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Enforce dhm_min_bitlen exactly, not just the byte size

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In a TLS client, enforce the Diffie-Hellman minimum parameter size
set with mbedtls_ssl_conf_dhm_min_bitlen() precisely. Before, the
minimum size was rounded down to the nearest multiple of 8.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2020-12-08 22:46:11 +01:00
parent 3e7b61c42b
commit 6527cd382b
2 changed files with 9 additions and 3 deletions
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4
ChangeLog.d/dhm_min_bitlen.txt Normal file
View file

@ -0,0 +1,4 @@
Bugfix
* In a TLS client, enforce the Diffie-Hellman minimum parameter size
set with mbedtls_ssl_conf_dhm_min_bitlen() precisely. Before, the
minimum size was rounded down to the nearest multiple of 8.

8
library/ssl_cli.c
View file

@ -2209,6 +2209,7 @@ static int ssl_parse_server_dh_params( mbedtls_ssl_context *ssl,
unsigned char *end ) unsigned char *end )
{ {
int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE; int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
size_t dhm_actual_bitlen;
/* /*
* Ephemeral DH parameters: * Ephemeral DH parameters:
@ -2226,10 +2227,11 @@ static int ssl_parse_server_dh_params( mbedtls_ssl_context *ssl,
return( ret ); return( ret );
} }
if( ssl->handshake->dhm_ctx.len * 8 < ssl->conf->dhm_min_bitlen ) dhm_actual_bitlen = mbedtls_mpi_bitlen( &ssl->handshake->dhm_ctx.P );
if( dhm_actual_bitlen < ssl->conf->dhm_min_bitlen )
{ {
MBEDTLS_SSL_DEBUG_MSG( 1, ( "DHM prime too short: %d < %d", MBEDTLS_SSL_DEBUG_MSG( 1, ( "DHM prime too short: %u < %u",
ssl->handshake->dhm_ctx.len * 8, (unsigned) dhm_actual_bitlen,
ssl->conf->dhm_min_bitlen ) ); ssl->conf->dhm_min_bitlen ) );
return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE ); return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
} }