Intercepted Transmission
Those monsters! They’ve kidnapped the Quokkas! Who in their right mind would capture those friendly little guys.. We’ve managed to intercept a CCIR476 transmission from the kidnappers, we think it contains the location of our friends! Can you help us decode it? We managed to decode the first two characters as ’##’
We’re told that the transmission is CCIR476 encoded. A quick search tells us that it is a radio communication protocol where each character is represented by a 7-bit code. Four of the bits are 1 and three are 0, to allow for single bit error correction.
All we need is a mapping of the 7-bit code to the character it represents, which we can find here.
There are two special control characters, LTRS and FIGS, which switch between letters and figures respectively, so we need to keep track of what mode we’re in.
binary = '101101001101101101001110100110110101110100110100101101101010110101110010110100101110100111001101100101101101101000111100011110011011010101011001011101101010010111011100100011110101010110110101011010111001011010110100101101101010110101101011001011010011101110001101100101110101101010110011011100001101101101101010101101101000111010110110010111010110101100101100110111101000101011101110001101101101001010111001011101110001010111001011100011011'
# 47 A — —
# 72 B ? ?
# 1D C : :
# 53 D 5 $
# 56 E 3 3
# 1B F 4 !
# 35 G 4 &
# 69 H 4 # or motor stop
# 4D I 8 8
# 17 J BELL ´
# 1E K ( (
# 65 L ) )
# 39 M . .
# 59 N , ,
# 71 0 9 9
# 2D P 0 0
# 2E Q 1 1
# 55 R 4 4
# 4B S ' BELL
# 74 T 5 5
# 4E U 7 7
# 3C V = ;
# 27 W 2 2
# 3A X / /
# 2B Y 6 6
# 63 Z + "
# 78 ← CR (Carriage return)
# 6C ≡ LF (Line feed)
# 5A ↓ LTRS (Letter shift)
# 36 ↑ FIGS (Figure shift)
# 5C SP (Space)
# 6A BLK (Blank)
LTRS = {
'47': 'A',
'72': 'B',
'1D': 'C',
'53': 'D',
'56': 'E',
'1B': 'F',
'35': 'G',
'69': 'H',
'4D': 'I',
'17': 'J',
'1E': 'K',
'65': 'L',
'39': 'M',
'59': 'N',
'71': 'O',
'2D': 'P',
'2E': 'Q',
'55': 'R',
'4B': 'S',
'74': 'T',
'4E': 'U',
'3C': 'V',
'27': 'W',
'3A': 'X',
'2B': 'Y',
'63': 'Z',
'78': '\r',
'6C': '\n',
'5A': 'LTRS',
'36': 'FIGS',
'5C': ' ',
'6A': '' # blank
}
FIGS = {
'47': 'A',
'72': '?',
'1D': ':',
'53': '$',
'56': '3',
'1B': '!',
'35': '&',
'69': '#',
'4D': '8',
'17': '´',
'1E': '(',
'65': ')',
'39': '.',
'59': ',',
'71': '9',
'2D': '0',
'2E': '1',
'55': '4',
'4B': '\a', # BELL
'74': '5',
'4E': '7',
'3C': ';',
'27': '2',
'3A': '/',
'2B': '6',
'63': '"',
'78': '\r',
'6C': '\n',
'5A': 'LTRS',
'36': 'FIGS',
'5C': ' ',
'6A': '' # blank
}
current = "LTRS"
flag = ""
# split into chunks of seven
chunks = [binary[i:i+7] for i in range(0, len(binary), 7)]
print(chunks)
for chunk in chunks:
h = hex(int(chunk, 2))[2:].upper()
if current == "LTRS":
char = LTRS.get(h)
if char == "FIGS":
current = "FIGS"
elif char == "LTRS":
pass
else:
flag += char
else:
char = FIGS.get(h)
if char == "FIGS":
pass
elif char == "LTRS":
current = "LTRS"
else:
flag += char
print(flag)