def find_spot(self, not_here=0):
# search for a blank sector to use
# - check randomly and pick first blank one (wear leveling, deniability)
# - we will write and then erase old slot
# - if "full", blow away a random one
from main import sf
options = [s for s in SLOTS if s != not_here]
tcc.random.shuffle(options)
buf = bytearray(16)
for pos in options:
sf.read(pos, buf)
if set(buf) == {0xff}:
# blank
return sf, pos
# No where to write! (probably a bug because we have lots of slots)
# ... so pick a random slot and kill what it had
#print("ERROR: nvram full?")
victem = options[0]
sf.sector_erase(victem)
sf.wait_done()
return sf, victem
def blank(self):
# erase current copy of values in nvram; older ones may exist still
# - use when clearing the seed value
from main import sf
if self.my_pos:
sf.wait_done()
sf.sector_erase(self.my_pos)
self.my_pos = 0
# act blank too, just in case.
self.current = {}
self.is_dirty = 0
def save(self):
# render as JSON, encrypt and write it.
self.current['_age'] = self.current.get('_age', 1) + 1
sf, pos = self.find_spot(self.my_pos)
aes = self.get_aes(tcc.AES.Encrypt, pos)
with SFFile(pos, max_size=4096, pre_erased=True) as fd:
chk = tcc.sha256()
# first the json data
d = ujson.dumps(self.current)
# pad w/ zeros
dat_len = len(d)
pad_len = (4096 - 32) - dat_len
assert pad_len >= 0, 'too big'
self.capacity = dat_len / 4096
fd.write(aes.update(d))
chk.update(d)
del d
while pad_len > 0:
here = min(32, pad_len)
pad = bytes(here)
fd.write(aes.update(pad))
chk.update(pad)
pad_len -= here
fd.write(aes.update(chk.digest()))
assert fd.tell() == 4096
# erase old copy of data
if self.my_pos and self.my_pos != pos:
sf.wait_done()
sf.sector_erase(self.my_pos)
sf.wait_done()
self.my_pos = pos
self.is_dirty = 0
def load(self):
# Search all slots for any we can read, decrypt that,
# and pick the newest one (in unlikely case of dups)
from main import sf
# reset
self.current = {}
self.my_pos = 0
self.is_dirty = 0
# 4k, but last 32 bytes are a SHA (itself encrypted)
global _tmp
buf = bytearray(4)
empty = 0
for pos in SLOTS:
gc.collect()
sf.read(pos, buf)
if buf[0] == buf[1] == buf[2] == buf[3] == 0xff:
# erased (probably)
empty += 1
continue
# check if first 2 bytes makes sense for JSON
aes = self.get_aes(tcc.AES.Encrypt, pos)
chk = aes.update(b'{"')
if chk != buf[0:2]:
# doesn't look like JSON meant for me
continue
# probably good, read it
aes = self.get_aes(tcc.AES.Encrypt, pos)
chk = tcc.sha256()
expect = None
with SFFile(pos, length=4096, pre_erased=True) as fd:
for i in range(4096 / 32):
b = aes.update(fd.read(32))
if i != 127:
_tmp[i * 32:(i * 32) + 32] = b
chk.update(b)
else:
expect = b
try:
# verify checksum in last 32 bytes
assert expect == chk.digest()
# loads() can't work from a byte array, and converting to
# bytes here would copy it; better to use file emulation.
d = ujson.load(BytesIO(_tmp))
except:
# One in 65k or so chance to come here w/ garbage decoded, so
# not an error.
continue
got_age = d.get('_age', 0)
if got_age > self.current.get('_age', -1):
# likely winner
self.current = d
self.my_pos = pos
#print("NV: data @ %d w/ age=%d" % (pos, got_age))
else:
# stale data seen; clean it up.
assert self.current['_age'] > 0
print("NV: cleanup @ %d" % pos)
sf.sector_erase(pos)
sf.wait_done()
# 4k is a large object, sigh, for us right now. cleanup
gc.collect()
# done, if we found something
if self.my_pos:
return
# nothing found.
self.my_pos = 0
self.current = self.default_values()
if empty == len(SLOTS):
# Whole thing is blank. Bad for plausible deniability. Write 3 slots
# with garbage. They will be wasted space until it fills.
blks = list(SLOTS)
tcc.random.shuffle(blks)
for pos in blks[0:3]:
for i in range(0, 4096, 256):
h = tcc.random.bytes(256)
sf.wait_done()
sf.write(pos + i, h)
