static void HMAC_init(const uECC_HashContext *hash_context, const uint8_t *K) {
    uint8_t *pad = hash_context->tmp + 2 * hash_context->result_size;
    unsigned i;
    for (i = 0; i < hash_context->result_size; ++i)
        pad[i] = K[i] ^ 0x36;
    for (; i < hash_context->block_size; ++i)
        pad[i] = 0x36;

    hash_context->init_hash(hash_context);
    hash_context->update_hash(hash_context, pad, hash_context->block_size);
}