Challenge Description
This challenge is based around a manual cipher applied to a handwritten letter from Bushy Evergreen to Shinny Upatree.
Encoded Transcript
The letter transcript reads as follows:
Wvzi Hsrmmb Fkzgivv,
Slkv blf'iv szermt z nviib wzb! R'n rm z yrg lu z hgrxpb hrgfzgrlm
zmw xlfow ivzoob fhv blfi svok.
Hl, Hftzikofn Nzib nzwv gsvhv znzarmt givzgh uli gsv luurxv
kzigb, zmw, dvoo, R xlfowm'g ivhrhg. R vmwvw fk vzgrmt zoo lu gsvn!
Mld gsviv'h mlgsrmt ovug uli gsv kzigb, zmw R'n rm z yrg lu z qzn
(mlg fmorpv gsv uroovw wlmfgh szsz). R mvvw gl hlig gsrh lfg yvuliv
Hftzikofn Nzib li Hzmgz urmw lfg, li gsv kzigb'h tlrmt gl yv z
wrhzhgvi.
Wl blf szev zmb rwvzh li girxph fk blfi hovvev gl svok nv
dsrk fk hlnv mvd givzgh rm grnv? R pmld R xzm xlfmg lm blf gl
pvvk gsrh yvgdvvm fh.
R vmxibkgvw gsrh nvhhztv drgs zgyzhs, hl slkvufoob nb orggov
hvxivg rh hzuv.
Gszmph z glm uli yvrmt hfxs z tivzg uirvmw. R ldv blf lmv!
Xsvvih,
Yfhsb Vevitivvm
K.H. Gszg xlwv blf dviv zhprmt zylfg gl fmolxp gsv wvevolknvmg nlwv
uli gsv Nztrxzo Rmufhrlm Nzxsrmv, N2C24 rh
NvgzXGU{ylgs_givzgh_zmw_nvhhztvh_hglovm}, slkv gszg svokh!
Analysis
The first thing to note is that the letter still appears to be an approximately english format, meaning that spaces and punctuation are preserved. Next, it appears that a lot the “words” repeat themselves, the only one letter words we see are “z” and “R”. “R” used by itself is always capitalized, meaning that it likely coresponds to the letter “I” in english! Because this appears to be a 1-1 substitution mapping, we have a few different possibilties regarding the cipher used. We could be using something like a ceaser cipher (though trying all 26 offsets shows this is not the case), a random substituion cipher, or something different. In our case, a common method used to encode messages is Atbash (which, also looks very similar to the challenge name…) Atbash converts letters into their backwards alphabet counterparts, A becomes Z, B becomes Y, and so on. Trying to decode the letter using atbash gives us the plaintext and the flag!
Decoded Message
Dear Shinny Upatree,
Hope you're having a merry day! I'm in a bit of a sticky situation
and could really use your help.
So, Sugarplum Mary made these amazing treats for the office
party, and, well, I couldn't resist. I ended up eating all of them!
Now there's nothing left for the party, and I'm in a bit of a jam
(not unlike the filled donuts haha). I need to sort this out before
Sugarplum Mary or Santa find out, or the party's going to be a
disaster.
Do you have any ideas or tricks up your sleeve to help me
whip up some new treats in time? I know I can count on you to
keep this between us.
I encrypted this message with atbash, so hopefully my little
secret is safe.
Thanks a ton for being such a great friend. I owe you one!
Cheers,
Bushy Evergreen
P.S. That code you were asking about to unlock the development mode
for the Magical Infusion Machine, M2X24 is
MetaCTF{both_treats_and_messages_stolen}, hope that helps!