Squashed 'src/secp256k1/' changes from 6c527ec..8225239

8225239 Merge #433: Make the libcrypto detection fail the newer API.
12de863 Make the libcrypto detection fail the newer API.
2928420 Merge #427: Remove Schnorr from travis as well
8eecc4a Remove Schnorr from travis as well
a8abae7 Merge #310: Add exhaustive test for group functions on a low-order subgroup
b4ceedf Add exhaustive test for verification
83836a9 Add exhaustive tests for group arithmetic, signing, and ecmult on a small group
20b8877 Add exhaustive test for group functions on a low-order subgroup
80773a6 Merge #425: Remove Schnorr experiment
e06e878 Remove Schnorr experiment
04c8ef3 Merge #407: Modify parameter order of internal functions to match API parameter order
6e06696 Merge #411: Remove guarantees about memcmp-ability
40c8d7e Merge #421: Update scalar_4x64_impl.h
a922365 Merge #422: Restructure nonce clearing
3769783 Restructure nonce clearing
0f9e69d Restructure nonce clearing
9d67afa Update scalar_4x64_impl.h
7d15cd7 Merge #413: fix auto-enabled static precompuatation
00c5d2e fix auto-enabled static precompuatation
91219a1 Remove guarantees about memcmp-ability
7a49cac Merge #410: Add string.h include to ecmult_impl
0bbd5d4 Add string.h include to ecmult_impl
353c1bf Fix secp256k1_ge_set_table_gej_var parameter order
541b783 Fix secp256k1_ge_set_all_gej_var parameter order
7d893f4 Fix secp256k1_fe_inv_all_var parameter order
c5b32e1 Merge #405: Make secp256k1_fe_sqrt constant time
926836a Make secp256k1_fe_sqrt constant time
e2a8e92 Merge #404: Replace 3M + 4S doubling formula with 2M + 5S one
8ec49d8 Add note about 2M + 5S doubling formula
5a91bd7 Merge #400: A couple minor cleanups
ac01378 build: add -DSECP256K1_BUILD to benchmark_internal build flags
a6c6f99 Remove a bunch of unused stdlib #includes
65285a6 Merge #403: configure: add flag to disable OpenSSL tests
a9b2a5d configure: add flag to disable OpenSSL tests
b340123 Merge #402: Add support for testing quadratic residues
e6e9805 Add function for testing quadratic residue field/group elements.
efd953a Add Jacobi symbol test via GMP
fa36a0d Merge #401: ecmult_const: unify endomorphism and non-endomorphism skew cases
c6191fd ecmult_const: unify endomorphism and non-endomorphism skew cases
0b3e618 Merge #378: .gitignore build-aux cleanup
6042217 Merge #384: JNI: align shared files copyright/comments to bitcoinj's
24ad20f Merge #399: build: verify that the native compiler works for static precomp
b3be852 Merge #398: Test whether ECDH and Schnorr are enabled for JNI
aa0b1fd build: verify that the native compiler works for static precomp
eee808d Test whether ECDH and Schnorr are enabled for JNI
7b0fb18 Merge #366: ARM assembly implementation of field_10x26 inner (rebase of #173)
001f176 ARM assembly implementation of field_10x26 inner
0172be9 Merge #397: Small fixes for sha256
3f8b78e Fix undefs in hash_impl.h
2ab4695 Fix state size in sha256 struct
6875b01 Merge #386: Add some missing `VERIFY_CHECK(ctx != NULL)`
2c52b5d Merge #389: Cast pointers through uintptr_t under JNI
43097a4 Merge #390: Update bitcoin-core GitHub links
31c9c12 Merge #391: JNI: Only call ecdsa_verify if its inputs parsed correctly
1cb2302 Merge #392: Add testcase which hits additional branch in secp256k1_scalar_sqr
d2ee340 Merge #388: bench_ecdh: fix call to secp256k1_context_create
093a497 Add testcase which hits additional branch in secp256k1_scalar_sqr
a40c701 JNI: Only call ecdsa_verify if its inputs parsed correctly
faa2a11 Update bitcoin-core GitHub links
47b9e78 Cast pointers through uintptr_t under JNI
f36f9c6 bench_ecdh: fix call to secp256k1_context_create
bcc4881 Add some missing `VERIFY_CHECK(ctx != NULL)` for functions that use `ARG_CHECK`
6ceea2c align shared files copyright/comments to bitcoinj's
70141a8 Update .gitignore
7b549b1 Merge #373: build: fix x86_64 asm detection for some compilers
bc7c93c Merge #374: Add note about y=0 being possible on one of the sextic twists
e457018 Merge #364: JNI rebased
86e2d07 JNI library: cleanup, removed unimplemented code
3093576a JNI library
bd2895f Merge pull request #371
e72e93a Add note about y=0 being possible on one of the sextic twists
3f8fdfb build: fix x86_64 asm detection for some compilers
e5a9047 [Trivial] Remove double semicolons
c18b869 Merge pull request #360
3026daa Merge pull request #302
03d4611 Add sage verification script for the group laws
a965937 Merge pull request #361
83221ec Add experimental features to configure
5d4c5a3 Prevent damage_array in the signature test from going out of bounds.
419bf7f Merge pull request #356
03d84a4 Benchmark against OpenSSL verification

git-subtree-dir: src/secp256k1
git-subtree-split: 8225239f490f79842a5a3b82ad6cc8aa11d5208e

1 files changed, 24 insertions, 45 deletions

diff --git a/src/ecmult_const_impl.h b/src/ecmult_const_impl.h
index 90ac94770e..0db314c48e 100644
--- a/src/ecmult_const_impl.h
+++ b/src/ecmult_const_impl.h
@@ -58,25 +58,27 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w) {
     int global_sign;
     int skew = 0;
     int word = 0;
+
     /* 1 2 3 */
     int u_last;
     int u;
 
-#ifdef USE_ENDOMORPHISM
     int flip;
     int bit;
     secp256k1_scalar neg_s;
     int not_neg_one;
-    /* If we are using the endomorphism, we cannot handle even numbers by negating
-     * them, since we are working with 128-bit numbers whose negations would be 256
-     * bits, eliminating the performance advantage. Instead we use a technique from
+    /* Note that we cannot handle even numbers by negating them to be odd, as is
+     * done in other implementations, since if our scalars were specified to have
+     * width < 256 for performance reasons, their negations would have width 256
+     * and we'd lose any performance benefit. Instead, we use a technique from
      * Section 4.2 of the Okeya/Tagaki paper, which is to add either 1 (for even)
-     * or 2 (for odd) to the number we are encoding, then compensating after the
-     * multiplication. */
-    /* Negative 128-bit numbers will be negated, since otherwise they are 256-bit */
+     * or 2 (for odd) to the number we are encoding, returning a skew value indicating
+     * this, and having the caller compensate after doing the multiplication. */
+
+    /* Negative numbers will be negated to keep their bit representation below the maximum width */
     flip = secp256k1_scalar_is_high(&s);
     /* We add 1 to even numbers, 2 to odd ones, noting that negation flips parity */
-    bit = flip ^ (s.d[0] & 1);
+    bit = flip ^ !secp256k1_scalar_is_even(&s);
     /* We check for negative one, since adding 2 to it will cause an overflow */
     secp256k1_scalar_negate(&neg_s, &s);
     not_neg_one = !secp256k1_scalar_is_one(&neg_s);
@@ -89,11 +91,6 @@ static int secp256k1_wnaf_const(int *wnaf, secp256k1_scalar s, int w) {
     global_sign = secp256k1_scalar_cond_negate(&s, flip);
     global_sign *= not_neg_one * 2 - 1;
     skew = 1 << bit;
-#else
-    /* Otherwise, we just negate to force oddness */
-    int is_even = secp256k1_scalar_is_even(&s);
-    global_sign = secp256k1_scalar_cond_negate(&s, is_even);
-#endif
 
     /* 4 */
     u_last = secp256k1_scalar_shr_int(&s, w);
@@ -127,15 +124,13 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
     secp256k1_ge tmpa;
     secp256k1_fe Z;
 
+    int skew_1;
+    int wnaf_1[1 + WNAF_SIZE(WINDOW_A - 1)];
 #ifdef USE_ENDOMORPHISM
     secp256k1_ge pre_a_lam[ECMULT_TABLE_SIZE(WINDOW_A)];
-    int wnaf_1[1 + WNAF_SIZE(WINDOW_A - 1)];
     int wnaf_lam[1 + WNAF_SIZE(WINDOW_A - 1)];
-    int skew_1;
     int skew_lam;
     secp256k1_scalar q_1, q_lam;
-#else
-    int wnaf[1 + WNAF_SIZE(WINDOW_A - 1)];
 #endif
 
     int i;
@@ -145,18 +140,10 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
 #ifdef USE_ENDOMORPHISM
     /* split q into q_1 and q_lam (where q = q_1 + q_lam*lambda, and q_1 and q_lam are ~128 bit) */
     secp256k1_scalar_split_lambda(&q_1, &q_lam, &sc);
-    /* no need for zero correction when using endomorphism since even
-     * numbers have one added to them anyway */
     skew_1   = secp256k1_wnaf_const(wnaf_1,   q_1,   WINDOW_A - 1);
     skew_lam = secp256k1_wnaf_const(wnaf_lam, q_lam, WINDOW_A - 1);
 #else
-    int is_zero = secp256k1_scalar_is_zero(scalar);
-    /* the wNAF ladder cannot handle zero, so bump this to one .. we will
-     * correct the result after the fact */
-    sc.d[0] += is_zero;
-    VERIFY_CHECK(!secp256k1_scalar_is_zero(&sc));
-
-    secp256k1_wnaf_const(wnaf, sc, WINDOW_A - 1);
+    skew_1   = secp256k1_wnaf_const(wnaf_1, sc, WINDOW_A - 1);
 #endif
 
     /* Calculate odd multiples of a.
@@ -179,21 +166,15 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
     /* first loop iteration (separated out so we can directly set r, rather
      * than having it start at infinity, get doubled several times, then have
      * its new value added to it) */
-#ifdef USE_ENDOMORPHISM
     i = wnaf_1[WNAF_SIZE(WINDOW_A - 1)];
     VERIFY_CHECK(i != 0);
     ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, i, WINDOW_A);
     secp256k1_gej_set_ge(r, &tmpa);
-
+#ifdef USE_ENDOMORPHISM
     i = wnaf_lam[WNAF_SIZE(WINDOW_A - 1)];
     VERIFY_CHECK(i != 0);
     ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a_lam, i, WINDOW_A);
     secp256k1_gej_add_ge(r, r, &tmpa);
-#else
-    i = wnaf[WNAF_SIZE(WINDOW_A - 1)];
-    VERIFY_CHECK(i != 0);
-    ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, i, WINDOW_A);
-    secp256k1_gej_set_ge(r, &tmpa);
 #endif
     /* remaining loop iterations */
     for (i = WNAF_SIZE(WINDOW_A - 1) - 1; i >= 0; i--) {
@@ -202,59 +183,57 @@ static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, cons
         for (j = 0; j < WINDOW_A - 1; ++j) {
             secp256k1_gej_double_nonzero(r, r, NULL);
         }
-#ifdef USE_ENDOMORPHISM
+
         n = wnaf_1[i];
         ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, n, WINDOW_A);
         VERIFY_CHECK(n != 0);
         secp256k1_gej_add_ge(r, r, &tmpa);
-
+#ifdef USE_ENDOMORPHISM
         n = wnaf_lam[i];
         ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a_lam, n, WINDOW_A);
         VERIFY_CHECK(n != 0);
         secp256k1_gej_add_ge(r, r, &tmpa);
-#else
-        n = wnaf[i];
-        VERIFY_CHECK(n != 0);
-        ECMULT_CONST_TABLE_GET_GE(&tmpa, pre_a, n, WINDOW_A);
-        secp256k1_gej_add_ge(r, r, &tmpa);
 #endif
     }
 
     secp256k1_fe_mul(&r->z, &r->z, &Z);
 
-#ifdef USE_ENDOMORPHISM
     {
         /* Correct for wNAF skew */
         secp256k1_ge correction = *a;
         secp256k1_ge_storage correction_1_stor;
+#ifdef USE_ENDOMORPHISM
         secp256k1_ge_storage correction_lam_stor;
+#endif
         secp256k1_ge_storage a2_stor;
         secp256k1_gej tmpj;
         secp256k1_gej_set_ge(&tmpj, &correction);
         secp256k1_gej_double_var(&tmpj, &tmpj, NULL);
         secp256k1_ge_set_gej(&correction, &tmpj);
         secp256k1_ge_to_storage(&correction_1_stor, a);
+#ifdef USE_ENDOMORPHISM
         secp256k1_ge_to_storage(&correction_lam_stor, a);
+#endif
         secp256k1_ge_to_storage(&a2_stor, &correction);
 
         /* For odd numbers this is 2a (so replace it), for even ones a (so no-op) */
         secp256k1_ge_storage_cmov(&correction_1_stor, &a2_stor, skew_1 == 2);
+#ifdef USE_ENDOMORPHISM
         secp256k1_ge_storage_cmov(&correction_lam_stor, &a2_stor, skew_lam == 2);
+#endif
 
         /* Apply the correction */
         secp256k1_ge_from_storage(&correction, &correction_1_stor);
         secp256k1_ge_neg(&correction, &correction);
         secp256k1_gej_add_ge(r, r, &correction);
 
+#ifdef USE_ENDOMORPHISM
         secp256k1_ge_from_storage(&correction, &correction_lam_stor);
         secp256k1_ge_neg(&correction, &correction);
         secp256k1_ge_mul_lambda(&correction, &correction);
         secp256k1_gej_add_ge(r, r, &correction);
-    }
-#else
-    /* correct for zero */
-    r->infinity |= is_zero;
 #endif
+    }
 }
 
 #endif