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#1 2013-06-18 21:55:52

marshmellow
Moderator
From: US
Registered: 2013-06-10
Posts: 2,301

unknown checkdigit on LF format

I am attempting to decode a lf card. I've identified the Facility Code and Card Number (standard 26 bit) and several parity bits.  but the last 8 bits appear to be a calculated checksum.

it is an FSK encoded card with 96 bits. the 8th bit in every byte is an odd parity.  the first 7 bytes are 0x01 (0's except just the odd parity).  the next 4 bytes is where the data resides and the last byte is the calculated checksum.


i have 5 sequential card numbers with the same facility code:

Card # 70:   00000001 00000001 00000001 00000001 00000001 00000001 00000001 01101110 00110001 00000010 00011001 10010100
Card # 71:   00000001 00000001 00000001 00000001 00000001 00000001 00000001 01101110 00110001 00000010 00011111 01001001
Card # 72:   00000001 00000001 00000001 00000001 00000001 00000001 00000001 01101110 00110001 00000010 00100011 01010100
Card # 73:   00000001 00000001 00000001 00000001 00000001 00000001 00000001 01101110 00110001 00000010 00100101 10001001
Card # 74:   00000001 00000001 00000001 00000001 00000001 00000001 00000001 01101110 00110001 00000010 00101001 00101010

Partial decrypt of Card # 70 (starting with byte 8 (counting from left)):
0 1(start of data indicator) 1(left even parity of next 12 bits ) 0111 (0 byte parity) 0011 (end of FC 115) 000 (1 byte parity) 0000001 (0 byte parity) 0001100 (end of card number 70) (1byte parity)  (1001010 checksum) (0 byte parity)

so:
01101110 00110001 00000010 00011001 10010100
FC: 0111   0011
Card#:             000  0000001   0001100   
Checksum:                                              1001010

any ideas on how to calculate this checksum?

Thanks.

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#2 2013-06-18 21:57:48

marshmellow
Moderator
From: US
Registered: 2013-06-10
Posts: 2,301

Re: unknown checkdigit on LF format

Oops in decrypt above listed right data odd parity as part of card number.

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