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Remove MBEDTLS_SSL_AEAD_RANDOM_IV feature

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In a USENIX WOOT '16 paper the authors warn about a security risk
of random Initialisation Vectors (IV) repeating values.

The MBEDTLS_SSL_AEAD_RANDOM_IV feature is affected by this risk and
it isn't compliant with RFC5116. Furthermore, strictly speaking it
is a different cipher suite from the TLS (RFC5246) point of view.

Removing the MBEDTLS_SSL_AEAD_RANDOM_IV feature to resolve the above
problems.

Hanno Böck, Aaron Zauner, Sean Devlin, Juraj Somorovsky and Philipp
Jovanovic, "Nonce-Disrespecting Adversaries: Practical Forgery Attacks
on GCM in TLS", USENIX WOOT '16
This commit is contained in:
Janos Follath 2016-09-08 10:44:16 +01:00 committed by Simon Butcher
parent 6d3e3389e5
commit 0be55a0549
4 changed files with 4 additions and 27 deletions
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4
ChangeLog
View file

@ -3,6 +3,10 @@ mbed TLS ChangeLog (Sorted per branch, date)
= mbed TLS 2.1.x branch released 2016-xx-xx
Security
* Remove MBEDTLS_SSL_AEAD_RANDOM_IV option, because it was not compliant
with RFC5116 and could lead to session key recovery in very long TLS
sessions. (H. Bock, A. Zauner, S. Devlin, J. Somorovsky, P. Jovanovic -
"Nonce-Disrespecting Adversaries Practical Forgery Attacks on GCM in TLS")
* Fix potential stack corruption in mbedtls_x509write_crt_der() and
mbedtls_x509write_csr_der() when the signature is copied to the buffer
without checking whether there is enough space in the destination. The

12
include/mbedtls/config.h
View file

@ -868,18 +868,6 @@
*/
//#define MBEDTLS_SHA256_SMALLER
/**
* \def MBEDTLS_SSL_AEAD_RANDOM_IV
*
* Generate a random IV rather than using the record sequence number as a
* nonce for ciphersuites using and AEAD algorithm (GCM or CCM).
*
* Using the sequence number is generally recommended.
*
* Uncomment this macro to always use random IVs with AEAD ciphersuites.
*/
//#define MBEDTLS_SSL_AEAD_RANDOM_IV
/**
* \def MBEDTLS_SSL_ALL_ALERT_MESSAGES
*

12
library/ssl_tls.c
View file

@ -1364,17 +1364,6 @@ static int ssl_encrypt_buf( mbedtls_ssl_context *ssl )
/*
* Generate IV
*/
#if defined(MBEDTLS_SSL_AEAD_RANDOM_IV)
ret = ssl->conf->f_rng( ssl->conf->p_rng,
ssl->transform_out->iv_enc + ssl->transform_out->fixed_ivlen,
ssl->transform_out->ivlen - ssl->transform_out->fixed_ivlen );
if( ret != 0 )
return( ret );
memcpy( ssl->out_iv,
ssl->transform_out->iv_enc + ssl->transform_out->fixed_ivlen,
ssl->transform_out->ivlen - ssl->transform_out->fixed_ivlen );
#else
if( ssl->transform_out->ivlen - ssl->transform_out->fixed_ivlen != 8 )
{
/* Reminder if we ever add an AEAD mode with a different size */
@ -1385,7 +1374,6 @@ static int ssl_encrypt_buf( mbedtls_ssl_context *ssl )
memcpy( ssl->transform_out->iv_enc + ssl->transform_out->fixed_ivlen,
ssl->out_ctr, 8 );
memcpy( ssl->out_iv, ssl->out_ctr, 8 );
#endif
MBEDTLS_SSL_DEBUG_BUF( 4, "IV used", ssl->out_iv,
ssl->transform_out->ivlen - ssl->transform_out->fixed_ivlen );

3
library/version_features.c
View file

@ -309,9 +309,6 @@ static const char *features[] = {
#if defined(MBEDTLS_SHA256_SMALLER)
"MBEDTLS_SHA256_SMALLER",
#endif /* MBEDTLS_SHA256_SMALLER */
#if defined(MBEDTLS_SSL_AEAD_RANDOM_IV)
"MBEDTLS_SSL_AEAD_RANDOM_IV",
#endif /* MBEDTLS_SSL_AEAD_RANDOM_IV */
#if defined(MBEDTLS_SSL_ALL_ALERT_MESSAGES)
"MBEDTLS_SSL_ALL_ALERT_MESSAGES",
#endif /* MBEDTLS_SSL_ALL_ALERT_MESSAGES */