Day 05: Image DNA

Description

Santa has thousands of Christmas balls in stock. They all look the same, but he can still tell them apart. Can you see the difference?

Solution

running strings on the images reveals a DNA sequence, hint?

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$ strings dec5-1.jpg
JFIF
$3br
[..]
f?*~
kh[F;-8B
VW}L
CTGTCGCGAGCGGATACATTCAAACAATCCTGGGTACAAAGAATAAAACCTGGGCAATAATTCACCCAAACAAGGAAAGTAGCGAAAAAGTTCCAGAGGCCAAA


$ strings dec5-2.jpg
JFIF
ICC_PROFILE
lcms
mntrRGB XYZ
[..]
Iwh]r
=nr_
ATATATAAACCAGTTAATCAATATCTCTATATGCTTATATGTCTCGTCCGTCTACGCACCTAATATAACGTCCATGCGTCACCCCTAGACTAATTACCTCATTC

Aha! we see two DNA strings, of equal length, and looks about the right length for a flag..

The challenge description refers to differences, so lets subtract these two strings from each other
We first convert to binary from this base-4 string

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    bin base4
A = 00  0
C = 01  1
G = 10  2
T = 11  3

In this scheme, each group of 4 characters maps to 8 bits, and therefore one ascii character.

Knowing that the result should start with HV20{, we experiment with the first few characters, and find
out that if we XOR the result of the above mapping, we get the right answer:

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CTGT CGCG: 01111011 01100110
ATAT ATAA: 00110011 00110000
xor:       01001000 01010110
ascii:     H        V

ok, this looks good! Let’s automate the rest in Python:

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import binascii

dna1="ATATATAAACCAGTTAATCAATATCTCTATATGCTTATATGTCTCGTCCGTCTACGCACCTAATATAACGTCCATGCGTCACCCCTAGACTAATTACCTCATTC"
dna2="CTGTCGCGAGCGGATACATTCAAACAATCCTGGGTACAAAGAATAAAACCTGGGCAATAATTCACCCAAACAAGGAAAGTAGCGAAAAAGTTCCAGAGGCCAAA"

# translate DNA strings to base-4 numbers
map={"A": "0", "C": "1", "G":"2", "T":"3"}
dna1b = dna1.translate(dna1.maketrans(map))
dna2b = dna2.translate(dna2.maketrans(map))

# xor the two numbers
flag = int(dna1b,4)^int(dna2b,4)

# convert back to ascii
print(binascii.unhexlify('%x' % flag))

This outputs the flag :)

Flag