rp2/moduos: Implement uos.urandom().

The implementation samples rosc.randombits at a frequency lower than the
oscillator frequency.  This gives better random values.  In addition, for
an 8-bit value 8 samples are taken and fed through a 8-bit CRC,
distributing the sampling over the byte.  The resulting sampling rate is
about 120k/sec.

The RNG does not include testing of error conditions, like the ROSC being
in sync with the sampling or completely failing.  Making the interim value
static causes it to perform a little bit better in short sync or drop-out
situations.

The output of uos.urandom() performs well with the NIST800-22 test suite.
In my trial it passed all tests of the sts 2.1.2 test suite.  I also ran a
test of the random data with the Common Criteria test suite AIS 31, and it
passed all tests too.

ports/rp2/main.c

@@ -30,6 +30,7 @@
 #include "py/runtime.h"
 #include "py/gc.h"
 #include "py/mperrno.h"
+#include "py/mphal.h"
 #include "py/stackctrl.h"
 #include "lib/mp-readline/readline.h"
 #include "lib/utils/gchelper.h"
@@ -168,10 +169,21 @@ void MP_WEAK __assert_func(const char *file, int line, const char *func, const c
 }
 #endif
 
+#define POLY (0xD5)
+
+uint8_t rosc_random_u8(size_t cycles) {
+    static uint8_t r;
+    for (size_t i = 0; i < cycles; ++i) {
+        r = ((r << 1) | rosc_hw->randombit) ^ (r & 0x80 ? POLY : 0);
+        mp_hal_delay_us_fast(1);
+    }
+    return r;
+}
+
 uint32_t rosc_random_u32(void) {
     uint32_t value = 0;
-    for (size_t i = 0; i < 32; ++i) {
-        value = value << 1 | rosc_hw->randombit;
+    for (size_t i = 0; i < 4; ++i) {
+        value = value << 8 | rosc_random_u8(32);
     }
     return value;
 }

ports/rp2/moduos.c

@@ -31,6 +31,8 @@
 #include "extmod/vfs_lfs.h"
 #include "genhdr/mpversion.h"
 
+uint8_t rosc_random_u8(size_t cycles);
+
 STATIC const qstr os_uname_info_fields[] = {
     MP_QSTR_sysname, MP_QSTR_nodename,
     MP_QSTR_release, MP_QSTR_version, MP_QSTR_machine
@@ -57,10 +59,22 @@ STATIC mp_obj_t os_uname(void) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(os_uname_obj, os_uname);
 
+STATIC mp_obj_t os_urandom(mp_obj_t num) {
+    mp_int_t n = mp_obj_get_int(num);
+    vstr_t vstr;
+    vstr_init_len(&vstr, n);
+    for (int i = 0; i < n; i++) {
+        vstr.buf[i] = rosc_random_u8(8);
+    }
+    return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr);
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(os_urandom_obj, os_urandom);
+
 STATIC const mp_rom_map_elem_t os_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_uos) },
 
     { MP_ROM_QSTR(MP_QSTR_uname), MP_ROM_PTR(&os_uname_obj) },
+    { MP_ROM_QSTR(MP_QSTR_urandom), MP_ROM_PTR(&os_urandom_obj) },
 
     #if MICROPY_VFS
     { MP_ROM_QSTR(MP_QSTR_chdir), MP_ROM_PTR(&mp_vfs_chdir_obj) },