derivepassphrase.git

1) # SPDX-FileCopyrightText: 2024 Marco Ricci <m@the13thletter.info>
2) #
3) # SPDX-License-Identifier: MIT
4) 
5) """A Python reimplementation of James Coglan's "sequin" Node.js module.
6) 
7) James Coglan's "sequin" Node.js module provides a pseudorandom number
8) generator (using rejection sampling on a stream of input numbers) that
9) attempts to minimize the amount of information it throws away:
10) (non-degenerate) rejected samples are fed into a stream of higher-order
11) numbers from which the next random number generation request will be
12) served.  The sequin module is used in Coglan's "vault" module (a
13) deterministic, stateless password manager that recomputes passwords
14) instead of storing them), and this reimplementation is used for
15) a similar purpose.
16) 
17) The main API is the [`Sequin`] [sequin.Sequin] class, which is
18) thoroughly documented.
19) 
20) """
21) 
22) from __future__ import annotations
23) 
24) import collections
25) 

26) from collections.abc import Iterator, Sequence
27) from typing_extensions import assert_type

28) 

29) __all__ = ('Sequin', 'SequinExhaustedError')

30) __author__ = 'Marco Ricci <m@the13thletter.info>'

31) # Use importlib.metadata.version(...) to query the version.

32) 
33) class Sequin:
34)     """Generate pseudorandom non-negative numbers in different ranges.
35) 
36)     Given a (presumed high-quality) uniformly random sequence of input
37)     bits, generate pseudorandom non-negative integers in a certain range
38)     on each call of the `generate` method.  (It is permissible to
39)     specify a different range per call to `generate`; this is the main
40)     use case.)  We use a modified version of rejection sampling, where
41)     rejected values are stored in "rejection queues" if possible, and
42)     these rejection queues re-seed the next round of rejection sampling.
43) 
44)     This is a Python reimplementation of James Coglan's [Node.js sequin
45)     module][JS_SEQUIN], as introduced in [his blog post][BLOG_POST].  It
46)     uses a [technique by Christian Lawson-Perfect][SEQUIN_TECHNIQUE].
47)     I do not know why the original module is called "sequin"; I presume
48)     it to be a pun on "sequence".
49) 
50)     [JS_SEQUIN]: https://www.npmjs.com/package/sequin
51)     [BLOG_POST]: https://blog.jcoglan.com/2012/07/16/designing-vaults-generator-algorithm/
52)     [SEQUIN_TECHNIQUE]: https://checkmyworking.com/2012/06/converting-a-stream-of-binary-digits-to-a-stream-of-base-n-digits/
53) 
54)     """
55)     def __init__(
56)         self,
57)         sequence: str | bytes | bytearray | Sequence[int],

58)         /, *, is_bitstring: bool = False

59)     ):
60)         """Initialize the Sequin.
61) 
62)         Args:
63)             sequence:
64)                 A sequence of bits, or things convertible to bits, to
65)                 seed the pseudorandom number generator.  Byte and text
66)                 strings are converted to 8-bit integer sequences.
67)                 (Conversion will fail if the text string contains
68)                 non-ISO-8859-1 characters.)  The numbers are then
69)                 converted to bits.

70)             is_bitstring:
71)                 If true, treat the input as a bitstring.  By default,
72)                 the input is treated as a string of 8-bit integers, from
73)                 which the individual bits must still be extracted.
74) 
75)         Raises:
76)             ValueError:
77)                 The sequence contains values outside the permissible
78)                 range.

79) 
80)         """
81)         def uint8_to_bits(value):
82)             """Yield individual bits of an 8-bit number, MSB first."""
83)             for i in (0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, 0x01):
84)                 yield 1 if value | i == value else 0
85)         if isinstance(sequence, str):

86)             try:
87)                 sequence = tuple(sequence.encode('iso-8859-1'))
88)             except UnicodeError as e:
89)                 raise ValueError('sequence item out of range') from e

90)         else:
91)             sequence = tuple(sequence)
92)         assert_type(sequence, tuple[int, ...])

