Fix leakage of projective coordinates in ECC
Fix side channel in ECC code that allowed an adversary with access to
precise enough timing and memory access information (typically an
untrusted operating system attacking a secure enclave) to fully recover
an ECDSA private key. Found and reported by Alejandro Cabrera Aldaya,
Billy Brumley and Cesar Pereida Garcia. CVE-2020-10932
See the comments in the code for how an attack would go.
For ECDSA, leaking a few bits of the scalar over several signatures
translates to full private key recovery using a lattice attack.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index 19942e5..3a9e501 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -2016,6 +2016,20 @@
final_norm:
#endif
+ /*
+ * Knowledge of the jacobian coordinates may leak the last few bits of the
+ * scalar [1], and since our MPI implementation isn't constant-flow,
+ * inversion (used for coordinate normalization) may leak the full value
+ * of its input via side-channels [2].
+ *
+ * [1] https://eprint.iacr.org/2003/191
+ * [2] https://eprint.iacr.org/2020/055
+ *
+ * Avoid the leak by randomizing coordinates before we normalize them.
+ */
+ if( f_rng != 0 )
+ MBEDTLS_MPI_CHK( ecp_randomize_jac( grp, RR, f_rng, p_rng ) );
+
MBEDTLS_ECP_BUDGET( MBEDTLS_ECP_OPS_INV );
MBEDTLS_MPI_CHK( ecp_normalize_jac( grp, RR ) );
@@ -2388,6 +2402,20 @@
MBEDTLS_MPI_CHK( mbedtls_mpi_safe_cond_swap( &R->Z, &RP.Z, b ) );
}
+ /*
+ * Knowledge of the projective coordinates may leak the last few bits of the
+ * scalar [1], and since our MPI implementation isn't constant-flow,
+ * inversion (used for coordinate normalization) may leak the full value
+ * of its input via side-channels [2].
+ *
+ * [1] https://eprint.iacr.org/2003/191
+ * [2] https://eprint.iacr.org/2020/055
+ *
+ * Avoid the leak by randomizing coordinates before we normalize them.
+ */
+ if( f_rng != NULL )
+ MBEDTLS_MPI_CHK( ecp_randomize_mxz( grp, R, f_rng, p_rng ) );
+
MBEDTLS_MPI_CHK( ecp_normalize_mxz( grp, R ) );
cleanup: