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Introduce configuration option for TLS 1.3 padding granularity

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TLS 1.3 record protection allows the addition of an arbitrary amount
of padding.

This commit introduces a configuration option

```
   MBEDTLS_SSL_TLS13_PADDING_GRANULARITY
```

The semantics of this option is that padding is chosen in a minimal
way so that the padded plaintext has a length which is a multiple of
MBEDTLS_SSL_TLS13_PADDING_GRANULARITY.

For example, setting MBEDTLS_SSL_TLS13_PADDING_GRANULARITY to 1024
means that padded plaintexts will have length 1024, 2048, ..., while
setting it to 1 means that no padding will be used.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2020-05-28 16:15:19 +01:00
parent b54094bd7c
commit 13996927cb
3 changed files with 41 additions and 8 deletions
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16
include/mbedtls/config.h
View File

@ -3554,6 +3554,22 @@
*/
//#define MBEDTLS_SSL_CID_PADDING_GRANULARITY 16
/** \def MBEDTLS_SSL_TLS13_PADDING_GRANULARITY
*
* This option controls the use of record plaintext padding
* in TLS 1.3.
*
* The padding will always be chosen so that the length of the
* padded plaintext is a multiple of the value of this option.
*
* Note: A value of \c 1 means that no padding will be used
* for outgoing records.
*
* Note: On systems lacking division instructions,
* a power of two should be preferred.
*/
//#define MBEDTLS_SSL_TLS13_PADDING_GRANULARITY 16
/** \def MBEDTLS_SSL_OUT_CONTENT_LEN
*
* Maximum length (in bytes) of outgoing plaintext fragments.

4
include/mbedtls/ssl.h
View File

@ -277,6 +277,10 @@
#define MBEDTLS_SSL_CID_PADDING_GRANULARITY 16
#endif
#if !defined(MBEDTLS_SSL_TLS13_PADDING_GRANULARITY)
#define MBEDTLS_SSL_TLS13_PADDING_GRANULARITY 16
#endif
/* \} name SECTION: Module settings */
/*

25
library/ssl_msg.c
View File

@ -343,6 +343,13 @@ static void ssl_read_memory( unsigned char *p, size_t len )
#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID) || \
defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)
static size_t ssl_compute_padding_length( size_t len,
size_t granularity )
{
return( ( granularity - ( len + 1 ) % granularity ) % granularity );
}
/* This functions transforms a (D)TLS plaintext fragment and a record content
* type into an instance of the (D)TLSInnerPlaintext structure. This is used
* in DTLS 1.2 + CID and within TLS 1.3 to allow flexible padding and to protect
@ -374,12 +381,10 @@ static void ssl_read_memory( unsigned char *p, size_t len )
static int ssl_build_inner_plaintext( unsigned char *content,
size_t *content_size,
size_t remaining,
uint8_t rec_type )
uint8_t rec_type,
size_t pad )
{
size_t len = *content_size;
size_t pad = ( MBEDTLS_SSL_CID_PADDING_GRANULARITY -
( len + 1 ) % MBEDTLS_SSL_CID_PADDING_GRANULARITY ) %
MBEDTLS_SSL_CID_PADDING_GRANULARITY;
/* Write real content type */
if( remaining == 0 )
@ -651,10 +656,14 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
#if defined(MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL)
if( transform->minor_ver == MBEDTLS_SSL_MINOR_VERSION_4 )
{
size_t padding =
ssl_compute_padding_length( rec->data_len,
MBEDTLS_SSL_TLS13_PADDING_GRANULARITY );
if( ssl_build_inner_plaintext( data,
&rec->data_len,
post_avail,
rec->type ) != 0 )
rec->type,
padding ) != 0 )
{
return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
}
@ -673,6 +682,9 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
if( rec->cid_len != 0 )
{
size_t padding =
ssl_compute_padding_length( rec->data_len,
MBEDTLS_SSL_CID_PADDING_GRANULARITY );
/*
* Wrap plaintext into DTLSInnerPlaintext structure.
* See ssl_build_inner_plaintext() for more information.
@ -683,7 +695,8 @@ int mbedtls_ssl_encrypt_buf( mbedtls_ssl_context *ssl,
if( ssl_build_inner_plaintext( data,
&rec->data_len,
post_avail,
rec->type ) != 0 )
rec->type,
padding ) != 0 )
{
return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );
}