93)         self.bases: dict[int, collections.deque[int]] = {}
94)         def gen() -> Iterator[int]:
95)             for num in sequence:
96)                 if num not in range(2 if is_bitstring else 256):
97)                     raise ValueError('sequence item out of range')
98)                 if is_bitstring:
99)                     yield num
100)                 else:
101)                     yield from uint8_to_bits(num)
102)         self.bases[2] = collections.deque(gen())

103) 
104)     def _all_or_nothing_shift(
105)         self, count: int, /, *, base: int = 2
106)     ) -> Sequence[int]:
107)         """Shift and return items if and only if there are enough.
108) 
109)         Args:
110)             count: Number of items to shift/consume.
111)             base: Use the base `base` sequence.
112) 
113)         Returns:
114)             If there are sufficient items in the sequence left, then
115)             consume them from the sequence and return them.  Otherwise,
116)             consume nothing, and return nothing.
117) 
118)         Notes:
119)             We currently remove now-empty sequences from the registry of
120)             sequences.
121) 
122)         Examples:

123)             >>> seq = Sequin([1, 0, 1, 0, 0, 1, 0, 0, 0, 1],
124)             ...              is_bitstring=True)

125)             >>> seq.bases
126)             {2: deque([1, 0, 1, 0, 0, 1, 0, 0, 0, 1])}
127)             >>> seq._all_or_nothing_shift(3)
128)             (1, 0, 1)
129)             >>> seq._all_or_nothing_shift(3)
130)             (0, 0, 1)
131)             >>> seq.bases[2]
132)             deque([0, 0, 0, 1])
133)             >>> seq._all_or_nothing_shift(5)
134)             ()
135)             >>> seq.bases[2]
136)             deque([0, 0, 0, 1])
137)             >>> seq._all_or_nothing_shift(4)
138)             (0, 0, 0, 1)
139)             >>> 2 in seq.bases  # now-empty sequences are removed
140)             False
141) 
142)         """
143)         try:
144)             seq = self.bases[base]
145)         except KeyError:
146)             return ()

147)         stash: collections.deque[int] = collections.deque()
148)         try:
149)             for i in range(count):
150)                 stash.append(seq.popleft())
151)         except IndexError:
152)             seq.extendleft(reversed(stash))

153)             return ()

154)         # Clean up queues.
155)         if not seq:
156)             del self.bases[base]
157)         return tuple(stash)

158) 
159)     @staticmethod
160)     def _big_endian_number(
161)         digits: Sequence[int], /, *, base: int = 2
162)     ) -> int:
163)         """Evaluate the given integer sequence as a big endian number.
164) 
165)         Args:
166)             digits: A sequence of integers to evaluate.
167)             base: The number base to evaluate those integers in.
168) 
169)         Raises:
170)             ValueError: `base` is an invalid base.
171)             ValueError: Not all integers are valid base `base` digits.
172) 
173)         Examples:
174)             >>> Sequin._big_endian_number([1, 2, 3, 4, 5, 6, 7, 8], base=10)
175)             12345678

176)             >>> Sequin._big_endian_number([1, 2, 3, 4, 5, 6, 7, 8], base=100)
177)             102030405060708

178)             >>> Sequin._big_endian_number([0, 0, 0, 0, 1, 4, 9, 7], base=10)
179)             1497
180)             >>> Sequin._big_endian_number([1, 0, 0, 1, 0, 0, 0, 0], base=2)
181)             144
182)             >>> Sequin._big_endian_number([1, 7, 5, 5], base=8) == 0o1755
183)             True
184) 
185)         """
186)         if base < 2:
187)             raise ValueError(f'invalid base: {base!r}')
188)         ret = 0
189)         allowed_range = range(base)
190)         n = len(digits)
191)         for i in range(n):
192)             i2 = (n - 1) - i
193)             x = digits[i]
194)             if not isinstance(x, int):
195)                 raise TypeError(f'not an integer: {x!r}')
196)             if x not in allowed_range:
197)                 raise ValueError(f'invalid base {base!r} digit: {x!r}')
198)             ret += (base ** i2) * x
199)         return ret
200) 
201)     def generate(self, n: int, /) -> int:
202)         """Generate a base `n` non-negative integer.
203) 
204)         We attempt to generate a value using rejection sampling.  If the
205)         generated sample is outside the desired range (i.e., is
206)         rejected), then attempt to reuse the sample by seeding
207)         a "higher-order" input sequence of uniformly random numbers (for
208)         a different base).
209) 
210)         Args:
211)             n:
212)                 Generate numbers in the range 0, ..., `n` - 1.

213)                 (Inclusive.)  Must be larger than 0.

214) 
215)         Returns:
216)             A pseudorandom number in the range 0, ..., `n` - 1.
217) 
218)         Raises:

219)             ValueError:
220)                 The range is empty.

221)             SequinExhaustedError:

222)                 The sequin is exhausted.
223) 

224)         Examples:
225)             >>> seq = Sequin([1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1],
226)             ...              is_bitstring=True)
227)             >>> seq2 = Sequin([1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1],
228)             ...               is_bitstring=True)
229)             >>> seq.generate(5)
230)             3
231)             >>> seq.generate(5)
232)             3
233)             >>> seq.generate(5)
234)             1
235)             >>> seq.generate(5)    # doctest: +IGNORE_EXCEPTION_DETAIL
236)             Traceback (most recent call last):
237)                 ...

