sw/device/lib/base/hardened.h

// Copyright lowRISC contributors.
// Licensed under the Apache License, Version 2.0, see LICENSE for details.
// SPDX-License-Identifier: Apache-2.0

#ifndef OPENTITAN_SW_DEVICE_LIB_BASE_HARDENED_H_
#define OPENTITAN_SW_DEVICE_LIB_BASE_HARDENED_H_

/**
 * @file
 * @brief Data Types for use in Hardened Code.
 */

/**
 * This is a boolean type for use in hardened contexts.
 *
 * The intention is that this is used instead of `<stdbool.h>`'s #bool, where a
 * higher hamming distance is required between the truthy and the falsey value.
 *
 * The values below were chosen at random, with some specific restrictions. They
 * have a Hamming Distance of 8, and they are 11-bit values so they can be
 * materialized with a single instruction on RISC-V. They are also specifically
 * not the complement of each other.
 */
typedef enum hardened_bool {
  /**
   * The truthy value, expected to be used like #true.
   */
  kHardenedBoolTrue = 0x739u,
  /**
   * The falsey value, expected to be used like #false.
   */
  kHardenedBoolFalse = 0x1d4u,
} hardened_bool_t;

/**
 * A byte-sized hardened boolean.
 *
 * This type is intended for cases where a byte-sized hardened boolean is
 * required, e.g. for the entries of the `CREATOR_SW_CFG_KEY_IS_VALID` OTP item.
 *
 * The values below were chosen to ensure that the hamming difference between
 * them is greater than 5 and they are not bitwise complements of each other.
 */
typedef enum hardened_byte_bool {
  /**
   * The truthy value.
   */
  kHardenedByteBoolTrue = 0xa5,
  /**
   * The falsy value.
   */
  kHardenedByteBoolFalse = 0x4b,
} hardened_byte_bool_t;

#endif  // OPENTITAN_SW_DEVICE_LIB_BASE_HARDENED_H_