Add Xor-shift based random bytes

A xorshift32 random generator is implemented. The algorithm can be
refered to https://en.wikipedia.org/wiki/Xorshift which is from
p. 4 of Marsaglia, "Xorshift RNGs". The generator is tested with
100,000,000 number generations and shows shannon entropy of 5.52 out of
5.54 expected value.

console.c

@@ -22,6 +22,7 @@ static cmd_element_t *cmd_list = NULL;
 static param_element_t *param_list = NULL;
 static bool block_flag = false;
 static bool prompt_flag = true;
+int prng = 0;
 
 /* Am I timing a command that has the console blocked? */
 static bool block_timing = false;
@@ -435,6 +436,7 @@ void init_cmd()
     add_param("error", &err_limit, "Number of errors until exit", NULL);
     add_param("echo", &echo, "Do/don't echo commands", NULL);
     add_param("entropy", &show_entropy, "Show/Hide Shannon entropy", NULL);
+    add_param("prng", &prng, "Activate default/xorshift prng", NULL);
 
     init_in();
     init_time(&last_time);

qtest.c

@@ -40,6 +40,9 @@ void timsort(void *priv, struct list_head *head, list_cmp_func_t cmp);
 extern double shannon_entropy(const uint8_t *input_data);
 extern int show_entropy;
 
+/* PRNG option */
+extern int prng;
+
 /* Our program needs to use regular malloc/free */
 #define INTERNAL 1
 #include "harness.h"
@@ -187,7 +190,11 @@ static void fill_rand_string(char *buf, size_t buf_size)
         len = rand() % buf_size;
 
     uint64_t randstr_buf_64[MAX_RANDSTR_LEN] = {0};
-    randombytes((uint8_t *) randstr_buf_64, len * sizeof(uint64_t));
+    if (prng == 1)
+        randombytes_xorshift((uint8_t *) randstr_buf_64,
+                             len * sizeof(uint64_t));
+    else
+        randombytes((uint8_t *) randstr_buf_64, len * sizeof(uint64_t));
     for (size_t n = 0; n < len; n++)
         buf[n] = charset[randstr_buf_64[n] % (sizeof(charset) - 1)];
 

random.c

@@ -293,3 +293,28 @@ int randombytes(uint8_t *buf, size_t n)
 #error "randombytes(...) is not supported on this platform"
 #endif
 }
+
+static uint32_t xorshift32()
+{
+    /* Algorithm "xor" from p. 4 of Marsaglia, "Xorshift RNGs" */
+    static uint32_t x = 1;
+    x ^= x << 13;
+    x ^= x >> 17;
+    x ^= x << 5;
+    return x;
+}
+
+static uint32_t xorshift8()
+{
+    return xorshift32() & 0xFF;
+}
+
+int randombytes_xorshift(uint8_t *buf, size_t n)
+{
+    while (n > 0) {
+        *buf = xorshift8();
+        buf++;
+        n--;
+    }
+    return 0;
+}

random.h

@@ -5,6 +5,7 @@
 #include <stdint.h>
 
 extern int randombytes(uint8_t *buf, size_t len);
+extern int randombytes_xorshift(uint8_t *buf, size_t n);
 
 static inline uint8_t randombit(void)
 {