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Fix wrong modulo call in ecp_double_add_mxz

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ecp_double_add_mxz wrongly does an MPI addition followed by a call to
MOD_MUL instead of MOD_ADD. This is more visible since the
mbedtls_mpi_xxx_mod functions have been added in commit 3b3b34f608
("Replace some macros by functions").

Fix that by using mbedtls_mpi_add_mod instead. The testsuite still
passes after that change.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
This commit is contained in:
Aurelien Jarno 2020-04-20 21:36:48 +02:00
parent 3a7ca1d0cb
commit 66deb38d64
2 changed files with 5 additions and 1 deletions
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4
ChangeLog.d/fix-ecp_double_add_mxz.txt Normal file
View file

@ -0,0 +1,4 @@
Changes
* Fix minor performance issue in operations on Curve25519 caused by using a
suboptimal modular reduction in one place. Found and fix contributed by
Aurelien Jarno in #3209.

2
library/ecp.c
View file

@ -2332,7 +2332,7 @@ static int ecp_double_add_mxz( const mbedtls_ecp_group *grp,
MBEDTLS_MPI_CHK( mbedtls_mpi_sub_mod( grp, &D, &Q->X, &Q->Z ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mod( grp, &DA, &D, &A ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mod( grp, &CB, &C, &B ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &S->X, &DA, &CB ) ); MOD_MUL( S->X );
MBEDTLS_MPI_CHK( mbedtls_mpi_add_mod( grp, &S->X, &DA, &CB ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mod( grp, &S->X, &S->X, &S->X ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_sub_mod( grp, &S->Z, &DA, &CB ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mod( grp, &S->Z, &S->Z, &S->Z ) );