Coverage for /pythoncovmergedfiles/medio/medio/usr/local/lib/python3.8/site-packages/rsa/randnum.py: 27%

1# Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu> 

2# 

3# Licensed under the Apache License, Version 2.0 (the "License"); 

4# you may not use this file except in compliance with the License. 

5# You may obtain a copy of the License at 

6# 

7# https://www.apache.org/licenses/LICENSE-2.0 

8# 

9# Unless required by applicable law or agreed to in writing, software 

10# distributed under the License is distributed on an "AS IS" BASIS, 

11# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 

12# See the License for the specific language governing permissions and 

13# limitations under the License. 

14 

15"""Functions for generating random numbers.""" 

16 

17# Source inspired by code by Yesudeep Mangalapilly <yesudeep@gmail.com> 

18 

19import os 

20import struct 

21 

22from rsa import common, transform 

23 

24 

25def read_random_bits(nbits: int) -> bytes: 

26 """Reads 'nbits' random bits. 

27 

28 If nbits isn't a whole number of bytes, an extra byte will be appended with 

29 only the lower bits set. 

30 """ 

31 

32 nbytes, rbits = divmod(nbits, 8) 

33 

34 # Get the random bytes 

35 randomdata = os.urandom(nbytes) 

36 

37 # Add the remaining random bits 

38 if rbits > 0: 

39 randomvalue = ord(os.urandom(1)) 

40 randomvalue >>= 8 - rbits 

41 randomdata = struct.pack("B", randomvalue) + randomdata 

42 

43 return randomdata 

44 

45 

46def read_random_int(nbits: int) -> int: 

47 """Reads a random integer of approximately nbits bits.""" 

48 

49 randomdata = read_random_bits(nbits) 

50 value = transform.bytes2int(randomdata) 

51 

52 # Ensure that the number is large enough to just fill out the required 

53 # number of bits. 

54 value |= 1 << (nbits - 1) 

55 

56 return value 

57 

58 

59def read_random_odd_int(nbits: int) -> int: 

60 """Reads a random odd integer of approximately nbits bits. 

61 

62 >>> read_random_odd_int(512) & 1 

63 1 

64 """ 

65 

66 value = read_random_int(nbits) 

67 

68 # Make sure it's odd 

69 return value | 1 

70 

71 

72def randint(maxvalue: int) -> int: 

73 """Returns a random integer x with 1 <= x <= maxvalue 

74 

75 May take a very long time in specific situations. If maxvalue needs N bits 

76 to store, the closer maxvalue is to (2 ** N) - 1, the faster this function 

77 is. 

78 """ 

79 

80 bit_size = common.bit_size(maxvalue) 

81 

82 tries = 0 

83 while True: 

84 value = read_random_int(bit_size) 

85 if value <= maxvalue: 

86 break 

87 

88 if tries % 10 == 0 and tries: 

89 # After a lot of tries to get the right number of bits but still 

90 # smaller than maxvalue, decrease the number of bits by 1. That'll 

91 # dramatically increase the chances to get a large enough number. 

92 bit_size -= 1 

93 tries += 1 

94 

95 return value