Parsing an EC key using secp256k1 and SpecifiedECDomain

keyparse.c

#include <stdint.h>
#include <stdbool.h>
#include <string.h>
#include <polarssl/pk.h>

/* Initialise DER encoded key string*/
const char privkey_str[] = "3081d302010104208ea106a78201d7bfc11af5c9230803db1eb104e0a4f47fa3af4a23a785584206a08185308182020101302c06072a8648ce3d0101022100fffffffffffffffffffffffffffffffffffffffffffffffffffffffefffffc2f300604010004010704210279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798022100fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141020101a12403220002a6ce40504bd48f6e71eaa27784705ba457463fb3b746e881350a09af2b850afc";

bool decode_hex(unsigned char *p, size_t max_len, const char *hexstr, size_t *out_len_)
{
static const unsigned char hexdigit_val[256] = {
        ['0'] = 0,
        ['1'] = 1,
        ['2'] = 2,
        ['3'] = 3,
        ['4'] = 4,
        ['5'] = 5,
        ['6'] = 6,
        ['7'] = 7,
        ['8'] = 8,
        ['9'] = 9,
        ['a'] = 0xa,
        ['b'] = 0xb,
        ['c'] = 0xc,
        ['d'] = 0xd,
        ['e'] = 0xe,
        ['f'] = 0xf,
        ['A'] = 0xa,
        ['B'] = 0xb,
        ['C'] = 0xc,
        ['D'] = 0xd,
        ['E'] = 0xe,
        ['F'] = 0xf,};

        if (!p || !hexstr)
                return false;
        if (!strncmp(hexstr, "0x", 2))
                hexstr += 2;
        if (strlen(hexstr) > (max_len * 2))
                return false;

        unsigned char *buf = p;
        size_t out_len = 0;

        while (*hexstr) {
                unsigned char c1 = (unsigned char) hexstr[0];
                unsigned char c2 = (unsigned char) hexstr[1];

                unsigned char v1 = hexdigit_val[c1];
                unsigned char v2 = hexdigit_val[c2];

                if (!v1 && (c1 != '0'))
                        return false;
                if (!v2 && (c2 != '0'))
                        return false;

                *buf = (v1 << 4) | v2;

                out_len++;
                buf++;
                hexstr += 2;
        }

        if (out_len_)
                *out_len_ = out_len;
        return true;
}

int main ()
{
	unsigned char privkey[sizeof privkey_str];
	size_t privkey_len = 0;
	int ret = 1;
	const ecp_curve_info *curve;

	fprintf(stdout,"Starting: Key is %s\n", privkey_str);

	/* Decode DER string to HEX */
	if (decode_hex(privkey, sizeof privkey, privkey_str, &privkey_len) != true) {
		fprintf(stderr,"Error: decode_hex failed\n");
		return ret;
	}

	fprintf(stdout,"decode_hex: OK\n");

	/* Parse DER encoded private key using PolarSSL */
	pk_context polar_ctx;

	pk_init(&polar_ctx);

	if ( (ret = pk_parse_key(&polar_ctx, privkey, privkey_len, NULL, 0)) != 0 ) {
		fprintf(stderr,"Error: PolarSSL pk_parse_key failed : %i\n", ret);
		return ret;
	}
	
	if (pk_can_do(&polar_ctx, POLARSSL_PK_ECDSA) != 1 ) {
		fprintf(stderr,"Error: PolarSSL pk_can_do failed\n");
		return ret;
	}

	fprintf(stdout,"PolarSSL pk_can_do: OK\n");

	curve = ecp_curve_info_from_grp_id( pk_ec( polar_ctx )->grp.id );
	if( curve == NULL ) {
		fprintf(stderr,"Error: PolarSSL curve_info failed\n");
		return ret;
	}

	fprintf(stdout, "PolarSSL EC key using curve: %s\n", curve->name);

	if (&polar_ctx) {
		pk_free(&polar_ctx);
	}

	fprintf(stdout,"PolarSSL pk_free: OK\n");

	fprintf(stdout,"Success: All keys parsed OK\n");
	return 0;
}