[bb] Xor Checksumming by Vic 3 Babes [ 1+ years ago ]

[BlitzBot]

Title : Xor Checksumming
Author : Vic 3 Babes
Posted : 1+ years ago

Description : Xor gets a bit of a rough deal in the Command Index - it's pretty useful for a few things, one of which is checksumming external files.

This example code shows how to xor checksum a bank to ensure no file-corruption for I/O error checking, or tampering has occurred - such as on highscore tables/graphics files.

It's better to load a file into a bank and xor checksum it, rather than doing it on disk.

Code :

Code: BlitzBasic

1. ;Use Xor Checksums to check for external file-corruption
2. ;or tampering - much easier to checksum a prefs file than
3. ;test the validity of every variable that it might hold
4. ;when you reload it.
5. AppTitle "Xor Checksumming"
6. Graphics 400,400,0,2
7.  
8. Const RANDLOW=$1234, RANDHIGH=$56789ABC
9. Const SIZEOFBANK=2000
10.  
11. Global bank=CreateBank(SIZEOFBANK)
12.  
13. SetBuffer BackBuffer()
14. SeedRnd MilliSecs()
15.  
16. ;fill example bank with random numbers
17. For offset=0 To SIZEOFBANK-4 Step 4
18.         PokeInt bank,offset,Rand($FFFFFFFF,$7FFFFFFF)
19. Next
20.  
21. ;get a checksum from the bank - but ignore the last
22. ;four bytes.
23.  
24. checksum=PeekInt(bank,0)
25. For offset=4 To SIZEOFBANK-8 Step 4
26.         checksum=checksum Xor PeekInt(bank,offset)
27. Next
28.  
29. ;use the checksum as a random seed
30. SeedRnd checksum
31. ;generate a random number from this seed
32. ;in the given range
33. checksum=Rand(RANDLOW, RANDHIGH)
34. ;poke it into the last 4 bytes of the bank
35. PokeInt bank,SIZEOFBANK-4,checksum
36. Text 0,0,"Random number from checksum seed: "+checksum
37.  
38. Text 0,30,"Press a key to see a demo of a seed bug"
39. Flip
40. WaitKey()
41.  
42. ;Now when you load your bank back in later, you
43. ;can simply reaquire the checksum with the second
44. ;For loop above - use it for the seed with SeedRnd.
45. ;Generate a random number between RANDLOW and RANDHIGH
46. ;and compare it with the random number you stuck at
47. ;the end of the bank - or wherever you stuck it.
48. ;This method guards against tampering as well as
49. ;file corruption.
50. ;
51. ;That's it for the algorithm - now a couple of things
52. ;to be aware of:
53. ;
54. ;1.     you can use byte or short checksumming, but make
55. ;       sure your bank is divisible by 2 or 4 if using
56. ;       shorts/integers - or take it into account when
57. ;       checksumming
58. ;2.     Blitz2D and Blitz+ don't generate the same sequence
59. ;       of random numbers from the same seeds.
60. ;3.     If you game/program relies on random numbers, you
61. ;       might want to preserve the seed with RndSeed or
62. ;       randomize it again with SeedRnd - because if your
63. ;       bank is always the same, then the same random
64. ;       numbers will be generated after loading.
65. ;4.     seed bug as at 01 Feb 07 - may have been fixed by
66. ;       the time you read this.
67. ;       Seeds of $FFFFFFFF or $7FFFFFFF always generate n1
68. ;       in Rand(n1,n2)
69. Cls
70. Text 0,0,"Print 15 random numbers between 10 and 50"
71. SeedRnd $7FFFFFFF
72. For randnum=1 To 15
73.         Text 0,randnum Shl 4,Rand(10,50)
74. Next
75. Text 0,260,"Press a key to end"
76. ;since one number is the highest positive integer
77. ;and the other is the lowest, I guess it's an
78. ;overflow thing
79. Flip
80. WaitKey
81. End

Comments :


Graythe(Posted 1+ years ago)

 Thanks - this really helped