Extract common code for computing X^3 + AX + B
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index 5f814f3..70b2283 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -1224,6 +1224,39 @@
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 @@
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 @@
/*
* 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;