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ECDSA: Fix side channel vulnerability

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The blinding applied to the scalar before modular inversion is
inadequate. Bignum is not constant time/constant trace, side channel
attacks can retrieve the blinded value, factor it (it is smaller than
RSA keys and not guaranteed to have only large prime factors). Then the
key can be recovered by brute force.

Reducing the blinded value makes factoring useless because the adversary
can only recover pk*t+z*N instead of pk*t.
This commit is contained in:
Janos Follath 2019-10-17 10:18:51 +01:00 committed by Simon Butcher
parent bb3d55665e
commit 6bd8c0ae2a
1 changed files with 1 additions and 0 deletions
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library/ecdsa.c
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@ -363,6 +363,7 @@ modn:
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 ) );