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.
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@ -30,6 +30,7 @@
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#include "py/runtime.h"
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#include "py/gc.h"
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#include "py/mperrno.h"
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#include "py/mphal.h"
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#include "py/stackctrl.h"
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#include "lib/mp-readline/readline.h"
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#include "lib/utils/gchelper.h"
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@ -168,10 +169,21 @@ void MP_WEAK __assert_func(const char *file, int line, const char *func, const c
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}
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#endif
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#define POLY (0xD5)
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uint8_t rosc_random_u8(size_t cycles) {
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static uint8_t r;
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for (size_t i = 0; i < cycles; ++i) {
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r = ((r << 1) | rosc_hw->randombit) ^ (r & 0x80 ? POLY : 0);
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mp_hal_delay_us_fast(1);
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}
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return r;
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}
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uint32_t rosc_random_u32(void) {
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uint32_t value = 0;
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for (size_t i = 0; i < 32; ++i) {
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value = value << 1 | rosc_hw->randombit;
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for (size_t i = 0; i < 4; ++i) {
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value = value << 8 | rosc_random_u8(32);
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}
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return value;
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}
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@ -31,6 +31,8 @@
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#include "extmod/vfs_lfs.h"
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#include "genhdr/mpversion.h"
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uint8_t rosc_random_u8(size_t cycles);
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STATIC const qstr os_uname_info_fields[] = {
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MP_QSTR_sysname, MP_QSTR_nodename,
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MP_QSTR_release, MP_QSTR_version, MP_QSTR_machine
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@ -57,10 +59,22 @@ STATIC mp_obj_t os_uname(void) {
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}
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STATIC MP_DEFINE_CONST_FUN_OBJ_0(os_uname_obj, os_uname);
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STATIC mp_obj_t os_urandom(mp_obj_t num) {
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mp_int_t n = mp_obj_get_int(num);
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vstr_t vstr;
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vstr_init_len(&vstr, n);
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for (int i = 0; i < n; i++) {
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vstr.buf[i] = rosc_random_u8(8);
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}
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return mp_obj_new_str_from_vstr(&mp_type_bytes, &vstr);
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}
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STATIC MP_DEFINE_CONST_FUN_OBJ_1(os_urandom_obj, os_urandom);
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STATIC const mp_rom_map_elem_t os_module_globals_table[] = {
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{ MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_uos) },
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{ MP_ROM_QSTR(MP_QSTR_uname), MP_ROM_PTR(&os_uname_obj) },
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{ MP_ROM_QSTR(MP_QSTR_urandom), MP_ROM_PTR(&os_urandom_obj) },
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#if MICROPY_VFS
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{ MP_ROM_QSTR(MP_QSTR_chdir), MP_ROM_PTR(&mp_vfs_chdir_obj) },
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