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Merge pull request #665 from ARMmbed/dev/yanesca/iotcrypt-958-ecdsa-side-channel-fix-2.16

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Backport 2.16: ECDSA side channel fix
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Jaeden Amero 2019-11-12 10:37:22 +00:00 committed by GitHub
commit 3f1c68a1e2
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7
ChangeLog
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@ -2,6 +2,13 @@ mbed TLS ChangeLog (Sorted per branch, date)
= 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.

1
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 ) );