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tls: code shrink curve25519

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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>
This commit is contained in:
Denys Vlasenko 2021-10-11 16:31:47 +02:00
parent 74ee3f2f73
commit 23aba8a9a6
1 changed files with 39 additions and 15 deletions
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54
networking/tls_fe.c
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@ -63,16 +63,22 @@ static void fprime_select(byte *dst, const byte *zero, const byte *one, byte con
}
#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 @@ static void fe_normalize(byte *x)
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 @@ static void curve25519(byte *result, const byte *e, const byte *q)
{
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 @@ static void curve25519(byte *result, const byte *e, const byte *q)
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 @@ static void curve25519(byte *result, const byte *e, const byte *q)
* 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 */