[crypto] Allow for an explicit representation of point at infinity

ECDSA requires the ability to add two arbitrary curve points, either
of which may legitimately be the point at infinity.

Update the API so that curves must choose an explicit affine
representation for the point at infinity, and provide a method to test
for this representation.  Multiplication and addition will now allow
this representation to be provided as an input, and will not fail if
the result is the point at infinity.  Callers must explicitly check
for the point at infinity where needed (e.g. after computing the ECDHE
shared secret curve point).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/x25519.c

@@ -783,17 +783,30 @@ static void x25519_reverse ( struct x25519_value *value ) {
 	} while ( ++low < --high );
 }
 
+/**
+ * Check if X25519 value is zero
+ *
+ * @v value		Value to check
+ * @ret is_zero		Value is zero
+ */
+int x25519_is_zero ( const struct x25519_value *value ) {
+	x25519_t point;
+
+	/* Check if value is zero */
+	bigint_init ( &point, value->raw, sizeof ( value->raw ) );
+	return bigint_is_zero ( &point );
+}
+
 /**
  * Calculate X25519 key
  *
  * @v base		Base point
  * @v scalar		Scalar multiple
  * @v result		Point to hold result (may overlap base point)
- * @ret rc		Return status code
  */
-int x25519_key ( const struct x25519_value *base,
-		 const struct x25519_value *scalar,
-		 struct x25519_value *result ) {
+void x25519_key ( const struct x25519_value *base,
+		  const struct x25519_value *scalar,
+		  struct x25519_value *result ) {
 	struct x25519_value *tmp = result;
 	union x25519_quad257 point;
 
@@ -814,9 +827,18 @@ int x25519_key ( const struct x25519_value *base,
 	/* Reverse result */
 	bigint_done ( &point.value, result->raw, sizeof ( result->raw ) );
 	x25519_reverse ( result );
+}
 
-	/* Fail if result was all zeros (as required by RFC8422) */
-	return ( bigint_is_zero ( &point.value ) ? -EPERM : 0 );
+/**
+ * Check if this is the point at infinity
+ *
+ * @v point		Curve point
+ * @ret is_infinity	This is the point at infinity
+ */
+static int x25519_curve_is_infinity ( const void *point ) {
+
+	/* We use all zeroes for the point at infinity (as per RFC8422) */
+	return x25519_is_zero ( point );
 }
 
 /**
@@ -830,7 +852,8 @@ int x25519_key ( const struct x25519_value *base,
 static int x25519_curve_multiply ( const void *base, const void *scalar,
 				   void *result ) {
 
-	return x25519_key ( base, scalar, result );
+	x25519_key ( base, scalar, result );
+	return 0;
 }
 
 /**
@@ -854,6 +877,7 @@ struct elliptic_curve x25519_curve = {
 	.pointsize = sizeof ( struct x25519_value ),
 	.keysize = sizeof ( struct x25519_value ),
 	.base = x25519_generator.raw,
+	.is_infinity = x25519_curve_is_infinity,
 	.multiply = x25519_curve_multiply,
 	.add = x25519_curve_add,
 };