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Implement TLS 1.3 key derivation function HKDF-Expand-Label

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This commit introduces a new file library/ssl_tls13_key.c
which will subsequently be populated with functionality relating
to the TLS 1.3 key schedule.

Those functions are expected to be internal and are documented
in the internal header library/ssl_tls13_keys.h.

The first function to be implemented is the key expansion
function `HKDF-Expand-Label`. See the documentation in
library/ssl_tls13_keys.h for more information.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2020-08-21 13:20:06 +01:00
parent 27a2688fbb
commit be9d6648f8
5 changed files with 292 additions and 0 deletions
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1
library/CMakeLists.txt
View file

@ -103,6 +103,7 @@ set(src_tls
ssl_srv.c
ssl_ticket.c
ssl_tls.c
ssl_tls13_keys.c
)
if(CMAKE_COMPILER_IS_GNUCC)

1
library/Makefile
View file

@ -162,6 +162,7 @@ OBJS_TLS= \
ssl_srv.o \
ssl_ticket.o \
ssl_tls.o \
ssl_tls13_keys.o \
# This line is intentionally left blank
.SILENT:

182
library/ssl_tls13_keys.c Normal file
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@ -0,0 +1,182 @@
/*
* 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.
*/
#if !defined(MBEDTLS_CONFIG_FILE)
#include "mbedtls/config.h"
#else
#include MBEDTLS_CONFIG_FILE
#endif
#if defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)
#include "mbedtls/hkdf.h"
#include "ssl_tls13_keys.h"
#include <stdint.h>
#include <string.h>
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. */
.finished = "finished",
.resumption = "resumption",
.traffic_upd = "traffic upd",
.export = "exporter",
.key = "key",
.iv = "iv",
.sn = "sn",
.c_hs_traffic = "c hs traffic",
.c_ap_traffic = "c ap traffic",
.c_e_traffic = "c e traffic",
.s_hs_traffic = "s hs traffic",
.s_ap_traffic = "s ap traffic",
.s_e_traffic = "s e traffic",
.exp_master = "exp master",
.res_master = "res master",
.ext_binder = "ext binder",
.res_binder = "res binder",
.derived = "derived"
};
/*
* 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
* <= MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN
* It is the caller's responsiblity to ensure this.
* - (ctx, clen): context + context length
* The context length MUST be
* <= MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN
* It is the caller's responsiblity 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.
*/
#define SSL_TLS1_3_KEY_SCHEDULE_MAX_HKDF_LABEL_LEN \
( 2 /* expansion length */ \
+ 1 /* label length */ \
+ MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN \
+ 1 /* context length */ \
+ 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 )
{
const char label_prefix[6] = { 't', 'l', 's', '1', '3', ' ' };
size_t total_label_len = sizeof( label_prefix ) + llen;
size_t total_hkdf_lbl_len =
2 /* length of expanded key material */
+ 1 /* label length */
+ total_label_len /* actual label, incl. prefix */
+ 1 /* context length */
+ clen; /* actual context */
unsigned char *p = dst;
/* Add total length. */
*p++ = 0;
*p++ = (unsigned char)( ( desired_length >> 0 ) & 0xFF );
/* Add label incl. prefix */
*p++ = (unsigned char)( total_label_len & 0xFF );
memcpy( p, label_prefix, sizeof(label_prefix) );
p += sizeof(label_prefix);
memcpy( p, label, llen );
p += llen;
/* Add context value */
*p++ = (unsigned char)( clen & 0xFF );
if( ctx != NULL )
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 ) );
}
#endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */

106
library/ssl_tls13_keys.h Normal file
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@ -0,0 +1,106 @@
/*
* 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.
*/
#if !defined(MBEDTLS_SSL_TLS1_3_KEYS_H)
#define MBEDTLS_SSL_TLS1_3_KEYS_H
#if defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)
#define MBEDTLS_SSL_TLS1_3_LABEL_LIST \
const unsigned char finished [ sizeof("finished") - 1 ]; \
const unsigned char resumption [ sizeof("resumption") - 1 ]; \
const unsigned char traffic_upd [ sizeof("traffic upd") - 1 ]; \
const unsigned char export [ sizeof("exporter") - 1 ]; \
const unsigned char key [ sizeof("key") - 1 ]; \
const unsigned char iv [ sizeof("iv") - 1 ]; \
const unsigned char sn [ sizeof("sn") - 1 ]; \
const unsigned char c_hs_traffic[ sizeof("c hs traffic") - 1 ]; \
const unsigned char c_ap_traffic[ sizeof("c ap traffic") - 1 ]; \
const unsigned char c_e_traffic [ sizeof("c e traffic") - 1 ]; \
const unsigned char s_hs_traffic[ sizeof("s hs traffic") - 1 ]; \
const unsigned char s_ap_traffic[ sizeof("s ap traffic") - 1 ]; \
const unsigned char s_e_traffic [ sizeof("s e traffic") - 1 ]; \
const unsigned char exp_master [ sizeof("exp master") - 1 ]; \
const unsigned char res_master [ sizeof("res master") - 1 ]; \
const unsigned char ext_binder [ sizeof("ext binder") - 1 ]; \
const unsigned char res_binder [ sizeof("res binder") - 1 ]; \
const unsigned char derived [ sizeof("derived") - 1 ]; \
union mbedtls_ssl_tls1_3_labels_union
{
MBEDTLS_SSL_TLS1_3_LABEL_LIST
};
struct mbedtls_ssl_tls1_3_labels_struct
{
MBEDTLS_SSL_TLS1_3_LABEL_LIST
};
extern const struct mbedtls_ssl_tls1_3_labels_struct mbedtls_ssl_tls1_3_labels;
#define MBEDTLS_SSL_TLS1_3_LBL_WITH_LEN( LABEL ) \
mbedtls_ssl_tls1_3_labels.LABEL, \
sizeof(mbedtls_ssl_tls1_3_labels.LABEL)
#define MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_LABEL_LEN \
sizeof( union mbedtls_ssl_tls1_3_labels_union )
/* The maximum length of HKDF contexts used in the TLS 1.3 standad.
* Since contexts are always hashes of message transcripts, this can
* be approximated from above by the maximum hash size. */
#define MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_CONTEXT_LEN \
MBEDTLS_MD_MAX_SIZE
/* Maximum desired length for expanded key material generated
* by HKDF-Expand-Label. */
#define MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN 255
/**
* \brief The \c HKDF-Expand-Label function from
* the TLS 1.3 standard RFC 8446.
*
* <tt>
* HKDF-Expand-Label( Secret, Label, Context, Length ) =
* HKDF-Expand( Secret, HkdfLabel, Length )
* </tt>
*
* \param hash_alg The identifier for the hash algorithm to use.
* \param secret The \c Secret argument to \c HKDF-Expand-Label.
* This must be a readable buffer of length \p slen Bytes.
* \param slen The length of \p secret in Bytes.
* \param label The \c Label argument to \c HKDF-Expand-Label.
* This must be a readable buffer of length \p llen Bytes.
* \param llen The length of \p label in Bytes.
* \param ctx The \c Context argument to \c HKDF-Expand-Label.
* This must be a readable buffer of length \p clen Bytes.
* \param clen The length of \p context in Bytes.
* \param buf The destination buffer to hold the expanded secret.
* This must be a writable buffe of length \p blen Bytes.
* \param blen The desired size of the expanded secret in Bytes.
*
* \returns \c 0 on success.
* \return A negative error code on failure.
*/
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 );
#endif /* MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL */
#endif /* MBEDTLS_SSL_TLS1_3_KEYS_H */

2
visualc/VS2010/mbedTLS.vcxproj
View file

@ -247,6 +247,7 @@
<ClInclude Include="..\..\library\psa_crypto_slot_management.h" />
<ClInclude Include="..\..\library\psa_crypto_storage.h" />
<ClInclude Include="..\..\library\ssl_invasive.h" />
<ClInclude Include="..\..\library\ssl_tls13_keys.h" />
<ClInclude Include="..\..\3rdparty\everest\include\everest\everest.h" />
<ClInclude Include="..\..\3rdparty\everest\include\everest\Hacl_Curve25519.h" />
<ClInclude Include="..\..\3rdparty\everest\include\everest\kremlib.h" />
@ -325,6 +326,7 @@
<ClCompile Include="..\..\library\ssl_srv.c" />
<ClCompile Include="..\..\library\ssl_ticket.c" />
<ClCompile Include="..\..\library\ssl_tls.c" />
<ClCompile Include="..\..\library\ssl_tls13_keys.c" />
<ClCompile Include="..\..\library\threading.c" />
<ClCompile Include="..\..\library\timing.c" />
<ClCompile Include="..\..\library\version.c" />