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Extract common code for computing X^3 + AX + B

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2022-12-20 10:02:52 +01:00 committed by Glenn Strauss
parent 452416121d
commit 8b6d14be8b
1 changed files with 37 additions and 27 deletions
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64
library/ecp.c
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@ -1224,6 +1224,39 @@ cleanup:
MBEDTLS_MPI_CHK( mbedtls_mpi_safe_cond_swap( (X), (Y), (cond) ) )
#if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED)
/*
* Computes the right-hand side of the Short Weierstrass equation
* RHS = X^3 + A X + B
*/
static int ecp_sw_rhs( const mbedtls_ecp_group *grp,
mbedtls_mpi *rhs,
const mbedtls_mpi *X )
{
int ret;
/* Compute X^3 + A X + B as X (X^2 + A) + B */
MPI_ECP_SQR( rhs, X );
/* Special case for A = -3 */
if( grp->A.p == NULL )
{
MPI_ECP_SUB_INT( rhs, rhs, 3 );
}
else
{
MPI_ECP_ADD( rhs, rhs, &grp->A );
}
MPI_ECP_MUL( rhs, rhs, X );
MPI_ECP_ADD( rhs, rhs, &grp->B );
cleanup:
return( ret );
}
/*
* Derive Y from X and a parity bit
*/
static int mbedtls_ecp_sw_derive_y( const mbedtls_ecp_group *grp,
const mbedtls_mpi *X,
mbedtls_mpi *Y,
@ -1246,18 +1279,8 @@ static int mbedtls_ecp_sw_derive_y( const mbedtls_ecp_group *grp,
mbedtls_mpi exp;
mbedtls_mpi_init( &exp );
/* use Y to store intermediate results */
/* y^2 = x^3 + ax + b = (x^2 + a)x + b */
/* x^2 */
MPI_ECP_MUL( Y, X, X );
/* x^2 + a */
if( !grp->A.p ) /* special case for A = -3; temporarily set exp = -3 */
MPI_ECP_LSET( &exp, -3 );
MPI_ECP_ADD( Y, Y, grp->A.p ? &grp->A : &exp );
/* (x^2 + a)x */
MPI_ECP_MUL( Y, Y, X );
/* (x^2 + a)x + b */
MPI_ECP_ADD( Y, Y, &grp->B );
/* use Y to store intermediate result, actually w above */
MBEDTLS_MPI_CHK( ecp_sw_rhs( grp, Y, X ) );
/* w = y^2 */ /* Y contains y^2 intermediate result */
/* exp = ((p+1)/4) */
@ -2698,23 +2721,10 @@ static int ecp_check_pubkey_sw( const mbedtls_ecp_group *grp, const mbedtls_ecp_
/*
* YY = Y^2
* RHS = X (X^2 + A) + B = X^3 + A X + B
* RHS = X^3 + A X + B
*/
MPI_ECP_SQR( &YY, &pt->Y );
MPI_ECP_SQR( &RHS, &pt->X );
/* Special case for A = -3 */
if( grp->A.p == NULL )
{
MPI_ECP_SUB_INT( &RHS, &RHS, 3 );
}
else
{
MPI_ECP_ADD( &RHS, &RHS, &grp->A );
}
MPI_ECP_MUL( &RHS, &RHS, &pt->X );
MPI_ECP_ADD( &RHS, &RHS, &grp->B );
MBEDTLS_MPI_CHK( ecp_sw_rhs( grp, &RHS, &pt->X ) );
if( MPI_ECP_CMP( &YY, &RHS ) != 0 )
ret = MBEDTLS_ERR_ECP_INVALID_KEY;