Read compressed point with polarssl

gistfile1.c

#include "polarssl/ecp.h"
#include <stdio.h>

static const unsigned char comp[4][33] = {
	{	0x02, 0x79, 0xBE, 0x66, 0x7E, 0xF9, 0xDC, 0xBB, 0xAC, 0x55, 0xA0, 0x62, 0x95, 0xCE, 0x87, 0x0B, 0x07, 0x02, 0x9B, 0xFC, 0xDB, 0x2D, 0xCE, 0x28, 0xD9, 0x59, 0xF2, 0x81, 0x5B, 0x16, 0xF8, 0x17, 0x98 },
	{	0x03, 0xF2, 0x87, 0x73, 0xC2, 0xD9, 0x75, 0x28, 0x8B, 0xC7, 0xD1, 0xD2, 0x05, 0xC3, 0x74, 0x86, 0x51, 0xB0, 0x75, 0xFB, 0xC6, 0x61, 0x0E, 0x58, 0xCD, 0xDE, 0xED, 0xDF, 0x8F, 0x19, 0x40, 0x5A, 0xA8 },
	{	0x02, 0xFE, 0x8D, 0x1E, 0xB1, 0xBC, 0xB3, 0x43, 0x2B, 0x1D, 0xB5, 0x83, 0x3F, 0xF5, 0xF2, 0x22, 0x6D, 0x9C, 0xB5, 0xE6, 0x5C, 0xEE, 0x43, 0x05, 0x58, 0xC1, 0x8E, 0xD3, 0xA3, 0xC8, 0x6C, 0xE1, 0xAF },
	{	0x03, 0x07, 0x92, 0x64, 0xC4, 0xB4, 0xBF, 0xCD, 0x7F, 0xE3, 0xA7, 0xB7, 0xB9, 0x2B, 0x6C, 0x43, 0x9F, 0x3A, 0x5B, 0x3A, 0xBC, 0xD2, 0x91, 0x89, 0xBF, 0x7B, 0x54, 0xD7, 0x81, 0xFF, 0x03, 0xD7, 0x22 }
};

static const unsigned char uncomp[4][65] = {
	{	0x04, 0x79, 0xBE, 0x66, 0x7E, 0xF9, 0xDC, 0xBB, 0xAC, 0x55, 0xA0, 0x62, 0x95, 0xCE, 0x87, 0x0B, 0x07, 0x02, 0x9B, 0xFC, 0xDB, 0x2D, 0xCE, 0x28, 0xD9, 0x59, 0xF2, 0x81, 0x5B, 0x16, 0xF8, 0x17, 0x98, 0x48, 0x3A, 0xDA, 0x77, 0x26, 0xA3, 0xC4, 0x65, 0x5D, 0xA4, 0xFB, 0xFC, 0x0E, 0x11, 0x08, 0xA8, 0xFD, 0x17, 0xB4, 0x48, 0xA6, 0x85, 0x54, 0x19, 0x9C, 0x47, 0xD0, 0x8F, 0xFB, 0x10, 0xD4, 0xB8 },
	{	0x04, 0xF2, 0x87, 0x73, 0xC2, 0xD9, 0x75, 0x28, 0x8B, 0xC7, 0xD1, 0xD2, 0x05, 0xC3, 0x74, 0x86, 0x51, 0xB0, 0x75, 0xFB, 0xC6, 0x61, 0x0E, 0x58, 0xCD, 0xDE, 0xED, 0xDF, 0x8F, 0x19, 0x40, 0x5A, 0xA8, 0x0A, 0xB0, 0x90, 0x2E, 0x8D, 0x88, 0x0A, 0x89, 0x75, 0x82, 0x12, 0xEB, 0x65, 0xCD, 0xAF, 0x47, 0x3A, 0x1A, 0x06, 0xDA, 0x52, 0x1F, 0xA9, 0x1F, 0x29, 0xB5, 0xCB, 0x52, 0xDB, 0x03, 0xED, 0x81 },
	{	0x04, 0xFE, 0x8D, 0x1E, 0xB1, 0xBC, 0xB3, 0x43, 0x2B, 0x1D, 0xB5, 0x83, 0x3F, 0xF5, 0xF2, 0x22, 0x6D, 0x9C, 0xB5, 0xE6, 0x5C, 0xEE, 0x43, 0x05, 0x58, 0xC1, 0x8E, 0xD3, 0xA3, 0xC8, 0x6C, 0xE1, 0xAF, 0x07, 0xB1, 0x58, 0xF2, 0x44, 0xCD, 0x0D, 0xE2, 0x13, 0x4A, 0xC7, 0xC1, 0xD3, 0x71, 0xCF, 0xFB, 0xFA, 0xE4, 0xDB, 0x40, 0x80, 0x1A, 0x25, 0x72, 0xE5, 0x31, 0xC5, 0x73, 0xCD, 0xA9, 0xB5, 0xB4 },
	{	0x04, 0x07, 0x92, 0x64, 0xC4, 0xB4, 0xBF, 0xCD, 0x7F, 0xE3, 0xA7, 0xB7, 0xB9, 0x2B, 0x6C, 0x43, 0x9F, 0x3A, 0x5B, 0x3A, 0xBC, 0xD2, 0x91, 0x89, 0xBF, 0x7B, 0x54, 0xD7, 0x81, 0xFF, 0x03, 0xD7, 0x22, 0x6F, 0x6F, 0x0E, 0x07, 0x84, 0xEA, 0xDA, 0x9F, 0x92, 0x99, 0x9E, 0xE9, 0xC4, 0x38, 0xD4, 0x7E, 0xAA, 0x2C, 0x80, 0x68, 0xF1, 0x84, 0x51, 0x97, 0xE3, 0x07, 0x1C, 0x74, 0xB0, 0x63, 0xC5, 0xE1 }
};

