void hmac_sha256(unsigned char *digest, const unsigned char *in_key, int key_length, const unsigned char *in_message, int message_length) {
	SHA256_CTX ctx;
	unsigned char key[64] = { 0 };
	unsigned char o_key_pad[64] = { 0 };
	unsigned char i_key_pad[64] = { 0 };

	unsigned char* key_pads[] = {o_key_pad, i_key_pad, NULL};
	unsigned char** cur_key_pad = key_pads;
	unsigned char key_pad_xor = 0x5c;

	if (key_length > 64) {
		sha256_init(&ctx);
		sha256_update(&ctx, in_key, key_length);
		sha256_final(&ctx, key);
	} else {
		const unsigned char* cur_key = in_key;
		const unsigned char* end_key = in_key + key_length;
		unsigned char* out_key = key;
		while (cur_key < end_key) {
			*(out_key++) = *(cur_key++);
		}
	}

	// o_key_pad ← keyxor [0x5c * blockSize]   //Outer padded key
	// i_key_pad ← keyxor [0x36 * blockSize]   //Inner padded key
	// key_pad_xor = 0x5c -> xor 0x6a -> 0x36
	for (; *cur_key_pad; cur_key_pad++) {
		const unsigned char* cur_key = key;
		const unsigned char* end_key = key + 64;
		unsigned char* out_key = *cur_key_pad;
		while (cur_key < end_key) {
			*(out_key++) = *(cur_key++) ^ key_pad_xor;
		}
		key_pad_xor ^= 0x6A;
	}

	sha256_init(&ctx);
	sha256_update(&ctx, i_key_pad, 64);
	if(message_length)
		sha256_update(&ctx, in_message, message_length);
	sha256_final(&ctx, key);

	sha256_init(&ctx);
	sha256_update(&ctx, o_key_pad, 64);
	sha256_update(&ctx, key, 32);
	sha256_final(&ctx, digest);
}