deprecate EC_POINT_make_affine and EC_POINTs_make_affine

[openssl.git] / crypto / ec / ecp_nistz256.c

diff --git a/crypto/ec/ecp_nistz256.c b/crypto/ec/ecp_nistz256.c
index f0f3ae32dd003304c3f5875ed1042eefe1ec7dc0..50b6d43b7c55d1c7467b5347dc2e5233732f182e 100644 (file)
--- a/crypto/ec/ecp_nistz256.c
+++ b/crypto/ec/ecp_nistz256.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2018 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 2014-2020 The OpenSSL Project Authors. All Rights Reserved.
  * Copyright (c) 2014, Intel Corporation. All Rights Reserved.
  * Copyright (c) 2015, CloudFlare, Inc.
  *
@@ -18,11 +18,17 @@
  *                          256 Bit Primes"
  */
 
+/*
+ * ECDSA low level APIs are deprecated for public use, but still ok for
+ * internal use.
+ */
+#include "internal/deprecated.h"
+
 #include <string.h>
 
 #include "internal/cryptlib.h"
 #include "crypto/bn.h"
-#include "ec_lcl.h"
+#include "ec_local.h"
 #include "internal/refcount.h"
 
 #if BN_BITS2 != 64
@@ -358,16 +364,47 @@ static void ecp_nistz256_point_add(P256_POINT *r,
     ecp_nistz256_sub(H, U2, U1);                /* H = U2 - U1 */
 
     /*
-     * This should not happen during sign/ecdh, so no constant time violation
+     * The formulae are incorrect if the points are equal so we check for
+     * this and do doubling if this happens.
+     *
+     * Points here are in Jacobian projective coordinates (Xi, Yi, Zi)
+     * that are bound to the affine coordinates (xi, yi) by the following
+     * equations:
+     *     - xi = Xi / (Zi)^2
+     *     - y1 = Yi / (Zi)^3
+     *
+     * For the sake of optimization, the algorithm operates over
+     * intermediate variables U1, U2 and S1, S2 that are derived from
+     * the projective coordinates:
+     *     - U1 = X1 * (Z2)^2 ; U2 = X2 * (Z1)^2
+     *     - S1 = Y1 * (Z2)^3 ; S2 = Y2 * (Z1)^3
+     *
+     * It is easy to prove that is_equal(U1, U2) implies that the affine
+     * x-coordinates are equal, or either point is at infinity.
+     * Likewise is_equal(S1, S2) implies that the affine y-coordinates are
+     * equal, or either point is at infinity.
+     *
+     * The special case of either point being the point at infinity (Z1 or Z2
+     * is zero), is handled separately later on in this function, so we avoid
+     * jumping to point_double here in those special cases.
+     *
+     * When both points are inverse of each other, we know that the affine
+     * x-coordinates are equal, and the y-coordinates have different sign.
+     * Therefore since U1 = U2, we know H = 0, and therefore Z3 = H*Z1*Z2
+     * will equal 0, thus the result is infinity, if we simply let this
+     * function continue normally.
+     *
+     * We use bitwise operations to avoid potential side-channels introduced by
+     * the short-circuiting behaviour of boolean operators.
      */
-    if (is_equal(U1, U2) && !in1infty && !in2infty) {
-        if (is_equal(S1, S2)) {
-            ecp_nistz256_point_double(r, a);
-            return;
-        } else {
-            memset(r, 0, sizeof(*r));
-            return;
-        }
+    if (is_equal(U1, U2) & ~in1infty & ~in2infty & is_equal(S1, S2)) {
+        /*
+         * This is obviously not constant-time but it should never happen during
+         * single point multiplication, so there is no timing leak for ECDH or
+         * ECDSA signing.
+         */
+        ecp_nistz256_point_double(r, a);
+        return;
     }
 
     ecp_nistz256_sqr_mont(Rsqr, R);             /* R^2 */
@@ -860,7 +897,8 @@ __owur static int ecp_nistz256_mult_precompute(EC_GROUP *group, BN_CTX *ctx)
              * It would be faster to use EC_POINTs_make_affine and
              * make multiple points affine at the same time.
              */
-            if (!EC_POINT_make_affine(group, P, ctx))
+            if (group->meth->make_affine == NULL
+                || !group->meth->make_affine(group, P, ctx))
                 goto err;
             if (!ecp_nistz256_bignum_to_field_elem(temp.X, P->X) ||
                 !ecp_nistz256_bignum_to_field_elem(temp.Y, P->Y)) {
@@ -1657,8 +1695,6 @@ const EC_METHOD *EC_GFp_nistz256_method(void)
         ec_GFp_simple_point_clear_finish,
         ec_GFp_simple_point_copy,
         ec_GFp_simple_point_set_to_infinity,
-        ec_GFp_simple_set_Jprojective_coordinates_GFp,
-        ec_GFp_simple_get_Jprojective_coordinates_GFp,
         ec_GFp_simple_point_set_affine_coordinates,
         ecp_nistz256_get_affine,
         0, 0, 0,