Flag: feel_so_bohemian_like_you
The challenge flavortext says "I got a letter from my friend Antonín. However, it's all Czech to me, maybe you can read it" and links to a text file containing a cipher:
E.ap ipc.bew
C dgoy ,aby yt n.y ötg rbt, yh. o.jp.y itp yh. .bjpözy.e ojptnn C o.by ötgv Go. cy ,co.nö abe et bty uck. cy yt abötb.v
Yh. o.jp.y coV inau{i..n'ot'xth.mcab'ncr.'ötg}
This appears to be a whitespace, curly-brace, and case-preserving substitution cipher. We could do some frequency analysis, but we get a lot of help from the fact that the ciphertext is a letter and appears to end with the flag; I ended up producing the mapping manually.
The mapping is:
. => e
' => _
, => w
a => a
b => n
c => i
d => j
e => d
g => u
h => h
i => f
j => c
k => v
n => l
ö => y
o => s
p => r
r => k
t => o
u => g
v => .
w => ,
y => t
z => p
producing:
E.ap ipc.bew
Dear friend,
C dgoy ,aby yt n.y ötg rbt, yh. o.jp.y itp yh. .bjpözy.e ojptnn C o.by ötgv Go. cy ,co.nö abe et bty uck. cy yt abötb.v
I just want to let you know the secret for the encrypted scroll I sent you. Use it wisely and do not give it to anyone.
Yh. o.jp.y coV inau{i..n'ot'xth.mcab'ncr.'ötg}
The secret is. flag{feel_so_ ohe ian_like_you}
Two letters in the flag never appear elsewhere, leaving .ohe.ian. Grepping the
SOWPODS Scrabble dictionary for that
pattern gives only 1 hit:
$ grep "^.OHE.IAN$" sowpods.txt
BOHEMIAN
The flag is thus feel_so_bohemian_like_you.