238)             SequinExhaustedError: Sequin is exhausted

239) 
240)             Using `n = 1` does not actually consume input bits:
241) 
242)             >>> seq2.generate(1)
243)             0
244) 
245)             But it still won't work on exhausted sequins:
246) 
247)             >>> seq.generate(1)    # doctest: +IGNORE_EXCEPTION_DETAIL
248)             Traceback (most recent call last):
249)                 ...

250)             SequinExhaustedError: Sequin is exhausted

251) 

252)         """

253)         if 2 not in self.bases:

254)             raise SequinExhaustedError('Sequin is exhausted')

255)         value = self._generate_inner(n, base=2)
256)         if value == n:

257)             raise SequinExhaustedError('Sequin is exhausted')

258)         return value
259) 
260)     def _generate_inner(
261)         self, n: int, /, *, base: int = 2
262)     ) -> int:
263)         """Recursive call to generate a base `n` non-negative integer.
264) 
265)         We first determine the correct exponent `k` to generate base `n`
266)         numbers from a stream of base `base` numbers, then attempt to
267)         take `k` numbers from the base `base` sequence (or bail if not
268)         possible).  If the resulting number `v` is out of range for
269)         base `n`, then push `v - n` onto the rejection queue for
270)         base `r` = `base` ** `k` - `n`, and attempt to generate the
271)         requested base `n` integer from the sequence of base `r` numbers
272)         next.  (This recursion is not attempted if `r` = 1.)  Otherwise,
273)         return the number.
274) 
275)         Args:
276)             n:
277)                 Generate numbers in the range 0, ..., `n` - 1.

278)                 (Inclusive.)  Must be larger than 0.

279)             base:
280)                 Use the base `base` sequence as a source for
281)                 pseudorandom numbers.
282) 
283)         Returns:
284)             A pseudorandom number in the range 0, ..., `n` - 1 if
285)             possible, or `n` if the stream is exhausted.
286) 

287)         Raises:
288)             ValueError:
289)                 The range is empty.
290) 

291)         Examples:
292)             >>> seq = Sequin([1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1],
293)             ...              is_bitstring=True)
294)             >>> seq2 = Sequin([1, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1],
295)             ...               is_bitstring=True)
296)             >>> seq._generate_inner(5)
297)             3
298)             >>> seq._generate_inner(5)
299)             3
300)             >>> seq._generate_inner(5)
301)             1
302)             >>> seq._generate_inner(5)  # error condition: sequin exhausted
303)             5
304) 
305)             Using `n = 1` does not actually consume input bits, and
306)             always works, regardless of sequin exhaustion:
307) 
308)             >>> seq2._generate_inner(1)
309)             0
310)             >>> seq._generate_inner(1)
311)             0
312) 
313)             Using an unsuitable range will raise:
314) 
315)             >>> seq2._generate_inner(0)    # doctest: +IGNORE_EXCEPTION_DETAIL
316)             Traceback (most recent call last):
317)                 ...
318)             ValueError: invalid target range
319) 

320)         """

321)         if n < 1:
322)             raise ValueError('invalid target range')
323)         if base < 2:
324)             raise ValueError(f'invalid base: {base!r}')

325)         # p = base ** k, where k is the smallest integer such that
326)         # p >= n.  We determine p and k inductively.
327)         p = 1
328)         k = 0
329)         while p < n:
330)             p *= base
331)             k += 1
332)         # The remainder r of p and n is used as the base for rejection
333)         # queue.
334)         r = p - n
335)         # The generated number v is initialized to n because of the
336)         # while loop below.
337)         v = n
338)         while v > n - 1:
339)             list_slice = self._all_or_nothing_shift(k, base=base)
340)             if not list_slice:

341)                 if n != 1:
342)                     return n
343)                 else:
344)                     v = 0

345)             v = self._big_endian_number(list_slice, base=base)
346)             if v > n - 1:
347)                 # If r is 0, then p == n, so v < n, or rather
348)                 # v <= n - 1.
349)                 assert r > 0
350)                 if r == 1:
351)                     continue
352)                 self._stash(v - n, base=r)
353)                 v = self._generate_inner(n, base=r)
354)         return v
355) 
356)     def _stash(self, value: int, /, *, base: int = 2) -> None:
357)         """Stash `value` on the base `base` sequence."""
358)         if base not in self.bases:
359)             self.bases[base] = collections.deque()
360)         self.bases[base].append(value)
361) 

362) class SequinExhaustedError(Exception):