Reply To: Extracurricular challenges

[upsidedown]

I’ll first offer some hints for my previous two ciphers: the plaintext of the 3×3 cipher is an excerpt from The Hitchhiker’s Guide to the Galaxy (as madness has hinted at 🙂 and the film that I was referring to for the 5×5 cipher is 2001: A Space Odyssey.

Third cipher: upsidedown-2025-3

Since it’s the third, I’ve composed three ciphers (written in the order they were applied to the plaintext):

1. Vigenere cipher, keyword length is not equal to the hill cipher block size
2. Autokey vigenere cipher, keyword length 6
3. 6×6 hill cipher

The alphabet is ABCDEFGHIJKLMNOPQRSTUVWXYZ?!’:.,_ (as before, underscores are used as spaces).

My program solves this in around 33 minutes (on 12 cores), so to save you the tedium I will reveal the first two columns of the inverse matrix. My program could break this in 2 minutes (on 1 core):

26 15  ?  ?  ?  ?
21 25  ?  ?  ?  ?
 1 22  ?  ?  ?  ?
13  6  ?  ?  ?  ?
27 15  ?  ?  ?  ?
19  1  ?  ?  ?  ?

I’ll also add that all three keys and the plaintext share a theme.

Here’s the ciphertext:

VHMI?XHCGVO,YOX?XLGUTPWKLGR?SSV,.FKNYJMC!?UADR!PAH,DU'OG:J,WBKO:SJHQXALWUQH
!UTU'BHOHA,F_EEARAYPONSNLW'XETFVMNOG:TEDICNDUHCK?R!KV?XWPM_TP'M_X!XQK:KGWWS
D_O:ZZEB.KS:!_'EAUUELK?!LJO.ZXK_.MRTD.IBL,DR!CFD!CV.NIQR.'LDCSALD:?NQFGEQHK
VLQFSUEGZXTBBZ,M:,'?BZOTP_TIKBZEGWJJE?YLBBB..P:TBY?ZYDFMERI._R!L?HIDWHKYFTO
Q,TTP.YJY,QHWJJE?YX.HV',KLR:WYOEILZJY!E:PJG'EO:HFTNHHXAM'Z,:ZQMQ.:TOJGAXGQ!
_VDO:AMEPRBZS?ZO,MNCOWSCM_:VEPZ?VEPS!NP!A?_!S!RIJDC''AXSGINJKCNU'OK__UFOV_:
DE'SBZ_VTER?VDUVBABM,XEYRL?PG:TAANBNJ:Z!QFPE'ZYVOZ,Z_TRGDHWGO',K,T_UZQNBOA.
N_FWFFFRLC,EC?QI'TNNVJQ.Y!LXK.A_JC!,.'NC?JCF:?:BG.:DH.Z:GEAOYV'FPGCIPIRHP,H
UYZKU?YE.:CIMOB.EO':ZQJJSYNIGB?GVPBNQQ,?GEN_,VQ!S!KCN.VCHRZ_NKTKALH_WWQN'.P
QMBCX!MMTS.!BCV.ZMX:WWRWMRTDIXZXH,KNKUTW,VJXV,BH.D!S:RSP?SWKHG!C,'RYWWFVZ,Y
Y!J!?QGAOBHKU!M:'IG:'VK,BSQJQIDXTPUM_MAKCLG.YDYXTV!QNA.M!!:FUASHBER,:MNUZNV
GV_TUQXNXCXAY,.:OWC!!IZXCJDQ,MFRQBBHHBA_JQBCQQBGLGGHLVS?_Z:OUC'DN?YF?X?MVFU
WTWDJAFFPS?.USH,TAO'WWIMER.TXEWBHOG'SSPXESRO?HUZHSEYYCK:AQKKSAQE?FL':XERC:I
