finished_key =
* HKDF-Expand-Label( BaseKey, "finished", "", Hash.length )
*/
ret = mbedtls_ssl_tls1_3_hkdf_expand_label(
md_type, base_key, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( finished ),
NULL, 0,
finished_key, md_size );
if( ret != 0 )
goto exit;
ret = mbedtls_md_hmac( md_info, finished_key, md_size, transcript, md_size, dst );
if( ret != 0 )
goto exit;
exit:
mbedtls_platform_zeroize( finished_key, sizeof( finished_key ) );
return( ret );
}
int mbedtls_ssl_tls1_3_create_psk_binder( mbedtls_ssl_context *ssl,
const mbedtls_md_type_t md_type,
unsigned char const *psk, size_t psk_len,
int psk_type,
unsigned char const *transcript,
unsigned char *result )
{
int ret = 0;
unsigned char binder_key[MBEDTLS_MD_MAX_SIZE];
unsigned char early_secret[MBEDTLS_MD_MAX_SIZE];
mbedtls_md_info_t const *md_info = mbedtls_md_info_from_type( md_type );
size_t const md_size = mbedtls_md_get_size( md_info );
#if !defined(MBEDTLS_DEBUG_C)
ssl = NULL; /* make sure we don't use it except for debug */
((void) ssl);
#endif
/* We should never call this function with an unknown hash,
* but add an assertion anyway. */
if( md_info == 0 )
return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
/*
* 0
* |
* v
* PSK -> HKDF-Extract = Early Secret
* |
* +-----> Derive-Secret(., "ext binder" | "res binder", "")
* | = binder_key
* v
*/
ret = mbedtls_ssl_tls1_3_evolve_secret( md_type,
NULL, /* Old secret */
psk, psk_len, /* Input */
early_secret );
if( ret != 0 )
{
MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_tls1_3_evolve_secret", ret );
goto exit;
}
if( psk_type == MBEDTLS_SSL_TLS1_3_PSK_RESUMPTION )
{
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
early_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( res_binder ),
NULL, 0, MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED,
binder_key, md_size );
MBEDTLS_SSL_DEBUG_MSG( 4, ( "Derive Early Secret with 'res binder'" ) );
}
else
{
ret = mbedtls_ssl_tls1_3_derive_secret( md_type,
early_secret, md_size,
MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( ext_binder ),
NULL, 0, MBEDTLS_SSL_TLS1_3_CONTEXT_UNHASHED,
binder_key, md_size );
MBEDTLS_SSL_DEBUG_MSG( 4, ( "Derive Early Secret with 'ext binder'" ) );
}
if( ret != 0 )
{
MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_tls1_3_derive_secret", ret );
goto exit;
}
/*
* The binding_value is computed in the same way as the Finished message
* but with the BaseKey being the binder_key.
*/
ret = ssl_tls1_3_calc_finished_core( md_type, binder_key, transcript, result );
if( ret != 0 )
goto exit;
MBEDTLS_SSL_DEBUG_BUF( 3, "psk binder", result, md_size );
exit:
mbedtls_platform_zeroize( early_secret, sizeof( early_secret ) );
mbedtls_platform_zeroize( binder_key, sizeof( binder_key ) );
return( ret );
}
#endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */
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