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Day 4: Christmas in Babylon
Challenge
Solution
The file is a zip file with a single code.txt file inside.
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$ head code.txt
using System;using System.Text;using static System.Console;void Rev(string s){var chars=Encoding.
ASCII.GetString(Convert.FromBase64String(s)).ToCharArray();Array.Reverse(chars);WriteLine(new string
(chars));}
Rev("KzgrKitoKysrKysreysrKysraSsvPiswK3krKz4oKysrKysrICsrPlQrKysrKysrKyt9KysrPlsrVCsrK3grKysr");
Rev("K2srICsrKysyKyt9KysrKysrPisrKytVKysrKysrKysrPisrNSsrKysrK3wrKysrNT4rKysrRCs2KytyKysqKz4r");
Rev("K0MrKysrKysrMj4rKytqKyArKysrKytMKysrICsrbys+KysrKysrKysrKys+KysgKytNKysraStQKysrditxKys+");
Rev("KysqKyArKysrK2c+KysrKysrKyt4KysrKyt1Kz4rKytTKytEK0UrKytpKysrdj5XKytnKysrKysrcisjKytCPkcr");
Rev("Kz4rKysrcCspK1E+KysrRisrKysrbCsrPitKKytpKysrKys+KysrK1QrKytXKysrPisrKysrKz5nKyorKyArUCsr");
Rev("KysrK3U+KysrKysrICsrK2orKysrPjErK3MrKysrKCsrK1UrMSsrKy8+UisrKysgKysrKytBKysrKytxKz4rK3Yr");
Rev("KysrKysrKysrKzg+K1orKysrKz5MKyttK1IrSCsrICsrKz4rQisrKysrKz4rICsgKysrKysrK2QrKysrICsrPjIr");
apparently that’s c# but we can just
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cat writeupfiles/04/code.txt | grep '^Rev' | sed 's/Rev("//g;s/".*//g' > tmp
for line in `cat tmp`; do echo "$line" | base64 -d | tac >> decode; echo >> decode; done;
to mimic what the C# is doing and we get
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$ head decode
+8+*+h++++++{+++++i+/>+0+y++>(++++++ ++>T+++++++++}+++>[+T+++x++++
+k+ ++++2++}++++++>++++U+++++++++>++5++++++|++++5>++++D+6++r++*+>+
+C+++++++2>+++j+ ++++++L+++ ++o+>+++++++++++>++ ++M+++i+P+++v+q++>
++*+ +++++g>++++++++x+++++u+>+++S++D+E+++i+++v>W++g++++++r+#++B>G+
+>++++p+)+Q>+++F+++++l++>+J++i+++++>++++T+++W+++>++++++>g+*++ +P++
++++u>++++++ +++j++++>1++s++++(+++U+1+++/>R++++ +++++A+++++q+>++v+
++++++++++8>+Z+++++>L++m+R+H++ +++>+B++++++>+ + +++++++d++++ ++>2+
v++ +&>9+ ++++++++R++++x+>++(+++>+++++++>++e++U+{++++>+++5+++> +Y+
+h+++n+ ++++w>+#+++s+K+t++l++>+}++++E++q++++>+++w+>+++n++j+j++>+++
++++Q++Y>+++8++ ++V+#+u+++a+>F++W+++A++d++E>z++++++++B++++>+}+L++W
surely that’s brainfuck, right?
But beef fails:
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$ beef writeupfiles/04/decode
beef: writeupfiles/04/decode: Unbalanced brackets
And so does bf on ubuntu. It looks like there’s extra weird characters but removing those also doesn’t help.
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