Merge pull request #665 from ARMmbed/dev/yanesca/iotcrypt-958-ecdsa-side-channel-fix-2.16
Backport 2.16: ECDSA side channel fix
diff --git a/ChangeLog b/ChangeLog
index c92e429..7074eb2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,13 @@
= mbed TLS 2.16.x branch released xxxx-xx-xx
+Security
+ * Fix side channel vulnerability in ECDSA. Our bignum implementation is not
+ constant time/constant trace, so side channel attacks can retrieve the
+ blinded value, factor it (as it is smaller than RSA keys and not guaranteed
+ to have only large prime factors), and then, by brute force, recover the
+ key. Reported by Alejandro Cabrera Aldaya and Billy Brumley.
+
Bugfix
* Remove redundant line for getting the bitlen of a bignum, since the variable
holding the returned value is overwritten a line after.
diff --git a/library/ecdsa.c b/library/ecdsa.c
index 2b48006..3cf3d7c 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -363,6 +363,7 @@
MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &e, &e, s ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &e, &e, &t ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( pk, pk, &t ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( pk, pk, &grp->N ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_inv_mod( s, pk, &grp->N ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( s, s, &e ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( s, s, &grp->N ) );