Merge branch 'iotssl-1272-fix-RSA-cache-attack-2.1-restricted' into mbedtls-2.1
* iotssl-1272-fix-RSA-cache-attack-2.1-restricted:
Add Changelog entry for RSA exponent blinding
Add exponent blinding to RSA with CRT
Add exponent blinding to RSA without CRT
diff --git a/ChangeLog b/ChangeLog
index 3e9bc2b..8adb468 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,13 @@
= mbed TLS 2.1.x branch released xxxx-xx-xx
+Security
+ * Add exponent blinding to RSA private operations as a countermeasure
+ against side-channel attacks like the cache attack described in
+ https://arxiv.org/abs/1702.08719v2.
+ Found and fix proposed by Michael Schwarz, Samuel Weiser, Daniel Gruss,
+ Clémentine Maurice and Stefan Mangard.
+
Bugfix
* Remove macros from compat-1.3.h that correspond to deleted items from most
recent versions of the library. Found by Kyle Keen.
diff --git a/library/rsa.c b/library/rsa.c
index e831875..a4afb2a 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -23,6 +23,11 @@
*
* http://theory.lcs.mit.edu/~rivest/rsapaper.pdf
* http://www.cacr.math.uwaterloo.ca/hac/about/chap8.pdf
+ * [3] Malware Guard Extension: Using SGX to Conceal Cache Attacks
+ * Michael Schwarz, Samuel Weiser, Daniel Gruss, Clémentine Maurice and
+ * Stefan Mangard
+ * https://arxiv.org/abs/1702.08719v2
+ *
*/
#if !defined(MBEDTLS_CONFIG_FILE)
@@ -351,6 +356,27 @@
}
/*
+ * Exponent blinding supposed to prevent side-channel attacks using multiple
+ * traces of measurements to recover the RSA key. The more collisions are there,
+ * the more bits of the key can be recovered. See [3].
+ *
+ * Collecting n collisions with m bit long blinding value requires 2^(m-m/n)
+ * observations on avarage.
+ *
+ * For example with 28 byte blinding to achieve 2 collisions the adversary has
+ * to make 2^112 observations on avarage.
+ *
+ * (With the currently (as of 2017 April) known best algorithms breaking 2048
+ * bit RSA requires approximately as much time as trying out 2^112 random keys.
+ * Thus in this sense with 28 byte blinding the security is not reduced by
+ * side-channel attacks like the one in [3])
+ *
+ * This countermeasure does not help if the key recovery is possible with a
+ * single trace.
+ */
+#define RSA_EXPONENT_BLINDING 28
+
+/*
* Do an RSA private key operation
*/
int mbedtls_rsa_private( mbedtls_rsa_context *ctx,
@@ -362,12 +388,34 @@
int ret;
size_t olen;
mbedtls_mpi T, T1, T2;
+ mbedtls_mpi P1, Q1, R;
+#if defined(MBEDTLS_RSA_NO_CRT)
+ mbedtls_mpi D_blind;
+ mbedtls_mpi *D = &ctx->D;
+#else
+ mbedtls_mpi DP_blind, DQ_blind;
+ mbedtls_mpi *DP = &ctx->DP;
+ mbedtls_mpi *DQ = &ctx->DQ;
+#endif
/* Make sure we have private key info, prevent possible misuse */
if( ctx->P.p == NULL || ctx->Q.p == NULL || ctx->D.p == NULL )
return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
mbedtls_mpi_init( &T ); mbedtls_mpi_init( &T1 ); mbedtls_mpi_init( &T2 );
+ mbedtls_mpi_init( &P1 ); mbedtls_mpi_init( &Q1 ); mbedtls_mpi_init( &R );
+
+
+ if( f_rng != NULL )
+ {
+#if defined(MBEDTLS_RSA_NO_CRT)
+ mbedtls_mpi_init( &D_blind );
+#else
+ mbedtls_mpi_init( &DP_blind );
+ mbedtls_mpi_init( &DQ_blind );
+#endif
+ }
+
#if defined(MBEDTLS_THREADING_C)
if( ( ret = mbedtls_mutex_lock( &ctx->mutex ) ) != 0 )
@@ -390,19 +438,60 @@
MBEDTLS_MPI_CHK( rsa_prepare_blinding( ctx, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &T, &T, &ctx->Vi ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &T, &T, &ctx->N ) );
+
+ /*
+ * Exponent blinding
+ */
+ MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( &P1, &ctx->P, 1 ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( &Q1, &ctx->Q, 1 ) );
+
+#if defined(MBEDTLS_RSA_NO_CRT)
+ /*
+ * D_blind = ( P - 1 ) * ( Q - 1 ) * R + D
+ */
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &R, RSA_EXPONENT_BLINDING,
+ f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &D_blind, &P1, &Q1 ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &D_blind, &D_blind, &R ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &D_blind, &D_blind, &ctx->D ) );
+
+ D = &D_blind;
+#else
+ /*
+ * DP_blind = ( P - 1 ) * R + DP
+ */
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &R, RSA_EXPONENT_BLINDING,
+ f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &DP_blind, &P1, &R ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &DP_blind, &DP_blind,
+ &ctx->DP ) );
+
+ DP = &DP_blind;
+
+ /*
+ * DQ_blind = ( Q - 1 ) * R + DQ
+ */
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( &R, RSA_EXPONENT_BLINDING,
+ f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &DQ_blind, &Q1, &R ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &DQ_blind, &DQ_blind,
+ &ctx->DQ ) );
+
+ DQ = &DQ_blind;
+#endif /* MBEDTLS_RSA_NO_CRT */
}
#if defined(MBEDTLS_RSA_NO_CRT)
- MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &T, &T, &ctx->D, &ctx->N, &ctx->RN ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &T, &T, D, &ctx->N, &ctx->RN ) );
#else
/*
- * faster decryption using the CRT
+ * Faster decryption using the CRT
*
* T1 = input ^ dP mod P
* T2 = input ^ dQ mod Q
*/
- MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &T1, &T, &ctx->DP, &ctx->P, &ctx->RP ) );
- MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &T2, &T, &ctx->DQ, &ctx->Q, &ctx->RQ ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &T1, &T, DP, &ctx->P, &ctx->RP ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &T2, &T, DQ, &ctx->Q, &ctx->RQ ) );
/*
* T = (T1 - T2) * (Q^-1 mod P) mod P
@@ -438,6 +527,17 @@
#endif
mbedtls_mpi_free( &T ); mbedtls_mpi_free( &T1 ); mbedtls_mpi_free( &T2 );
+ mbedtls_mpi_free( &P1 ); mbedtls_mpi_free( &Q1 ); mbedtls_mpi_free( &R );
+
+ if( f_rng != NULL )
+ {
+#if defined(MBEDTLS_RSA_NO_CRT)
+ mbedtls_mpi_free( &D_blind );
+#else
+ mbedtls_mpi_free( &DP_blind );
+ mbedtls_mpi_free( &DQ_blind );
+#endif
+ }
if( ret != 0 )
return( MBEDTLS_ERR_RSA_PRIVATE_FAILED + ret );