Sesame-Device/include/fresh.h

#pragma once

#include <stdint.h>

struct TimeConfiguration {

    /**
     * @brief The timezone offset in seconds
     */
    int32_t offsetToGMT;

    /**
     * @brief The daylight savings offset in seconds
     */
    int32_t offsetDaylightSavings;

    /**
     * @brief The url of the NTP server
     */
    const char* ntpServerUrl;

    /**
     * @brief The allowed discrepancy between the time of a received message
     * and the device time (in seconds)
     * 
     * A stricter (lower) value better prevents against replay attacks,
     * but may lead to issues when dealing with slow networks and other
     * routing delays.
     */
    uint32_t allowedTimeOffset;
};

class TimeCheck {

public:

    /**
     * @brief Create a time checker instance
     */
    TimeCheck();

    /**
     * @brief Set the configuration
     */
    void configure(TimeConfiguration configuration);

    /**
     * @brief Configure the NTP server to get the current time
     */
    void startNTP();

    /**
     * @brief Print the current time to the serial output
     * 
     * The time must be initialized by calling `configureNTP()` before use.
     */
    void printLocalTime();

    /** 
     * Gets the current epoch time
     */
    uint32_t getEpochTime();

    /**
     * @brief Check wether the time of a message is within the allowed bounds regarding freshness.
     * 
     * The timestamp is used to ensure 'freshness' of the messages,
     * i.e. that they are not unreasonably delayed or captured and
     * later replayed by an attacker.
     * 
     * @param messageTime The timestamp of the message (seconds since epoch) 
     * @return true The time is within the acceptable offset of the local time
     * @return false The message time is invalid
     */
    bool isMessageTimeAcceptable(uint32_t messageTime);

private:

    TimeConfiguration config;
};