Enforce dhm_min_bitlen exactly, not just the byte size
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>
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index bd7f281..073311b 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -2209,6 +2209,7 @@
unsigned char *end )
{
int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
+ size_t dhm_actual_bitlen;
/*
* Ephemeral DH parameters:
@@ -2226,10 +2227,11 @@
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",
- ssl->handshake->dhm_ctx.len * 8,
+ MBEDTLS_SSL_DEBUG_MSG( 1, ( "DHM prime too short: %u < %u",
+ (unsigned) dhm_actual_bitlen,
ssl->conf->dhm_min_bitlen ) );
return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
}