Merge BoringSSL 'd605df5': Use packed representation for large Curve25519 table

Author: briansmith

crypto/curve25519/curve25519.c

@@ -180,21 +180,29 @@ static void fe_0(fe *h) {
   OPENSSL_memset(h, 0, sizeof(fe));
 }
 
+#if defined(OPENSSL_SMALL)
+
 static void fe_loose_0(fe_loose *h) {
   OPENSSL_memset(h, 0, sizeof(fe_loose));
 }
 
+#endif
+
 // h = 1
 static void fe_1(fe *h) {
   OPENSSL_memset(h, 0, sizeof(fe));
   h->v[0] = 1;
 }
 
+#if defined(OPENSSL_SMALL)
+
 static void fe_loose_1(fe_loose *h) {
   OPENSSL_memset(h, 0, sizeof(fe_loose));
   h->v[0] = 1;
 }
 
+#endif
+
 // h = f + g
 // Can overlap h with f or g.
 static void fe_add(fe_loose *h, const fe *f, const fe *g) {
@@ -319,11 +327,6 @@ static void fe_copy(fe *h, const fe *f) {
 static void fe_copy_lt(fe_loose *h, const fe *f) {
   fe_limbs_copy(h->v, f->v);
 }
-#if !defined(OPENSSL_SMALL)
-static void fe_copy_ll(fe_loose *h, const fe_loose *f) {
-  fe_limbs_copy(h->v, f->v);
-}
-#endif // !defined(OPENSSL_SMALL)
 
 static void fe_loose_invert(fe *out, const fe_loose *z) {
   fe t0;
@@ -532,12 +535,16 @@ static void ge_p3_0(ge_p3 *h) {
   fe_0(&h->T);
 }
 
+#if defined(OPENSSL_SMALL)
+
 static void ge_precomp_0(ge_precomp *h) {
   fe_loose_1(&h->yplusx);
   fe_loose_1(&h->yminusx);
   fe_loose_0(&h->xy2d);
 }
 
+#endif
+
 // r = p
 static void ge_p3_to_p2(ge_p2 *r, const ge_p3 *p) {
   fe_copy(&r->X, &p->X);
@@ -664,16 +671,6 @@ static void x25519_ge_sub(ge_p1p1 *r, const ge_p3 *p, const ge_cached *q) {
   fe_add(&r->T, &trZ, &trT);
 }
 
-static uint8_t equal(signed char b, signed char c) {
-  uint8_t ub = b;
-  uint8_t uc = c;
-  uint8_t x = ub ^ uc;  // 0: yes; 1..255: no
-  uint32_t y = x;       // 0: yes; 1..255: no
-  y -= 1;               // 4294967295: yes; 0..254: no
-  y >>= 31;             // 1: yes; 0: no
-  return y;
-}
-
 static void cmov(ge_precomp *t, const ge_precomp *u, uint8_t b) {
   fe_cmov(&t->yplusx, &u->yplusx, b);
   fe_cmov(&t->yminusx, &u->yminusx, b);
@@ -722,7 +719,7 @@ static void x25519_ge_scalarmult_small_precomp(
     ge_precomp_0(&e);
 
     for (j = 1; j < 16; j++) {
-      cmov(&e, &multiples[j-1], equal(index, j));
+      cmov(&e, &multiples[j-1], 1&constant_time_eq_w(index, j));
     }
 
     ge_cached cached;
@@ -742,35 +739,36 @@ void x25519_ge_scalarmult_base(ge_p3 *h, const uint8_t a[32]) {
 
 #else
 
-static uint8_t negative(signed char b) {
-  uint32_t x = b;
-  x >>= 31;  // 1: yes; 0: no
-  return x;
-}
+static void table_select(ge_precomp *t, const int pos, const signed char b) {
+  uint8_t bnegative = constant_time_msb_w(b);
+  uint8_t babs = b - ((bnegative & b) << 1);
+
+  uint8_t t_bytes[3][32] = {
+      {constant_time_is_zero_w(b) & 1}, {constant_time_is_zero_w(b) & 1}, {0}};
+#if defined(__clang__) // materialize for vectorization, 6% speedup
+  __asm__("" : "+m" (t_bytes) : /*no inputs*/);
+#endif
+  static_assert(sizeof(t_bytes) == sizeof(k25519Precomp[pos][0]), "");
+  for (int i = 0; i < 8; i++) {
+    constant_time_conditional_memxor(t_bytes, k25519Precomp[pos][i],
+                                     sizeof(t_bytes),
+                                     constant_time_eq_w(babs, 1 + i));
+  }
+
+  fe yplusx, yminusx, xy2d;
+  fe_frombytes_strict(&yplusx, t_bytes[0]);
+  fe_frombytes_strict(&yminusx, t_bytes[1]);
+  fe_frombytes_strict(&xy2d, t_bytes[2]);
+
+  fe_copy_lt(&t->yplusx, &yplusx);
+  fe_copy_lt(&t->yminusx, &yminusx);
+  fe_copy_lt(&t->xy2d, &xy2d);
 
-static void table_select(ge_precomp *t, int pos, signed char b) {
   ge_precomp minust;
-  uint8_t bnegative = negative(b);
-  uint8_t babs = b - ((uint8_t)((-bnegative) & b) << 1);
-
-  ge_precomp_0(t);
-  cmov(t, &k25519Precomp[pos][0], equal(babs, 1));
-  cmov(t, &k25519Precomp[pos][1], equal(babs, 2));
-  cmov(t, &k25519Precomp[pos][2], equal(babs, 3));
-  cmov(t, &k25519Precomp[pos][3], equal(babs, 4));
-  cmov(t, &k25519Precomp[pos][4], equal(babs, 5));
-  cmov(t, &k25519Precomp[pos][5], equal(babs, 6));
-  cmov(t, &k25519Precomp[pos][6], equal(babs, 7));
-  cmov(t, &k25519Precomp[pos][7], equal(babs, 8));
-  fe_copy_ll(&minust.yplusx, &t->yminusx);
-  fe_copy_ll(&minust.yminusx, &t->yplusx);
-
-  // NOTE: the input table is canonical, but types don't encode it
-  fe tmp;
-  fe_carry(&tmp, &t->xy2d);
-  fe_neg(&minust.xy2d, &tmp);
-
-  cmov(t, &minust, bnegative);
+  fe_copy_lt(&minust.yplusx, &yminusx);
+  fe_copy_lt(&minust.yminusx, &yplusx);
+  fe_neg(&minust.xy2d, &xy2d);
+  cmov(t, &minust, bnegative>>7);
 }
 
 // h = a * B