trezor-firmware/bignum.c

/**
 * Copyright (c) 2013 Tomas Dzetkulic
 * Copyright (c) 2013 Pavol Rusnak
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES
 * OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
 * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */

#include "bignum.h"
#include "secp256k1.h"

inline uint32_t read_be(const uint8_t *data)
{
	return (((uint32_t)data[0]) << 24) |
	       (((uint32_t)data[1]) << 16) |
	       (((uint32_t)data[2]) << 8)  |
	       (((uint32_t)data[3]));
}

inline void write_be(uint8_t *data, uint32_t x)
{
	data[0] = x >> 24;
	data[1] = x >> 16;
	data[2] = x >> 8;
	data[3] = x;
}

void bn_read_be(const uint8_t *in_number, bignum256 *out_number)
{
	int i;
	uint64_t temp = 0;
	for (i = 0; i < 8; i++) {
		temp += (((uint64_t)read_be(in_number + (7 - i) * 4)) << (2 * i));
		out_number->val[i]= temp & 0x3FFFFFFF;
		temp >>= 30;
	}
	out_number->val[8] = temp;
}

void bn_write_be(const bignum256 *in_number, uint8_t *out_number)
{
	int i, shift = 30 + 16 - 32;
	uint64_t temp = in_number->val[8];
	for (i = 0; i < 8; i++) {
		temp <<= 30;
		temp |= in_number->val[7 - i];
		write_be(out_number + i * 4, temp >> shift);
		shift -= 2;
	}
}

int bn_is_zero(const bignum256 *a)
{
	int i;
	for (i = 0; i < 9; i++) {
		if (a->val[i] != 0) return 0;
	}
	return 1;
}

int bn_is_less(const bignum256 *a, const bignum256 *b)
{
	int i;
	for (i = 8; i >= 0; i--) {
		if (a->val[i] < b->val[i]) return 1;
		if (a->val[i] > b->val[i]) return 0;
	}
	return 0;
}

// assumes x < 2*prime, result < prime
void bn_mod(bignum256 *x, const bignum256 *prime)
{
	int i = 8;
	uint32_t temp;
	// compare numbers
	while (i >= 0 && prime->val[i] == x->val[i]) i--;
	// if equal
	if (i == -1) {
		// set x to zero
		for (i = 0; i < 9; i++) {
			x->val[i] = 0;
		}
	} else {
		// if x is greater
		if (x->val[i] > prime->val[i]) {
			// substract p from x
			temp = 0x40000000u;
			for (i = 0; i < 9; i++) {
				temp += x->val[i] - prime->val[i];
				x->val[i] = temp & 0x3FFFFFFF;
				temp >>= 30;
				temp += 0x3FFFFFFFu;
			}
		}
	}
}

// x = k * x
// both inputs and result may be bigger than prime but not bigger than 2 * prime
void bn_multiply(const bignum256 *k, bignum256 *x, const bignum256 *prime)
{
	int i, j;
	uint64_t temp = 0;
	uint32_t res[18], coef;

	// compute lower half of long multiplication
	for (i = 0; i < 9; i++)
	{
		for (j = 0; j <= i; j++) {
			temp += k->val[j] * (uint64_t)x->val[i - j];
		}
		res[i] = temp & 0x3FFFFFFFu;
		temp >>= 30;
	}
	// compute upper half
	for (; i < 17; i++)
	{
		for (j = i - 8; j < 9 ; j++) {
			temp += k->val[j] * (uint64_t)x->val[i - j];
		}
		res[i] = temp & 0x3FFFFFFFu;
		temp >>= 30;
	}
	res[17] = temp;
	// compute modulo p division is only estimated so this may give result greater than prime but not bigger than 2 * prime
	for (i = 16; i >= 8; i--) {
		// estimate (res / prime)
		coef = (res[i] >> 16) + (res[i + 1] << 14);
		// substract (coef * prime) from res
		temp = 0x1000000000000000llu + res[i - 8] - prime->val[0] * (uint64_t)coef;
		res[i - 8] = temp & 0x3FFFFFFF;
		for (j = 1; j < 9; j++) {
			temp >>= 30;
			temp += 0xFFFFFFFC0000000llu + res[i - 8 + j] - prime->val[j] * (uint64_t)coef;
			res[i - 8 + j] = temp & 0x3FFFFFFF;
		}
	}
	// store the result
	for (i = 0; i < 9; i++) {
		x->val[i] = res[i];
	}
}

