100 points
Category: Cryptography
Tags : #rot #substitution #cypher
You will find the flag after decrypting this file Download the encrypted flag.
Given the name of this challenge, a rotation based substitution cypher is suspected.
Contents of the encrypted flag file :
$ cat encrypted.txt
xqkwKBN{z0bib1wv_l3kzgxb3l_25l7k61j}
Encrypted text appears to follow the flag structure (i.e. picoCTF{flag-value-here}) without additional data obfuscating the flag.
Hence for a rotational substitution cypher we should be able to build up our translation table, using values we know from the flag structure :
a b c d e f g h i j k l m n o p q r s t u v w x y z
T c F i o p
Translation table is case insensitive, now filling in the rest from the known letters and hence rotation point.
a b c d e f g h i j k l m n o p q r s t u v w x y z
s T u v w x y z a b c d e F g h i j k l m n o p q r
Now just substitute the encrypted text using this translation table :
xqkwKBN{z0bib1wv_l3kzgxb3l_25l7k61j}
=>
picoCTF{ }
Flag value left un-substituted for the purposes of this write up.
NOTE: if you really don't want to work this out by hand there are various ROT websites that you can cycle through the various rotations until the plain text is clear.