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Add comment on input/output aliasing in ecp_add_mixed()

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Signed-off-by: Hanno Becker <hanno.becker@arm.com>
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Hanno Becker 2022-01-04 05:01:53 +00:00
parent ce29ae84dd
commit 838b715fcc
1 changed files with 7 additions and 0 deletions
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7
library/ecp.c
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@ -1508,6 +1508,10 @@ static int ecp_add_mixed( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
#else #else
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED; int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
mbedtls_mpi T1, T2, T3, T4; mbedtls_mpi T1, T2, T3, T4;
/* NOTE: Aliasing between input and output is allowed, so one has to make
* sure that at the point X,Y,Z are written, {P,Q}->{X,Y,Z} are no
* longer read from. */
mbedtls_mpi * const X = &R->X; mbedtls_mpi * const X = &R->X;
mbedtls_mpi * const Y = &R->Y; mbedtls_mpi * const Y = &R->Y;
mbedtls_mpi * const Z = &R->Z; mbedtls_mpi * const Z = &R->Z;
@ -1551,6 +1555,7 @@ static int ecp_add_mixed( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
} }
} }
/* {P,Q}->Z no longer used, so OK to write to Z even if there's aliasing. */
MPI_ECP_MUL( Z, &P->Z, &T1 ); MPI_ECP_MUL( Z, &P->Z, &T1 );
MPI_ECP_MUL( &T3, &T1, &T1 ); MPI_ECP_MUL( &T3, &T1, &T1 );
MPI_ECP_MUL( &T4, &T3, &T1 ); MPI_ECP_MUL( &T4, &T3, &T1 );
@ -1559,12 +1564,14 @@ static int ecp_add_mixed( const mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
MPI_ECP_MOV( &T1, &T3 ); MPI_ECP_MOV( &T1, &T3 );
MPI_ECP_SHIFT_L( &T1, 1 ); MPI_ECP_SHIFT_L( &T1, 1 );
/* {P,Q}->X no longer used, so OK to write to X even if there's aliasing. */
MPI_ECP_MUL( X, &T2, &T2 ); MPI_ECP_MUL( X, &T2, &T2 );
MPI_ECP_SUB( X, X, &T1 ); MPI_ECP_SUB( X, X, &T1 );
MPI_ECP_SUB( X, X, &T4 ); MPI_ECP_SUB( X, X, &T4 );
MPI_ECP_SUB( &T3, &T3, X ); MPI_ECP_SUB( &T3, &T3, X );
MPI_ECP_MUL( &T3, &T3, &T2 ); MPI_ECP_MUL( &T3, &T3, &T2 );
MPI_ECP_MUL( &T4, &T4, &P->Y ); MPI_ECP_MUL( &T4, &T4, &P->Y );
/* {P,Q}->Y no longer used, so OK to write to Y even if there's aliasing. */
MPI_ECP_SUB( Y, &T3, &T4 ); MPI_ECP_SUB( Y, &T3, &T4 );
cleanup: cleanup: