tls: code shrink curve25519
function old new delta
fe_select 39 - -39
curve25519 849 800 -49
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-88) Total: -88 bytes
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/networking/tls_fe.c b/networking/tls_fe.c
index ecb4102..3a0a677 100644
--- a/networking/tls_fe.c
+++ b/networking/tls_fe.c
@@ -63,16 +63,22 @@
}
#endif
+#if 0 /* constant-time */
static void fe_select(byte *dst,
- const byte *zero, const byte *one,
+ const byte *src,
byte condition)
{
const byte mask = -condition;
int i;
for (i = 0; i < F25519_SIZE; i++)
- dst[i] = zero[i] ^ (mask & (one[i] ^ zero[i]));
+ dst[i] = dst[i] ^ (mask & (src[i] ^ dst[i]));
}
+#else
+# define fe_select(dst, src, condition) do { \
+ if (condition) lm_copy(dst, src); \
+} while (0)
+#endif
#if 0 //UNUSED
static void raw_add(byte *x, const byte *p)
@@ -225,7 +231,7 @@
minusp[31] = (byte)c;
/* Load x-p if no underflow */
- fe_select(x, minusp, x, (c >> 15) & 1);
+ fe_select(x, minusp, !(c & (1<<15)));
}
static void lm_add(byte* r, const byte* a, const byte* b)
@@ -548,26 +554,32 @@
{
int i;
- struct {
+ struct Z {
/* for bbox's special case of q == NULL meaning "use basepoint" */
/*static const*/ uint8_t basepoint9[CURVE25519_KEYSIZE]; // = {9};
/* from wolfssl-3.15.3/wolfssl/wolfcrypt/fe_operations.h */
/*static const*/ byte f25519_one[F25519_SIZE]; // = {1};
- /* Current point: P_m */
- byte xm[F25519_SIZE];
- byte zm[F25519_SIZE]; // = {1};
/* Predecessor: P_(m-1) */
byte xm1[F25519_SIZE]; // = {1};
byte zm1[F25519_SIZE]; // = {0};
+ /* Current point: P_m */
+ byte xm[F25519_SIZE];
+ byte zm[F25519_SIZE]; // = {1};
+ /* Temporaries */
+ byte xms[F25519_SIZE];
+ byte zms[F25519_SIZE];
} z;
+ uint8_t *XM1 = (uint8_t*)&z + offsetof(struct Z,xm1); // gcc 11.0.0 workaround
#define basepoint9 z.basepoint9
#define f25519_one z.f25519_one
-#define xm z.xm
-#define zm z.zm
#define xm1 z.xm1
#define zm1 z.zm1
+#define xm z.xm
+#define zm z.zm
+#define xms z.xms
+#define zms z.zms
memset(&z, 0, sizeof(z));
f25519_one[0] = 1;
zm[0] = 1;
@@ -583,8 +595,8 @@
for (i = 253; i >= 0; i--) {
const int bit = (e[i >> 3] >> (i & 7)) & 1;
- byte xms[F25519_SIZE];
- byte zms[F25519_SIZE];
+// byte xms[F25519_SIZE];
+// byte zms[F25519_SIZE];
/* From P_m and P_(m-1), compute P_(2m) and P_(2m-1) */
xc_diffadd(xm1, zm1, q, f25519_one, xm, zm, xm1, zm1);
@@ -597,10 +609,22 @@
* bit = 1 --> (P_(2m+1), P_(2m))
* bit = 0 --> (P_(2m), P_(2m-1))
*/
- fe_select(xm1, xm1, xm, bit);
- fe_select(zm1, zm1, zm, bit);
- fe_select(xm, xm, xms, bit);
- fe_select(zm, zm, zms, bit);
+#if 0
+ fe_select(xm1, xm, bit);
+ fe_select(zm1, zm, bit);
+ fe_select(xm, xms, bit);
+ fe_select(zm, zms, bit);
+#else
+// same as above in about 50 bytes smaller code, but
+// requires that in-memory order is exactly xm1,zm1,xm,zm,xms,zms
+ if (bit) {
+ //memcpy(xm1, xm, 4 * F25519_SIZE);
+ //^^^ gcc 11.0.0 warns of overlapping memcpy
+ //memmove(xm1, xm, 4 * F25519_SIZE);
+ //^^^ gcc 11.0.0 warns of out-of-bounds access to xm1[]
+ memmove(XM1, XM1 + 2 * F25519_SIZE, 4 * F25519_SIZE);
+ }
+#endif
}
/* Freeze out of projective coordinates */