VERIFY_CHECK precondition for secp256k1_fe_set_int.

Author: roconnor-blockstream

src/field.h

@@ -50,7 +50,7 @@ static int secp256k1_fe_normalizes_to_zero(const secp256k1_fe *r);
  *  without constant-time guarantee. */
 static int secp256k1_fe_normalizes_to_zero_var(const secp256k1_fe *r);
 
-/** Set a field element equal to a small integer. Resulting field element is normalized; it has
+/** Set a field element equal to a small, non-negative integer. Resulting field element is normalized; it has
  *  magnitude 0 if a == 0, and magnitude 1 otherwise. */
 static void secp256k1_fe_set_int(secp256k1_fe *r, int a);
 

src/field_10x26_impl.h

@@ -264,6 +264,7 @@ static int secp256k1_fe_normalizes_to_zero_var(const secp256k1_fe *r) {
 }
 
 SECP256K1_INLINE static void secp256k1_fe_set_int(secp256k1_fe *r, int a) {
+    VERIFY_CHECK(0 <= a && a <= 0x7FFF);
     r->n[0] = a;
     r->n[1] = r->n[2] = r->n[3] = r->n[4] = r->n[5] = r->n[6] = r->n[7] = r->n[8] = r->n[9] = 0;
 #ifdef VERIFY

src/field_5x52_impl.h

@@ -227,6 +227,7 @@ static int secp256k1_fe_normalizes_to_zero_var(const secp256k1_fe *r) {
 }
 
 SECP256K1_INLINE static void secp256k1_fe_set_int(secp256k1_fe *r, int a) {
+    VERIFY_CHECK(0 <= a && a <= 0x7FFF);
     r->n[0] = a;
     r->n[1] = r->n[2] = r->n[3] = r->n[4] = 0;
 #ifdef VERIFY