Add note in coreSNTP demo about Network Time Security (NTS) (#648)

This PR makes the following changes in the coreSNTP demo:

To suggest the most secure way of using SNTP communication, we are adding a documentation note about the Network Time Security in the SNTP demo.
Hygiene update of using the FreeRTOS/backoffAlgorithm utility library for calculating time polling period interval backoff when time requests are rejected by a time server.

FreeRTOS-Plus/Demo/coreSNTP_Windows_Simulator/DemoTasks/SNTPClientTask.c

@@ -31,13 +31,22 @@
  *
  * This file contains the SNTP client (daemon) task as well as functionality for
  * maintaining wall-clock or UTC time in RAM. The SNTP client periodically synchronizes
- * system clock with an SNTP/NTP servers. Any other task running an application in the
+ * system clock with SNTP/NTP server(s). Any other task running an application in the
  * system can query the system time. For an example of an application task querying time
  * from the system, refer to the SampleAppTask.c file in this project.
  *
- * !!! NOTE !!!
+ * This demo shows how the coreSNTP library can be used to communicate with SNTP/NTP
- * This SNTP demo does not authenticate the server nor the client.
+ * servers in a mutually authenticated through the use of symmetric-key based AES-128-CMAC
- * Hence, this demo should not be used as production ready code.
+ * algorithm. To run this demo with an SNTP/NTP server in authenticated mode, the AES-128-CMAC
+ * symmetric key needs to be pre-shared between the client (i.e. this demo) and the server.
+ *
+ * !!!Note!!!:
+ * Even though this demo shows the use of AES-128-CMAC, a symmetric-key cryptographic based
+ * solution, for authenticating SNTP communication between the demo (SNTP client) and
+ * SNTP/NTP server, we instead RECOMMEND that production devices use the most secure authentication
+ * mechanism alternative available with the Network Time Security (NTS) protocol, an asymmetric-key
+ * cryptographic protocol. For more information, refer to the NTS specification here:
+ * https://datatracker.ietf.org/doc/html/rfc8915
  */
 
 /* Standard includes. */
@@ -70,6 +79,9 @@
 #include "core_pkcs11_config.h"
 #include "core_pkcs11.h"
 
+/* Backoff Algorithm include. */
+#include "backoff_algorithm.h"
+
 /*-----------------------------------------------------------*/
 
 /* Compile time error for undefined configs. */
@@ -170,6 +182,25 @@
  */
 #define SNTP_PACKET_AUTHENTICATED_MODE_SIZE     ( SNTP_PACKET_BASE_SIZE + SNTP_PACKET_SYMMETRIC_KEY_ID_LENGTH + pkcs11AES_CMAC_SIGNATURE_LENGTH )
 
+/**
+ * @brief The maximum poll period that the SNTP client can use as back-off on receiving a rejection from a time server.
+ *
+ * @note This demo performs back-off in polling rate from time server ONLY for the case when a single time server being
+ * is configured through the democonfigLIST_OF_TIME_SERVERS macro.
+ * This is because when more than one time server is configured, the coreSNTP library automatically handles the case
+ * of server rejection of time request by rotating to the next configured server for subsequent time polling requests.
+ */
+#define SNTP_DEMO_POLL_MAX_BACKOFF_DELAY_SEC    UINT16_MAX
+
+/**
+ * @brief The maximum number of times of retrying time requests at exponentially backed-off polling frequency
+ * from a server that rejects time requests.
+ *
+ * @note This macro is only relevant for the case when a single time server is configured in
+ * the demo through, democonfigLIST_OF_TIME_SERVERS.
+ */
+#define SNTP_DEMO_MAX_SERVER_BACKOFF_RETRIES    10
+
 /*-----------------------------------------------------------*/
 
 /**
@@ -593,6 +624,31 @@ static bool createUdpSocket( Socket_t * pSocket );
  */
 static void closeUdpSocket( Socket_t * pSocket );
 
+/**
+ * @brief Utility to calculate new poll period with exponential backoff and jitter
+ * algorithm.
+ *
+ * @note The demo applies time polling frequency backoff only when a single time server
+ * is configured, through the democonfigLIST_OF_SERVERS macro, and the single server
+ * rejects time requests.
+ *
+ * @param[in, out] pContext The context representing the back-off parameters. This
+ * context is initialized by the function whenever the caller indicates it with the
+ * @p shouldInitializeContext flag.
+ * @param[in] shouldInitializeContext Flag to indicate if the passed context should be
+ * initialized to start a new sequence of backed-off time request retries.
+ * @param[in] minPollPeriod The minimum poll period
+ * @param[in] pPollPeriod The new calculated poll period.
+ *
+ * @return Return #true if a new poll interval is calculated to retry time request
+ * from the server; #false otherwise to indicate exhaustion of time request retry attempts
+ * with the server.
+ */
+static bool calculateBackoffForNextPoll( BackoffAlgorithmContext_t * pContext,
+                                         bool shouldInitializeContext,
+                                         uint32_t minPollPeriod,
+                                         uint32_t * pPollPeriod );
+
 /*------------------------------------------------------------------------------*/
 
 static uint32_t translateYearToUnixSeconds( uint16_t year )
@@ -1379,6 +1435,46 @@ static void closeUdpSocket( Socket_t * pSocket )
 
 /*-----------------------------------------------------------*/
 
+static bool calculateBackoffForNextPoll( BackoffAlgorithmContext_t * pBackoffContext,
+                                         bool shouldInitializeContext,
+                                         uint32_t minPollPeriod,
+                                         uint32_t * pPollPeriod )
+{
+    uint16_t newPollPeriod = 0U;
+    BackoffAlgorithmStatus_t status;
+
+    configASSERT( pBackoffContext != NULL );
+    configASSERT( pPollPeriod != NULL );
+
+    if( shouldInitializeContext == true )
+    {
+        /* Initialize reconnect attempts and interval.*/
+        BackoffAlgorithm_InitializeParams( &pBackoffContext,
+                                           minPollPeriod,
+                                           SNTP_DEMO_POLL_MAX_BACKOFF_DELAY_SEC,
+                                           SNTP_DEMO_MAX_SERVER_BACKOFF_RETRIES );
+    }
+
+    /* Generate a random number and calculate the new backoff poll period to wait before the next
+     * time poll attempt. */
+    status = BackoffAlgorithm_GetNextBackoff( &pBackoffContext, generateRandomNumber(), &newPollPeriod );
+
+    if( status == BackoffAlgorithmRetriesExhausted )
+    {
+        LogError( ( "All backed-off attempts of polling time server have expired: MaxAttempts=%d",
+                    SNTP_DEMO_MAX_SERVER_BACKOFF_RETRIES ) );
+    }
+    else
+    {
+        /* Store the calculated backoff period as the new poll period. */
+        *pPollPeriod = newPollPeriod;
+    }
+
+    return( status == BackoffAlgorithmSuccess );
+}
+
+/*-----------------------------------------------------------*/
+
 void sntpTask( void * pParameters )
 {
     SntpContext_t clientContext;
@@ -1411,6 +1507,13 @@ void sntpTask( void * pParameters )
      * between client and server, if the server supports authentication. */
     static SntpAuthContext_t authContext;
 
+    /* Context used for calculating backoff that is applied to polling interval when the configured
+     * time server rejects time request.
+     * Note: Backoff is applied to polling interval ONLY when a single server is configured in the demo
+     * because in the case of multiple server configurations, the coreSNTP library handles server
+     * rejection by rotating server. */
+    static BackoffAlgorithmContext_t backoffContext;
+
     /* Initialize the authentication context for information for the first time server and its
      * keys configured in the demo. */
     populateAuthContextForServer( pTimeServers[ 0 ], &authContext );
@@ -1426,6 +1529,7 @@ void sntpTask( void * pParameters )
     if( initStatus == true )
     {
         SntpStatus_t status;
+        bool backoffModeFlag = false;
 
         /* Calculate Poll interval of SNTP client based on desired accuracy and clock tolerance of the system. */
         status = Sntp_CalculatePollInterval( democonfigSYSTEM_CLOCK_TOLERANCE_PPM,
@@ -1433,7 +1537,7 @@ void sntpTask( void * pParameters )
                                              &systemClock.pollPeriod );
         configASSERT( status == SntpSuccess );
 
