Coverage for pyguymer3/sha256_of_GZ.py: 95%

1#!/usr/bin/env python3 

2 

3# Define function ... 

4def sha256_of_GZ( 

5 fname, 

6 /, 

7 *, 

8 chunksize = 1048576, 

9 ignoreModificationTime = True, 

10): 

11 """Find the SHA-256 hash of a GZ file 

12 

13 This function returns the SHA-256 hash of the passed GZ file as if the first 

14 "Modification Time" field is set to zero. Using this function it is possible 

15 to discover that the only binary difference between two different GZ files 

16 is the first "Modification Time" field. 

17 

18 If this function is told not to ignore the first "Modification Time" field 

19 then this function will return the SHA-256 identically to any other method. 

20 

21 Parameters 

22 ---------- 

23 fname : str 

24 the input GZ file name 

25 chunksize : int, optional 

26 the size of the chunks of any files which are read in (in bytes) 

27 ignoreModificationTime : bool, optional 

28 ignore the first "Modification Time" field 

29 

30 Returns 

31 ------- 

32 hexdigest : str 

33 The hash hexdigest of the input GZ file. 

34 

35 Notes 

36 ----- 

37 The following websites have some very useful information on how to parse GZ 

38 files: 

39 

40 - https://en.wikipedia.org/wiki/Gzip#File_format 

41 - https://tools.ietf.org/html/rfc1952.html#page-5 

42 

43 Copyright 2017 Thomas Guymer [1]_ 

44 

45 References 

46 ---------- 

47 .. [1] PyGuymer3, https://github.com/Guymer/PyGuymer3 

48 """ 

49 

50 # Import standard modules ... 

51 import hashlib 

52 import struct 

53 

54 # ************************************************************************** 

55 

56 # Construct a hash object ... 

57 hObj = hashlib.sha256() 

58 

59 # Open input GZ file read-only ... 

60 with open(fname, "rb") as fObj: 

61 # Attempt to read 2 bytes and pass them to the hash object ... 

62 src = fObj.read(2) 

63 hObj.update(src) 

64 

65 # Check that this is a GZ file ... 

66 if src != b"\x1f\x8b": 

67 raise Exception(f"\"{fname}\" is not a GZ") from None 

68 

69 # Pass 2 bytes to the hash object ... 

70 hObj.update(fObj.read(2)) 

71 

72 # Check what the user wants to do ... 

73 if ignoreModificationTime: 

74 # Pass 0 as a little-endian un-signed 32-bit integer to the hash 

75 # object ... 

76 fObj.read(4) 

77 hObj.update(struct.pack(">I", 0)) 

78 else: 

79 # Pass 4 bytes to the hash object ... 

80 hObj.update(fObj.read(4)) 

81 

82 # Pass the rest of the file to the hash object using chunks ... 

83 while True: 

84 chunk = fObj.read(chunksize) 

85 if len(chunk) == 0: 

86 break 

87 hObj.update(chunk) 

88 

89 # Return hash hexdigest ... 

90 return hObj.hexdigest()