Set random NTP port by default

[luigigubello]

Added method to set a (pseudo)random local port, so that the board needs not to use always the same embedded local port for receiving NTP packets.

At the moment, ArduinoIoTCloud library uses the fixed port 8888 for receiving NTP packets via unencrypted UDP connection. Changing randomly port to receive NTP packets doesn't improve cryptographic security and it is not the final solution, but it adds a layer to make harder the attacker's job.

How it works

NTPUtils.h

extern const int  MIN_NTP_PORT;
extern const int  MAX_NTP_PORT;

NTPUtils.cpp

unsigned long NTPUtils::getTime(UDP & udp)
{
  NTPUtils randomPort;
  int _randomPort = randomPort.setRandomPort(MIN_NTP_PORT, MAX_NTP_PORT);
  udp.begin(_randomPort);
  [...]
}

int NTPUtils::setRandomPort(int minValue, int maxValue) {
  if (has_crypto) {
    return ECCX08.random(minValue, maxValue);
  } else {
    randomSeed(analogRead(0));
    return random(minValue, maxValue);
  }
}

The user sets the variables MIN_NTP_PORT and MIN_NTP_PORT and the ArduinoIoTCloud library will use this range to calculate a random port for receiving NTP packets.

How to use

In ThingProperties.h add the lines:

int MIN_NTP_PORT = 49152;
int MAX_NTP_PORT = 65535;

The user can set arbitrary ports, 49152 and 65535 are based on IANA Standard.

[codecov-commenter]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #198   +/-   ##
=======================================
  Coverage   95.27%   95.27%           
=======================================
  Files          25       25           
  Lines         889      889           
=======================================
  Hits          847      847           
  Misses         42       42           

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