-        LogDebug( ( "SNTP client polling interval calculated as %lus", systemClock.pollPeriod ) );
+        LogDebug( ( "Minimum SNTP client polling interval calculated as %lus", systemClock.pollPeriod ) );
 
         LogInfo( ( "Initialized SNTP Client context. Starting SNTP client loop to poll time every %lu seconds",
                    systemClock.pollPeriod ) );
@@ -1475,17 +1579,38 @@ void sntpTask( void * pParameters )
              * a single time server. */
             if( ( status == SntpRejectedResponse ) && ( numOfServers == 1 ) )
             {
+                bool backoffStatus = false;
+
+                /* Determine if this is the first back-off attempt we are making since the most recent server rejection
+                 * for time request. */
+                bool firstBackoffAttempt = false;
+
+                if( backoffModeFlag == false )
+                {
+                    firstBackoffAttempt = true;
+
+                    /* Set the flag to indicate we are in back-off retry mode for requesting time from the server. */
+                    backoffModeFlag = true;
+                }
+
                 LogInfo( ( "The single configured time server, %s, rejected time request. Backing-off before ",
                            "next time poll....", strlen( pTimeServers[ 0 ] ) ) );
 
                 /* Add exponential back-off to polling period. */
-                systemClock.pollPeriod *= 2;
+                backoffStatus = calculateBackoffForNextPoll( &backoffContext,
+                                                             firstBackoffAttempt,
+                                                             systemClock.pollPeriod,
+                                                             &systemClock.pollPeriod );
+                configASSERT( backoffStatus == true );
 
                 /* Wait for the increased poll interval before retrying request for time from server. */
                 vTaskDelay( pdMS_TO_TICKS( systemClock.pollPeriod * 1000 ) );
             }
             else
             {
+                /* Reset flag to indicate that we are not backing-off for the next time poll. */
+                backoffModeFlag = false;
+
                 /* Wait for the poll interval period before the next iteration of time synchronization. */
                 vTaskDelay( pdMS_TO_TICKS( systemClock.pollPeriod * 1000 ) );
             }

FreeRTOS-Plus/Demo/coreSNTP_Windows_Simulator/demo_config.h

@@ -109,6 +109,13 @@ extern void vLoggingPrintf( const char * pcFormatString,
  * It is RECOMMENDED to use an authentication mechanism for protecting devices against server spoofing
  * attacks.
  *
+ * @note Even though this demo shows the use of AES-128-CMAC, a symmetric-key cryptographic based
+ * solution, for authenticating SNTP communication between the demo (SNTP client) and
+ * SNTP/NTP server, we instead RECOMMEND that production devices use the most secure authentication
+ * mechanism alternative available with the Network Time Security (NTS) protocol, an asymmetric-key
+ * cryptographic protocol. For more information, refer to the NTS specification here:
+ * https://datatracker.ietf.org/doc/html/rfc8915
+ *
  * @note Please provide the 128-bit keys as comma separated list of hexadecimal strings in the order matching
  * the list of time servers configured in democonfigLIST_OF_TIME_SERVERS configuration. If a time server does
  * not support authentication, then NULL should be used to indicate use of no authentication mechanism for the

lexicon.txt

@@ -705,6 +705,7 @@ filteration
 findfirst
 findobjects
 fips
+firstbackoffattempt 
 fixme
 flase
 flasg
@@ -1226,6 +1227,7 @@ mingw
 minilistitem
 minimise
 minimised
+minpollperiod 
 mips
 misc
 misra
@@ -1325,6 +1327,7 @@ nsec
 nt
 ntp
 ntpdemo
+nts
 nullptr
 num
 numaker
@@ -1611,6 +1614,7 @@ ppass
 ppassword
 ppc
 ppcmessagetodisplay
+ppollperiod 
 ppr
 pprivatekey
 ppublishinfo
@@ -2206,6 +2210,7 @@ shadownamelength
 shadowstatus
 shasum
 shigherprioritytaskwoken
+shouldinitializecontext
 shouldn
 shtml
 sice