From f60041688c8af902c59930afd25952f1003bcdcb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?=
<manuel.pegourie-gonnard@arm.com>
Date: Wed, 25 Mar 2020 12:41:29 +0100
Subject: [PATCH] Fix leakage of projective coordinates in ECC
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See the comments in the code for how an attack would go, and the ChangeLog
entry for an impact assessment. (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>
---
ChangeLog | 7 +++++++
library/ecp.c | 28 ++++++++++++++++++++++++++++
2 files changed, 35 insertions(+)
diff --git a/ChangeLog b/ChangeLog
index 6ec1e7ec9..64c72a56d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,13 @@ mbed TLS ChangeLog (Sorted per branch, date)
= mbed TLS x.x.x branch released xxxx-xx-xx
+Security
+ * 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
+
Bugfix
* Fix compilation failure when both MBEDTLS_SSL_PROTO_DTLS and
MBEDTLS_SSL_HW_RECORD_ACCEL are enabled.
diff --git a/library/ecp.c b/library/ecp.c
index 040c20bd3..725e176df 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1938,6 +1938,20 @@ static int ecp_mul_comb_after_precomp( const mbedtls_ecp_group *grp,
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 ) );
@@ -2308,6 +2322,20 @@ static int ecp_mul_mxz( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
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: