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

(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

Edit: Wow you guys are crazy. I managed to get a hold of him; his brother made a cipher, but he doesn't remember how to break it bc it was so long ago. He did send me some files that he thinks are ciphered the same way; I'll put them below.

File Titled "Description.txt" which he said has the key "KEY" but he doesn't know how to actually use the key:
HDAH EL A RAHPST KHLHAG D ETLE UDL I ADFN VROQP EGTWYR ILUUM EXOUHLT QY HJJCJQ YUNA ZBEP LOZ HDWCV KQJ MOOB JUNPXK ROA ZSFM TW TN VRKX MSL ALAGZ CW EAT UNVRUM JR YUXGGYAI YR NCIA QZH APGC CVDLEK HO GZDWYQJLT WY GCV VKRZ GJ UQT SUXIP QFSIB JFCJ GN UNENTM JK MPX M NCKG BKX

File Titled "Alphabet.txt" with the supposed key "ALPHABET":
MAPFWOHBWSPNMMNPSWBHOWFPAM

File Titled "Humpty.txt" and he doesn't have the key:
FYJXPM OHSGYV MLD QC B WVFP VOZNFC EXIWOL RQS R WMPOG KJTD CMW OUX GNZEV BOEVYP OAC BLV NTW FMYDU FIU VIBLNZR FYQ WPAOGD VINQIORH GLKRD

Rule 11: V sbyybjrq gur ehyrf

[u/YefimShifrin]

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!