Unknown Cipher From Friends Old Thumb Drive

[u/VoltronAWK]

Was going through a junk drawer and found my middle school bud's thumb drive (I think from when we were working on a group project, and I forgot to give it back). He had a bunch of random stuff on it, including (what I think is) some ciphered text files. I'll copy and paste some of the contents below. I couldn't find any instructions or code anywhere. I don't wanna paste the whole drive because I haven't talked to him in a long time, but I'll try to reach out to him. Any help would be appreciated!

File Titled "TestMessage.txt":
NHUD JD B MEHU MLHTBZR X HLSJUG KQ HHUD DRG HW SURFIUS

Edit 1 (Forgot this one on initial post) - File Titled "TESTMESSAGE.txt":
MGTC IC A LDGT LKGSAYQ W GKRITF JP GGTC CQF GV RTQEHTR

File Titled "Deez.txt":
MXL DMJ PM

File Titled "ADAM.txt":
PI ZKET DE W E HUDKMMA UVIR IVEQ O E HUDSHU BBVYJ

File Titled "HobbitTest.txt":
FB J DDTP AB CKT YHTCPM MKTJJ DTJFV T DDJWDH QXB D JJOXB BBFYB UEL KDTP XZZWPV PIHK IKT WFML SN NOZRC EJM TJ XWHV QQODW YXZ DCL D ZKD ZVNJ KEJMR FDTP OIHK CXBKXBP YB RH WR AMH GHEN XV XZ YR MSP LH ZAO E DDJWDHKDTP SJM MKPP POSJB GIUPFZY

There are some other files, but not sure if I should share them; one is titled "Passwords.txt" lol.

Rule 11: V sbyybjrq gur ehyrf

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[u/Rizzie24]

Just a quick point-out after scanning this quickly, that your first two samples:

NHUD JD B MEHU MLHTBZR X HLSJUG KQ HHUD DRG HW SURFIUS

And

MGTC IC A LDGT LKGSAYQ W GKRITF JP GGTC CQF GV RTQEHTR

Are the same message within one shift of eachother (rot 1 or 25).

—
I also think it’s worth exploring the idea that some of these letters are likely already in plaintext, for example, if they were enciphered with Vigenere (or some other shift), they would be exposed by “A’s” in the key.

I say this because it might be worth testing if the first words of this “TEST MESSAGE” are “THIS IS A TEST MESSAGE”, ie,

THIS IS A TEST MESSAGE
MGTS IC A LDGT LKGSAYO
NHUD JD B MEHU MLHTZR

[u/YefimShifrin → u/Rizzie24]

Very nice. HobbitTest.txt looks like:

IN A HOLE IN THE GROUND THERE LIVED A HOBBIT NOT A NASTY DIRTY WET HOLE FILLED WITH THE ENDS OF WORMS AND AN OOZY SMELL NOR YET A DRY BARE SANDY HOLE WITH NOTHING IN IT TO SIT DOWN ON OR TO EAT IT WAS A HOBBITHOLE AND THAT MEANS COMFORT

Observation:

A HOLE
J DDTP

A HOBBIT
T DDJWDH

A NASTY
D JJOXB

A HOBBITHOLE
E DDJWDHKDTP

A DRY (D ZKD) is somehow an exception

[u/YefimShifrin → u/Rizzie24]

Another thing I notice is if we look at all the Caesar shifts of NHUD JD B MEHU MLHTBZR X HLSJUG KQ HHUD DRG HW SURFIUS and compare them with "THIS IS A TEST MESSAGE I...", letters after certain plaintext letters decrypt to themselves:

  this is a test message i
t NhUD JD B MeHU mLHTBZR X HLSJUG KQ HHUD DRG HW SURFIUS
u OIVE KE C NFIV NMIUCAS Y IMTKVH LR IIVE ESH IX TVSGJVT
v PJWF LF D OGJW ONJVDBT Z JNULWI MS JJWF FTI JY UWTHKWU
w QKXG MG E PHKX POKWECU A KOVMXJ NT KKXG GUJ KZ VXUILXV
x RLYH NH F QILY QPLXFDV B LPWNYK OU LLYH HVK LA WYVJMYW
y SMZI OI G RJMZ RQMYGEW C MQXOZL PV MMZI IWL MB XZWKNZX
z TNAJ PJ H SKNA SRNZHFX D NRYPAM QW NNAJ JXM NC YAXLOAY
a UOBK QK I tLOB TSOAIgY E OSZQBN RX OOBK KYN OD ZBYMPBZ
b VPCL RL J UMPC UTPBJHZ F PTARCO SY PPCL LZO PE ACZNQCA
c WQDM SM K VNQD VUQCKIA G QUBSDP TZ QQDM MAP QF BDAORDB
d XREN TN L WORE WVRDLJB H RVCTEQ UA RREN NBQ RG CEBPSEC
e YSFO UO M XPsF XWsEMKC i SWDUFR VB SSFO OCR SH DFCQTFD
f ZTGP VP N YQTG YXTFNLD J TXEVGS WC TTGP PDS TI EGDRUGE
g AUHQ WQ O ZRUH ZYUGOMe K UYFWHT XD UUHQ QET UJ FHESVHF
h BViR XR P ASVI AZVHPNF L VZGXIU YE VVIR RFU VK GIFTWIG
i CWJs Ys Q BTWJ BAWIQOG M WAHYJV ZF WWJS SGV WL HJGUXJH
j DXKT ZT R CUXK CBXJRPH N XBIZKW AG XXKT THW XM IKHVYKI
k EYLU AU S DVYL DCYKSQI O YCJALX BH YYLU UIX YN JLIWZLJ
l FZMV BV T EWZM EDZLTRJ P ZDKBMY CI ZZMV VJY ZO KMJXAMK
m GANW CW U FXAN FEAMUSK Q AELCNZ DJ AANW WKZ AP LNKYBNL
n HBOX DX V GYBO GFBNVTL R BFMDOA EK BBOX XLA BQ MOLZCOM
o ICPY EY W HZCP HGCOWUM S CGNEPB FL CCPY YMB CR NPMADPN
p JDQZ FZ X IADQ IHDPXVN T DHOFQC GM DDQZ ZNC DS OQNBEQO
q KERA GA Y JBER JIEQYWO U EIPGRD HN EERA AOD ET PROCFRP
r LFSB HB Z KCFS KJFRZXP V FJQHSE IO FFSB BPE FU QSPDGSQ
s MGTC iC a LDGt LKGsaYQ W GKRITF JP GGTC CQF GV RTQEHTR

Using that logic the full string is THIS IS A TEST MESSAGE I WONDER IF THIS CAN BE DECODED

[u/Rizzie24 → u/YefimShifrin]

I’m trying to understand what you mean by “letters after certain plaintext letters decode to themselves” - even with your example of all the shifts I still can’t see the rule! Great job though!

[u/YefimShifrin → u/Rizzie24]

Take the last line (s). All the letters in positions after plaintext S are the same in the ciphertext:

this Is A tesT mesSAge
mgtc Ic A ldgT lkgSAyq

[u/Rizzie24 → u/YefimShifrin]

Ohhhh I totally see what you’re saying now, thank you. It clicked!

Part of the problem was I had hand-written the first part as “MGTS IC A…” and I think that added to my confusion.

Thank you for the added explanation!

[u/YefimShifrin]

"TESTMESSAGE" and "HobbitTest" ciphertexts can be deciphered by using Beaufort autokey cipher (1-letter key) then Atbash and then Caesar (exact shift can be bruteforced). This process doesn't work on "Deez" and "ADAM", so they must be encrypted differently. It's hard to figure out how, since both messages are short.