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Add fallback to non-compliant truncated HMAC for compatibiltiy

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In case truncated HMAC must be used but the Mbed TLS peer hasn't been updated
yet, one can use the compile-time option MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT to
temporarily fall back to the old, non-compliant implementation of the truncated
HMAC extension.
This commit is contained in:
Hanno Becker 2017-11-20 16:36:41 +00:00
parent e9dcb843b2
commit 053b3459d4
3 changed files with 28 additions and 0 deletions
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4
include/mbedtls/check_config.h
View File

@ -77,6 +77,10 @@
#error "MBEDTLS_DHM_C defined, but not all prerequisites"
#endif
#if defined(MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT) && !defined(MBEDTLS_SSL_TRUNCATED_HMAC)
#error "MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT defined, but not all prerequisites"
#endif
#if defined(MBEDTLS_ECDH_C) && !defined(MBEDTLS_ECP_C)
#error "MBEDTLS_ECDH_C defined, but not all prerequisites"
#endif

16
include/mbedtls/config.h
View File

@ -1183,6 +1183,22 @@
*/
#define MBEDTLS_SSL_TRUNCATED_HMAC
/**
* \def MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
*
* Fallback to old, non-conforming implementation of the truncated
* HMAC extension which also truncates the HMAC key.
*
* \warning This should only be enabled temporarily when the use
* of truncated HMAC is mandatory *and* the peer is an Mbed TLS
* stack that doesn't use the fixed implementation yet.
*
* Uncomment to fallback to old, non-compliant truncated HMAC implementation.
*
* Requires: MBEDTLS_SSL_TRUNCATED_HMAC
*/
//#define MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT
/**
* \def MBEDTLS_THREADING_ALT
*

8
library/ssl_tls.c
View File

@ -714,7 +714,15 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
* so we only need to adjust the length here.
*/
if( session->trunc_hmac == MBEDTLS_SSL_TRUNC_HMAC_ENABLED )
{
transform->maclen = MBEDTLS_SSL_TRUNCATED_HMAC_LEN;
#if defined(MBEDTLS_SSL_TRUNCATED_HMAC_COMPAT)
/* Fall back to old, non-compliant version of the truncated
* HMAC implementation which also truncates the key. */
mac_key_len = transform->maclen;
#endif
}
#endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
/* IV length */