ecc-eh-to-a, eddsa-sign: Parameterize hard-coded value

This allows the same code to be reused in curve448 and Ed448.
Signed-off-by: Daiki Ueno <dueno@redhat.com>

ecc-eh-to-a.c

@@ -73,7 +73,7 @@ ecc_eh_to_a (const struct ecc_curve *ecc,
 	     at all? Full reduction mod p is maybe sufficient. */
 	  unsigned shift;
 	  assert (ecc->p.bit_size == 255);
-	  shift = 252 - GMP_NUMB_BITS * (ecc->p.size - 1);
+	  shift = ecc->q.bit_size - 1 - GMP_NUMB_BITS * (ecc->p.size - 1);
 	  cy = mpn_submul_1 (r, ecc->q.m, ecc->p.size,
 			     r[ecc->p.size-1] >> shift);
 	  assert (cy < 2);

eddsa-sign.c

@@ -92,7 +92,7 @@ _eddsa_sign (const struct ecc_curve *ecc,
     unsigned shift;
     mp_limb_t cy;
     assert (ecc->p.bit_size == 255);
-    shift = 252 - GMP_NUMB_BITS * (ecc->p.size - 1);
+    shift = ecc->q.bit_size - 1 - GMP_NUMB_BITS * (ecc->p.size - 1);
     cy = mpn_submul_1 (sp, ecc->q.m, ecc->p.size,
 		       sp[ecc->p.size-1] >> shift);
     assert (cy < 2);