typedef struct hmac_context {
  void (*init_hash)(const struct hmac_context *context);
  void (*update_hash)(const struct hmac_context *context,
                      const uint8_t *message,
                      unsigned message_size);
  void (*finish_hash)(const struct hmac_context *context, uint8_t *hash_result);
  unsigned block_size; /* Hash function block size in bytes, eg 64 for SHA-256. */
  unsigned result_size; /* Hash function result size in bytes, eg 32 for SHA-256. */
  uint8_t *tmp; /* Must point to a buffer of at least (2 * result_size + block_size) bytes. */
} hmac_context;


typedef struct hmac_sha256_context {
  hmac_context hmac_ctx;
  sha256_context sha_ctx;
  uint8_t tmp[32 * 2 + 64];
} hmac_sha256_context;

void
hmac_sha256_init (hmac_sha256_context *ctx, const uint8_t *K);

void
hmac_sha256_update (const hmac_sha256_context *ctx, const uint8_t *message,
                    unsigned message_size);

void
hmac_sha256_finish (const hmac_sha256_context *ctx, const uint8_t *K,
                    uint8_t *hash_result);