// result is smaller than 2*prime
void bn_fast_mod(bignum256 *x, const bignum256 *prime)
{
	int j;
	uint32_t coef;
	uint64_t temp;

	coef = x->val[8] >> 16;
	if (!coef) return;
	// substract (coef * prime) from x
	temp = 0x1000000000000000llu + x->val[0] - prime->val[0] * (uint64_t)coef;
	x->val[0] = temp & 0x3FFFFFFF;
	for (j = 1; j < 9; j++) {
		temp >>= 30;
		temp += 0xFFFFFFFC0000000llu + x->val[j] - prime->val[j] * (uint64_t)coef;
		x->val[j] = temp & 0x3FFFFFFF;
	}
}

#ifndef INVERSE_FAST

#ifdef USE_PRECOMPUTED_IV
#warning USE_PRECOMPUTED_IV will not be used, please undef
#endif

// in field G_prime, small but slow
void bn_inverse(bignum256 *x, const bignum256 *prime)
{
	uint32_t i, j, limb;
	bignum256 res;
	res.val[0] = 1;
	for (i = 1; i < 9; i++) {
		res.val[i] = 0;
	}
	for (i = 0; i < 9; i++) {
		limb = prime->val[i];
		// this is not enough in general but fine for secp256k1 because prime->val[0] > 1
		if (i == 0) limb -= 2;
		for (j = 0; j < 30; j++) {
			if (i == 8 && limb == 0) break;
			if (limb & 1) {
				multiply(x, &res, prime);
			}
			limb >>= 1;
			multiply(x, x, prime);
		}
	}
	bn_mod(&res, prime);
	memcpy(x, &res, sizeof(bignum256));
}

#else

// in field G_prime, big but fast
void bn_inverse(bignum256 *x, const bignum256 *prime)
{
	int i, j, k, len1, len2, mask;
	uint32_t u[9], v[9], s[10], r[10], temp, temp2;
	bn_fast_mod(x, prime);
	bn_mod(x, prime);
	for (i = 0; i < 9; i++) {
		u[i] = prime->val[i];
		v[i] = x->val[i];
	}
	len1 = 9;
	s[0] = 1;
	r[0] = 0;
	len2 = 1;
	k = 0;
	for (;;) {
		for (i = 0; i < len1; i++) {
			if (v[i]) break;
		}
		if (i == len1) break;
		for (;;) {
			for (i = 0; i < 30; i++) {
				if (u[0] & (1 << i)) break;
			}
			if (i == 0) break;
			mask = (1 << i) - 1;
			for (j = 0; j + 1 < len1; j++) {
				u[j] = (u[j] >> i) | ((u[j + 1] & mask) << (30 - i));
			}
			u[j] = (u[j] >> i);
			mask = (1 << (30 - i)) - 1;
			s[len2] = s[len2 - 1] >> (30 - i);
			for (j = len2 - 1; j > 0; j--) {
				s[j] = (s[j - 1] >> (30 - i)) | ((s[j] & mask) << i);
			}
			s[0] = (s[0] & mask) << i;
			if (s[len2]) {
				r[len2] = 0;
				len2++;
			}
			k += i;
		}
		for (;;) {
			for (i = 0; i < 30; i++) {
				if (v[0] & (1 << i)) break;
			}
			if (i == 0) break;
			mask = (1 << i) - 1;
			for (j = 0; j + 1 < len1; j++) {
				v[j] = (v[j] >> i) | ((v[j + 1] & mask) << (30 - i));
			}
			v[j] = (v[j] >> i);
			mask = (1 << (30 - i)) - 1;
			r[len2] = r[len2 - 1] >> (30 - i);
			for (j = len2 - 1; j > 0; j--) {
				r[j] = (r[j - 1] >> (30 - i)) | ((r[j] & mask) << i);
			}
			r[0] = (r[0] & mask) << i;
			if (r[len2]) {
				s[len2] = 0;
				len2++;
			}
			k += i;
		}
		
