South Sudan - Decode the RFID Tag (rev)

668B

C5_rawbits.txt

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Here's the first few lines of the text file:

Technical Specifications:
Using Clock:64
Invert:0
Bits Found:625

Encoding:
ASK/Manchester - Clock: 64


Decoded bitstream:
0000111010111001
1001000011111011
1101110011101111
0011111111111001
0000111010111001
1001000011111011
1101110011101111
0011111111111001
0000111010111001

After doing some OSINT, this appears to be manchester encoding. Using https://rodyne.com/?p=641 the delimiter for the data seems to be 9 consecutive 1 bits. I also found this writeup on a very similar ctf challenge https://nacayoshi00.wordpress.com/2017/06/.

To get the message, after the sequence of 9 1's we need to split the bitstream into groups of five bits, and take the first 4 bits of every group as a character. We should not take into account the column parity row. Here's the solve script:

str = [0b11001,0b00001,0b11010,0b11100,0b11001,0b00001,0b11110,0b11110,0b11100,0b11101]
for i in str:
    print(hex(i>>1))

# CSG_FLAG{c0dec0ffee}