.,.RWT!,QZZCSURUKBPTMZZUQGCOMLFM_ADO_B'IITZA:_YZ_KXJNKHDWG?_VPUDPXOXGGLMDHD
JNJK,'RJUDYIJ'PXDRMORTAHW,VJXV'TQ'E?W?!A!IDRTQ'DWNJ.UXHBKO:EV:,VSX?ELO'_ZGE
THYTGTDIDZMKQT'KVEFDBS.TJQWKSPMQR:XWK'KS'KMZOKRU'GVYPKNLYHNBJRWA,YE,FHK.HVW
KOIXQPCAKMPB:!SDAEG:'KX_MYSEQR_HQ?CB:!RFQWFTPWESTQQ.VFLXJQVMSZU!DQ'MNBUG?C'
!CSC.MJ'VN:.BDPWQOIUKMWPYSJJF'TGPZFTSHPI'FOXU,DMYIA?:.:PFA'ZOU'!?SI'AZ,SGHW
'IUE?YPT'JQRHDIHMW:OAPMTE:MHXELV?SDCJO','T_ACZC'P:,WF:AWEJKK.IS_EVJKZASWTRI
':DHMQTXKFFZ:C!UVBX_LF?X:AKY!DNPKLN:J!MC,HI:O.QN!!IBU,YUO_MHIGJYOM!PIX!CNAD
AXBNDOC?I'_!IK?::WQJ.,EO.FZTV?LOZXLQJ''XEZJ:ON_SDDOQUHTUFF,:IR,A:VTFX.YVWNG
MEJDK_HVQWXWAWDNZPI?PO,IXX:HIQNCGLPL!BUN:RLIEAP',TGJKEK'F_:EB!!_CD',_V'K'C:
LHP_X_JZWSBAE:KXMYE?NYXKVQLMMZ:S_QVPGR,HQR?Z:'FEVMAFJFVDCEHTZERSLPANUSR!ECJ
GE.SYKMNK'_:IFTYS,'CFNY!BBLAWQ,CG'N'IXQCQ'EO:VHDJR?W_O:O!_GBQVIFDHNOTTFIGRS
XYSAFAFDOKOI'KETZDJA?P'QRVJBU_YHASDFSU.HFZKTC!AJYEQIOQCGCARK_KVKXGTNXA.'M.Z
M!EJQRUREXQH:K:K,!APBIJ!CCVZQNUWHD:.SRJ,Q','!UHJU_FBTJ!ZJ?CD!IKJFWSWX.J,SHL
E'GCK?XRKFTAEABM_M_.E.O:I?PSNC_?L,QWAYVKBQW::YUDEXNBA.J:UFI.PELCRR!ATZDG,Z?
SPO,X'G:WEFECP!DCQTNSIS:BX_KZ!A!GJ:UJ!DO!NJZNWPQJWGM.LJN',CMEZ'Z.O?!WBFOAX.
AWI:GBCCKQA:?T!YEIJEPUEV:!MQMBGQF!PUQQHVN'F,,CXVA'BNF_._CSHVSJ'VXHA:BRGG?PE
QEB.OEVKDE,T!EEGNLOV_.AGQJZREXHJH'IWPGYPSK.?VPQGYGJ?YNHIWBWIEPYORGNIQ:?WCK,
XZBMXRGQHYEYSNM.'OOY'JFKTRC_:QJ?!TSN_JI'PS,AWR'BECYN:LHM_HUYSRWZ_YKFFMAZLKL
P,E_Y?XXLIYO,GLXXN!K:PJOHJXO!?CFBJK,HZXZJ'.A!?.W!VQRHPRVL,IGQSVFBXSQBTIBRHW
QZVV_TSMSBV,PJT:BBBBJR:OTL'JUOLSL_S?PFM.TK'LXIA?MYVWZNNW!E'R?AOGF!!OQYYRCRZ
SZYUQKKQJOLQVRLR!MGGZKD_:UB_YEBZJ'TZEAPZNENTFGPT.NVN'.R!RUGO_DPHJDKAI!,LXBS
ZMQO:XGN.QOHQU::GNXO',.SONQOSKXV_QV.HMAPIOL,.SFJKWARWTAO'FMC:,VMDBZP'R,UVRX
SSBBVOH_KD'QYZ?EBEDVAOTVGYW,O,FVTRVEQOXBYWP_NTEBC,FLRVRJ!YYHLDKWFQGKEJAR:L_
S?FNWNAGQTSVKD.,F''JWKIGXXQ!A?WZIV_DO,BXSLOW,OU_QLQY?:T!KCL'QEKAUOSUX:YCFT.
?VS,DDRBB?:T!?O.P__IPHNFVBONCQSYAZUITM!SYG,IKS.AMJAGGEWAY.SDGW._JLK!!GSIWDF
SWMT:APEYKVTHDBDBDUWVPCILGBACN:KX.Y.ZE!E.'Q,SVU!_BJRPTRZXAIO,TY_QUCPQLFRFKY
C'IMRRNLC.,B!VBFTYX:OZ.PYGV:YYSBIXWG!IED,D_WGA,HTADY:C,OEWITAOPTFXDGNUKEKVB