From 54587fcf9bca776ed2c9f8f4aee94fa6780d9f08 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= Date: Wed, 1 Apr 2020 11:25:51 +0200 Subject: [PATCH] Fix leakage of projective coordinates in ECC MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- ChangeLog | 7 +++++++ library/ecp.c | 28 ++++++++++++++++++++++++++++ 2 files changed, 35 insertions(+) diff --git a/ChangeLog b/ChangeLog index b4d2d4481..690d6935b 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 d1ea7487a..108695bf5 100644 --- a/library/ecp.c +++ b/library/ecp.c @@ -1444,6 +1444,20 @@ static int ecp_mul_comb( mbedtls_ecp_group *grp, mbedtls_ecp_point *R, * Now get m * P from M * P and normalize it */ MBEDTLS_MPI_CHK( ecp_safe_invert_jac( grp, R, ! m_is_odd ) ); + + /* + * 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, R, f_rng, p_rng ) ); MBEDTLS_MPI_CHK( ecp_normalize_jac( grp, R ) ); cleanup: @@ -1664,6 +1678,20 @@ 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: