Seed random number generator used for generating request IDs when sending requests over FIFO based USB transport

examples/htool_usb.c

@@ -21,6 +21,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "../libhoth_usb.h"
 #include "ec_util.h"
@@ -324,8 +325,21 @@ struct libhoth_device* htool_libhoth_usb_device(void) {
   if (!ctx || !usb_dev) {
     return NULL;
   }
-  struct libhoth_usb_device_init_options opts = {.usb_device = usb_dev,
-                                                 .usb_ctx = ctx};
+
+  struct timespec monotonic_time;
+  // `clock_gettime` function is guaranteed by POSIX standard on compliant
+  // systems. But it may have implementation defined resolution. So xor with PID
+  // as well
+  if (clock_gettime(CLOCK_MONOTONIC, &monotonic_time) != 0) {
+    fprintf(stderr, "Could not get clock time to generate PRNG seed\n");
+    return NULL;
+  }
+  uint32_t prng_seed =
+      monotonic_time.tv_sec ^ monotonic_time.tv_nsec ^ getpid();
+
+  struct libhoth_usb_device_init_options opts = {
+      .usb_device = usb_dev, .usb_ctx = ctx, .prng_seed = prng_seed};
+
   int rv = libhoth_usb_open(&opts, &result);
   if (rv) {
     // TODO: Convert error-code to a string

libhoth_usb.c

@@ -142,7 +142,8 @@ int libhoth_usb_open(const struct libhoth_usb_device_init_options* options,
       status = libhoth_usb_mailbox_open(usb_dev, config_descriptor);
       break;
     case LIBHOTH_USB_INTERFACE_TYPE_FIFO:
-      status = libhoth_usb_fifo_open(usb_dev, config_descriptor);
+      status =
+          libhoth_usb_fifo_open(usb_dev, config_descriptor, options->prng_seed);
       break;
     default:
       status = LIBHOTH_ERR_INTERFACE_NOT_FOUND;

libhoth_usb.h

@@ -17,6 +17,7 @@
 
 #include <libusb.h>
 #include <stddef.h>
+#include <stdint.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -30,6 +31,9 @@ struct libhoth_usb_device_init_options {
   // The libusb context to use for operations. Can be NULL for the default
   // context.
   libusb_context* usb_ctx;
+  // Seed value to use for Pseudo-random number generator for communicating with
+  // RoT over USB FIFO interface
+  uint32_t prng_seed;
 };
 
 // Note that the options struct only needs to to live for the duration of

libhoth_usb_device.h

@@ -59,6 +59,7 @@ struct libhoth_usb_fifo {
   int all_transfers_completed;
   bool in_transfer_completed;
   bool out_transfer_completed;
+  uint32_t prng_state;
 };
 
 struct libhoth_usb_interface_info {
@@ -78,7 +79,8 @@ struct libhoth_usb_device {
 };
 
 int libhoth_usb_fifo_open(struct libhoth_usb_device *dev,
-                          const struct libusb_config_descriptor *descriptor);
+                          const struct libusb_config_descriptor *descriptor,
+                          uint32_t prng_seed);
 int libhoth_usb_fifo_send_request(struct libhoth_usb_device *dev,
                                   const void *request, size_t request_size);
 int libhoth_usb_fifo_receive_response(struct libhoth_usb_device *dev,

libhoth_usb_fifo.c

@@ -83,8 +83,20 @@ static void fifo_transfer_callback(struct libusb_transfer *transfer) {
   }
 }
 
+// 32-bits XOR shift algorithm from "Xorshift RNGs" by George Marsaglia
+static uint32_t libhoth_generate_pseudorandom_u32(uint32_t *seed) {
+  *seed ^= (*seed << 13);
+  // The paper seems to have a typo in the algorithm presented on Pg 4, missing
+  // the ^ operation for second assignment. Pg 3 shows 8 shift operations that
+  // can serve as basis for xorshift. All of them have ^= operation.
+  *seed ^= (*seed >> 17);
+  *seed ^= (*seed << 5);
+  return *seed;
+}
+
 int libhoth_usb_fifo_open(struct libhoth_usb_device *dev,
-                          const struct libusb_config_descriptor *descriptor) {
+                          const struct libusb_config_descriptor *descriptor,
+                          uint32_t prng_seed) {
   int status = LIBHOTH_OK;
   if (dev == NULL || descriptor == NULL ||
       dev->info.type != LIBHOTH_USB_INTERFACE_TYPE_FIFO) {
@@ -143,6 +155,7 @@ int libhoth_usb_fifo_open(struct libhoth_usb_device *dev,
     status = LIBHOTH_ERR_MALLOC_FAILED;
     goto err_out;
   }
+  drvdata->prng_state = prng_seed;
   return LIBHOTH_OK;
 err_out:
   if (drvdata->in_buffer != NULL) free(drvdata->in_buffer);
@@ -167,7 +180,8 @@ int libhoth_usb_fifo_send_request(struct libhoth_usb_device *dev,
   // Prepare the buffer with a request ID
   struct libhoth_usb_fifo *drvdata = &dev->driver_data.fifo;
   for (int i = 0; i < LIBHOTH_USB_FIFO_REQUEST_ID_SIZE; i++) {
-    drvdata->out_buffer[i] = (uint8_t)rand();
+    drvdata->out_buffer[i] =
+        (uint8_t)libhoth_generate_pseudorandom_u32(&drvdata->prng_state);
   }
 
   memcpy(drvdata->out_buffer + LIBHOTH_USB_FIFO_REQUEST_ID_SIZE, request,