Longitudinal Redundancy Check fails
- by PaulH
I have an application that decodes data from a magnetic stripe reader. But, I'm having difficulty getting my calculated LRC check byte to match the one on the cards. If I were to grab 3 cards each with 3 tracks, I would guess the algorithm below would work on 4 of the 9 tracks in those cards.
The algorithm I'm using looks like this (C#):
private static char GetLRC(string s, int start, int end)
{
int result = 0;
for (int i = start; i <= end; i++)
{
result ^= Convert.ToByte(s[i]);
}
return Convert.ToChar(result);
}
This is an example of track 3 data that fails the check. On this card, track 2 matched, but track 1 also failed.
0 1 2 3 4 5 6 7 8 9 A B C D E F
00 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5
10 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 7
20 7 7 7 7 7 7 7 7 7 8 8 8 8 8 8 8
30 8 8 8 9 9 9 9 9 9 9 9 9 9 0 0 0
40 0 0 0 0 0 0 0 1 2 3 4 1 1 1 1 1
50 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3
60 3 3 3 3 3 3 3 3
The sector delimiter is ';' and it ends with a '?'.
The LRC byte from this track is 0x30. Unfortunately, the algorithm above computes an LRC of 0x00 per the following calculation (apologies for its length. I want to be thorough):
00 ^ 3b = 3b ';'
3b ^ 33 = 08
08 ^ 34 = 3c
3c ^ 34 = 08
08 ^ 34 = 3c
3c ^ 34 = 08
08 ^ 34 = 3c
3c ^ 34 = 08
08 ^ 34 = 3c
3c ^ 34 = 08
08 ^ 34 = 3c
3c ^ 34 = 08
08 ^ 35 = 3d
3d ^ 35 = 08
08 ^ 35 = 3d
3d ^ 35 = 08
08 ^ 35 = 3d
3d ^ 35 = 08
08 ^ 35 = 3d
3d ^ 35 = 08
08 ^ 35 = 3d
3d ^ 35 = 08
08 ^ 36 = 3e
3e ^ 36 = 08
08 ^ 36 = 3e
3e ^ 36 = 08
08 ^ 36 = 3e
3e ^ 36 = 08
08 ^ 36 = 3e
3e ^ 36 = 08
08 ^ 36 = 3e
3e ^ 36 = 08
08 ^ 37 = 3f
3f ^ 37 = 08
08 ^ 37 = 3f
3f ^ 37 = 08
08 ^ 37 = 3f
3f ^ 37 = 08
08 ^ 37 = 3f
3f ^ 37 = 08
08 ^ 37 = 3f
3f ^ 37 = 08
08 ^ 38 = 30
30 ^ 38 = 08
08 ^ 38 = 30
30 ^ 38 = 08
08 ^ 38 = 30
30 ^ 38 = 08
08 ^ 38 = 30
30 ^ 38 = 08
08 ^ 38 = 30
30 ^ 38 = 08
08 ^ 39 = 31
31 ^ 39 = 08
08 ^ 39 = 31
31 ^ 39 = 08
08 ^ 39 = 31
31 ^ 39 = 08
08 ^ 39 = 31
31 ^ 39 = 08
08 ^ 39 = 31
31 ^ 39 = 08
08 ^ 30 = 38
38 ^ 30 = 08
08 ^ 30 = 38
38 ^ 30 = 08
08 ^ 30 = 38
38 ^ 30 = 08
08 ^ 30 = 38
38 ^ 30 = 08
08 ^ 30 = 38
38 ^ 30 = 08
08 ^ 31 = 39
39 ^ 32 = 0b
0b ^ 33 = 38
38 ^ 34 = 0c
0c ^ 31 = 3d
3d ^ 31 = 0c
0c ^ 31 = 3d
3d ^ 31 = 0c
0c ^ 31 = 3d
3d ^ 31 = 0c
0c ^ 31 = 3d
3d ^ 31 = 0c
0c ^ 31 = 3d
3d ^ 31 = 0c
0c ^ 32 = 3e
3e ^ 32 = 0c
0c ^ 32 = 3e
3e ^ 32 = 0c
0c ^ 32 = 3e
3e ^ 32 = 0c
0c ^ 32 = 3e
3e ^ 32 = 0c
0c ^ 32 = 3e
3e ^ 32 = 0c
0c ^ 33 = 3f
3f ^ 33 = 0c
0c ^ 33 = 3f
3f ^ 33 = 0c
0c ^ 33 = 3f
3f ^ 33 = 0c
0c ^ 33 = 3f
3f ^ 33 = 0c
0c ^ 33 = 3f
3f ^ 3f = 00 '?'
If anybody can point out how to fix my algorithm, I would appreciate it.
Thanks,
PaulH