		i = len1 - 1;
		while (i > 0 && u[i] == v[i]) i--;
		if (u[i] > v[i]) {
			temp = 0x40000000u + u[0] - v[0];
			u[0] = (temp >> 1) & 0x1FFFFFFF;
			temp >>= 30;
			for (i = 1; i < len1; i++) {
				temp += 0x3FFFFFFFu + u[i] - v[i];
				u[i - 1] += (temp & 1) << 29;
				u[i] = (temp >> 1) & 0x1FFFFFFF;
				temp >>= 30;
			}
			temp = temp2 = 0;
			for (i = 0; i < len2; i++) {
				temp += s[i] + r[i];
				temp2 += s[i] << 1;
				r[i] = temp & 0x3FFFFFFF;
				s[i] = temp2 & 0x3FFFFFFF;
				temp >>= 30;
				temp2 >>= 30;
			}
			if (temp != 0 || temp2 != 0) {
				r[len2] = temp;
				s[len2] = temp2;
				len2++;
			}
		} else {
			temp = 0x40000000u + v[0] - u[0];
			v[0] = (temp >> 1) & 0x1FFFFFFF;
			temp >>= 30;
			for (i = 1; i < len1; i++) {
				temp += 0x3FFFFFFFu + v[i] - u[i];
				v[i - 1] += (temp & 1) << 29;
				v[i] = (temp >> 1) & 0x1FFFFFFF;
				temp >>= 30;
			}
			temp = temp2 = 0;
			for (i = 0; i < len2; i++) {
				temp += s[i] + r[i];
				temp2 += r[i] << 1;
				s[i] = temp & 0x3FFFFFFF;
				r[i] = temp2 & 0x3FFFFFFF;
				temp >>= 30;
				temp2 >>= 30;
			}
			if (temp != 0 || temp2 != 0) {
				s[len2] = temp;
				r[len2] = temp2;
				len2++;
			}
		}
		if (u[len1 - 1] == 0 && v[len1 - 1] == 0) len1--;
		k++;
	}
	i = 8;
	while (i > 0 && r[i] == prime->val[i]) i--;
	if (r[i] >= prime->val[i]) {
		temp = 1;
		for (i = 0; i < 9; i++) {
			temp += 0x3FFFFFFF + r[i] - prime->val[i];
			r[i] = temp & 0x3FFFFFFF;
			temp >>= 30;
		}
	}
	temp = 1;
	for (i = 0; i < 9; i++) {
		temp += 0x3FFFFFFF + prime->val[i] - r[i];
		r[i] = temp & 0x3FFFFFFF;
		temp >>= 30;
	}
	int done = 0;
#ifdef USE_PRECOMPUTED_IV
	if (prime == &prime256k1) {
		for (j = 0; j < 9; j++) {
			x->val[j] = r[j];
		}
		bn_multiply(secp256k1_iv + k - 256, x, prime);
		bn_fast_mod(x, prime);
		done = 1;
	}
#endif
	if (!done) {
		for (j = 0; j < k; j++) {
			if (r[0] & 1) {
				temp = r[0] + prime->val[0];
				r[0] = (temp >> 1) & 0x1FFFFFFF;
				temp >>= 30;
				for (i = 1; i < 9; i++) {
					temp += r[i] + prime->val[i];
					r[i - 1] += (temp & 1) << 29;
					r[i] = (temp >> 1) & 0x1FFFFFFF;
					temp >>= 30;
				}
			} else {
				for (i = 0; i < 8; i++) {
					r[i] = (r[i] >> 1) | ((r[i + 1] & 1) << 29);
				}
				r[8] = r[8] >> 1;
			}
		}
		for (j = 0; j < 9; j++) {
			x->val[j] = r[j];
		}
	}
}
#endif


void bn_addmod(bignum256 *a, const bignum256 *b, const bignum256 *prime)
{
	int i;
	uint32_t carry = 0;
	for (i = 0; i < 9; i++) {
		a->val[i] += b->val[i] + carry;
		if (a->val[i] > 0x3FFFFFFF) {
			carry = a->val[i] >> 30;
			a->val[i] &= 0x3FFFFFFF;
		} else {
			carry = 0;
		}
	}
	bn_mod(a, prime);
}

// res = a - b
// b < 2*prime; result not normalized
void bn_substract(const bignum256 *a, const bignum256 *b, bignum256 *res)
{
	int i;
	uint32_t temp = 0;
	for (i = 0; i < 9; i++) {
		temp += a->val[i] + 2u *prime256k1.val[i] - b->val[i];
		res->val[i] = temp & 0x3FFFFFFF;
		temp >>= 30;
	}
}