int uECC_verify()

in ext/tinycrypt/src/ecc_dsa.c [192:294]
81 lines of code
8 McCabe index (conditional complexity)

int uECC_verify(const uint8_t *public_key, const uint8_t *message_hash,
		unsigned hash_size, const uint8_t *signature,
	        uECC_Curve curve)
{

	uECC_word_t u1[NUM_ECC_WORDS], u2[NUM_ECC_WORDS];
	uECC_word_t z[NUM_ECC_WORDS];
	uECC_word_t sum[NUM_ECC_WORDS * 2];
	uECC_word_t rx[NUM_ECC_WORDS];
	uECC_word_t ry[NUM_ECC_WORDS];
	uECC_word_t tx[NUM_ECC_WORDS];
	uECC_word_t ty[NUM_ECC_WORDS];
	uECC_word_t tz[NUM_ECC_WORDS];
	const uECC_word_t *points[4];
	const uECC_word_t *point;
	bitcount_t num_bits;
	bitcount_t i;

	uECC_word_t _public[NUM_ECC_WORDS * 2];
	uECC_word_t r[NUM_ECC_WORDS], s[NUM_ECC_WORDS];
	wordcount_t num_words = curve->num_words;
	wordcount_t num_n_words = BITS_TO_WORDS(curve->num_n_bits);

	rx[num_n_words - 1] = 0;
	r[num_n_words - 1] = 0;
	s[num_n_words - 1] = 0;

	uECC_vli_bytesToNative(_public, public_key, curve->num_bytes);
	uECC_vli_bytesToNative(_public + num_words, public_key + curve->num_bytes,
			       curve->num_bytes);
	uECC_vli_bytesToNative(r, signature, curve->num_bytes);
	uECC_vli_bytesToNative(s, signature + curve->num_bytes, curve->num_bytes);

	/* r, s must not be 0. */
	if (uECC_vli_isZero(r, num_words) || uECC_vli_isZero(s, num_words)) {
		return 0;
	}

	/* r, s must be < n. */
	if (uECC_vli_cmp_unsafe(curve->n, r, num_n_words) != 1 ||
	    uECC_vli_cmp_unsafe(curve->n, s, num_n_words) != 1) {
		return 0;
	}

	/* Calculate u1 and u2. */
	uECC_vli_modInv(z, s, curve->n, num_n_words); /* z = 1/s */
	u1[num_n_words - 1] = 0;
	bits2int(u1, message_hash, hash_size, curve);
	uECC_vli_modMult(u1, u1, z, curve->n, num_n_words); /* u1 = e/s */
	uECC_vli_modMult(u2, r, z, curve->n, num_n_words); /* u2 = r/s */

	/* Calculate sum = G + Q. */
	uECC_vli_set(sum, _public, num_words);
	uECC_vli_set(sum + num_words, _public + num_words, num_words);
	uECC_vli_set(tx, curve->G, num_words);
	uECC_vli_set(ty, curve->G + num_words, num_words);
	uECC_vli_modSub(z, sum, tx, curve->p, num_words); /* z = x2 - x1 */
	XYcZ_add(tx, ty, sum, sum + num_words, curve);
	uECC_vli_modInv(z, z, curve->p, num_words); /* z = 1/z */
	apply_z(sum, sum + num_words, z, curve);

	/* Use Shamir's trick to calculate u1*G + u2*Q */
	points[0] = 0;
	points[1] = curve->G;
	points[2] = _public;
	points[3] = sum;
	num_bits = smax(uECC_vli_numBits(u1, num_n_words),
	uECC_vli_numBits(u2, num_n_words));

	point = points[(!!uECC_vli_testBit(u1, num_bits - 1)) |
                       ((!!uECC_vli_testBit(u2, num_bits - 1)) << 1)];
	uECC_vli_set(rx, point, num_words);
	uECC_vli_set(ry, point + num_words, num_words);
	uECC_vli_clear(z, num_words);
	z[0] = 1;

	for (i = num_bits - 2; i >= 0; --i) {
		uECC_word_t index;
		curve->double_jacobian(rx, ry, z, curve);

		index = (!!uECC_vli_testBit(u1, i)) | ((!!uECC_vli_testBit(u2, i)) << 1);
		point = points[index];
		if (point) {
			uECC_vli_set(tx, point, num_words);
			uECC_vli_set(ty, point + num_words, num_words);
			apply_z(tx, ty, z, curve);
			uECC_vli_modSub(tz, rx, tx, curve->p, num_words); /* Z = x2 - x1 */
			XYcZ_add(tx, ty, rx, ry, curve);
			uECC_vli_modMult_fast(z, z, tz, curve);
		}
  	}

	uECC_vli_modInv(z, z, curve->p, num_words); /* Z = 1/Z */
	apply_z(rx, ry, z, curve);

	/* v = x1 (mod n) */
	if (uECC_vli_cmp_unsafe(curve->n, rx, num_n_words) != 1) {
		uECC_vli_sub(rx, rx, curve->n, num_n_words);
	}

	/* Accept only if v == r. */
	return (int)(uECC_vli_equal(rx, r, num_words) == 0);
}