Sesame-Device/include/fresh.h

#pragma once

#include <stdint.h>
#include "config.h"

class TimeCheck {

public:

    /**
     * @brief Create a time checker instance
     * 
     * Specify 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.
     * 
     * @param offset The allowed time discrepancy in both directions (seconds)
     */
    TimeCheck(uint32_t allowedTimeOffset = 60);

    /**
     * @brief Configure an NTP server to get the current time
     * 
     * @param offsetToGMT The timezone offset in seconds
     * @param offsetDaylightSavings The daylight savings offset in seconds
     * @param serverUrl The url of the NTP server
     */
    void configureNTP(int32_t offsetToGMT, int32_t offsetDaylightSavings, const char* serverUrl);

    /**
     * @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 The allowed time discrepancy (in seconds)
     * 
     * Specifies 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.
     * 
     * @param offset The offset in both directions (seconds)
     */
    void setMessageTimeAllowedOffset(uint32_t offset);

    /**
     * @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:
    
    /**
     * @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 allowedOffset;
};