// Single purpose read point in binary format, only support compressed secp256k1 point
int ecp_point_read_binary2(const ecp_group *group, ecp_point *point, const unsigned char *buffer, size_t ilen) {
	int ret;
	unsigned char parity;
	size_t plen;
	mpi e, y2;

	mpi_init(&e); mpi_init(&y2);

	ret = ecp_point_read_binary(group, point, buffer, ilen);
	if (POLARSSL_ERR_ECP_FEATURE_UNAVAILABLE != ret) {
		return ret;
	}

	if (POLARSSL_ECP_DP_SECP256K1 != group->id) {
		return POLARSSL_ERR_ECP_FEATURE_UNAVAILABLE;
	}

	if (0x02 == buffer[0]) {
		parity = 0;
	} else if (0x03 == buffer[0]) {
		parity = 1;
	} else {
		return POLARSSL_ERR_ECP_BAD_INPUT_DATA;
	}

	plen = mpi_size(&group->P);

	if (ilen != plen + 1) {
		return POLARSSL_ERR_ECP_BAD_INPUT_DATA;
	}

	MPI_CHK(mpi_read_binary(&point->X, buffer + 1, plen));
	MPI_CHK(mpi_lset(&point->Z, 1));

	// Set y2 = X^3 + B
	MPI_CHK(mpi_mul_mpi(&y2, &point->X, &point->X));
	MPI_CHK(mpi_mod_mpi(&y2, &y2, &group->P));
	MPI_CHK(mpi_mul_mpi(&y2, &y2, &point->X));
	MPI_CHK(mpi_add_mpi(&y2, &y2, &group->B));
	MPI_CHK(mpi_mod_mpi(&y2, &y2, &group->P));

	// Compute square root of y2
	MPI_CHK(mpi_add_int(&e, &group->P, 1));
	MPI_CHK(mpi_shift_r(&e, 2));
	MPI_CHK(mpi_exp_mod(&point->Y, &y2, &e, &group->P, NULL));

	// Set parity
	if (mpi_get_bit(&point->Y, 0) != parity) {
		MPI_CHK(mpi_sub_mpi(&point->Y, &group->P, &point->Y));
	}

cleanup:
	mpi_free(&e);
	mpi_free(&y2);

	return ret;
}

int main() {
	int ret;
	ecp_group group;
	ecp_point point;
	unsigned char buffer[65];
	size_t i, olen = 0;

	ecp_group_init(&group);
	ecp_point_init(&point);

	MPI_CHK(ecp_use_known_dp(&group, POLARSSL_ECP_DP_SECP256K1));

	for (i = 0; i < 4; i++) {
		MPI_CHK(ecp_point_read_binary2(&group, &point, comp[i], sizeof(comp[i])));
		MPI_CHK(ecp_point_write_binary(&group, &point, POLARSSL_ECP_PF_UNCOMPRESSED, &olen, buffer, sizeof(buffer)));

		if (memcmp(buffer, uncomp[i], sizeof(buffer))) {
			printf ("Failed. Point %d not same\n", i);
		} else {
			printf ("Point %d OK\n", i);
		}

		ecp_point_free(&point);
	}

cleanup:
	ecp_group_free(&group);
	ecp_point_free(&point);

	if (ret) {
		printf("failed: -0x%04X\n", -ret);
	}

	return ret;
}