Dr Andrew Scott G7VAV

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CSC370.2: Security and Cryptography
Recommended Texts
Schneier
Bruce Schneier,
Applied Cryptography: Protocols, Algorithms and Source Code in C
John Wiley & Sons, 16 November 1995, Paperback: 758 pages
ISBN:0471117099
Anderson
Ross J. Anderson,
Security Engineering: A Guide to Building Dependable Distributed Systems
John Wiley & Sons, 11 April 2008, Hardcover: 1080 pages
ISBN:0470068523
Howard and LeBlanc
Michael Howard and David LeBlanc,
Writing Secure Code (2nd Edition)
Microsoft Press, 1 December 2002, Paperback: 798 pages
ISBN:0735617228
Neumann
Peter G. Neumann,
Computer-Related Risks (ACM Press)
Addison Wesley, 24 November 1994, Paperback: 384 pages
ISBN:020155805X

Can you crack the code?
The following two ciphers are passages from well known English texts. Use your knowlegde of the respective cipher systems and the frequency tables linked below to decipher them.
Sadly, no prizes for finding the answers -- it's just for fun. The texts and keys are chosen to make the task relatively easy.
You can download a copy of these along with some basic information about the ciphers by clicking on the thumbnail image to the right.
Caesar Cipher
One of the simplest forms of cipher is the Caesar Shift cipher. This is a simple mono-alphabetic substitution cipher where each letter in an input message is replaced by another letter from a single cipher alphabet. The cipher alphabet is formed by shifting or rotating the characters A-Z by a known number of places. The cipher key is simply the number of places the characters have been moved, 1 to 25.
Breaking such a code can be done by noting the widely differing usage frequencies of letters in English, or even just brute force. To make the task of breaking such a code more complex the cipher alphabet could have the letters randomly shuffled giving more than 400,000,000,000,000,000,000,000,000 possible alphabets rather than 25. The following is a simple shift substitution of a well known English text. Use the Caesar Shift Cipher code wheel, below right, to help decode the cipher.
FANQA DZAFF ANQFT MFUEF TQCGQ EFUAZ ITQFT QDFUE ZANXQ DUZFT QYUZP FAEGR RQDFT
The wheels can be
printed on light card,
cut out and joined
together using a split
fastner.
QEXUZ SEMZP MDDAI EARAG FDMSQ AGERA DFGZQ ADFAF MWQMD YEMSM UZEFM EQMAR FDAGN
XQEMZ PNKAB BAEUZ SQZPF TQYFA PUQFA EXQQB ZAYAD QMZPN KMEXQ QBFAE MKIQQ ZPFTQ
TQMDF MOTQM ZPFTQ FTAGE MZPZM FGDMX ETAOW EFTMF RXQET UETQU DFAFU EMOAZ EGYYM
FUAZP QHAGF XKFAN QIUET PFAPU QFAEX QQBFA EXQQB BQDOT MZOQF APDQM YMKFT QDQEF
TQDGN RADUZ FTMFE XQQBA RPQMF TITMF PDQMY EYMKO AYQIT QZIQT MHQET GRRXQ PARRF
TUEYA DFMXO AUXYG EFSUH QGEBM GEQFT QDQEF TQDQE BQOFF TMFYM WQEOM XMYUF KAREA
XAZSX URQRA DITAI AGXPN QMDFT QITUB EMZPE OADZE ARFUY QFTAB BDQEE ADEID AZSFT
QBDAG PYMZE OAZFG YQXKF TQBMZ SEARP QEBUE PXAHQ FTQXM IEPQX MKFTQ UZEAX QZOQA
RARRU OQMZP FTQEB GDZEF TMFBM FUQZF YQDUF ARFTG ZIADF TKFMW QEITQ ZTQTU YEQXR
YUSTF TUECG UQFGE YMWQI UFTMN MDQNA PWUZI TAIAG XPFTQ EQRMD PQXEN QMDFA SDGZF
MZPEI QMFGZ PQDMI QMDKX URQNG FFTMF FTQPD QMPAR EAYQF TUZSM RFQDP QMFTF TQGZP
UEOAH QDPOA GZFDK RDAYI TAEQN AGDZZ AFDMH QXXQD DQFGD ZEBGL LXQEF TQIUX XMZPY
MWQEG EDMFT QDNQM DFTAE QUXXE IQTMH QFTMZ RXKFA AFTQD EFTMF IQWZA IZAFA RFTGE
OAZEO UQZOQ PAQEY MWQOA IMDPE ARGEM XXMZP FTGEF TQZMF UHQTG QARDQ EAXGF UAZUE
EUOWX UQPAQ DIUFT FTQBM XQOME FARFT AGSTF MZPQZ FQDBD UEQEA RSDQM FBUFT MZPYA
YQZFI UFTFT UEDQS MDPFT QUDOG DDQZF EFGDZ MIDKM ZPXAE QFTQZ MYQAR MOFUA ZEARF
KAGZA IFTQR MUDAB TQXUM ZKYBT UZFTK ADUEA ZENQM XXYKE UZEDQ YQYND QPMZP DQIEO
Vigenère Cipher
The Caesar cipher is easy to crack using simple frequency analysis, or even just a brute force attack. To make things harder one can use a poly-alphabetic cipher where a different alphabet is used to encipher each character – approximating a one time pad. For practical purposes a repeating set of alphabets are used; the first letter of each alphabet forming the key or keyword.
As the cipher alphabet changes for each input character, simple frequency analysis no longer works. A method of deciphering this type of system is thought to have been first discovered by Charles Babbage – although, as the cipher was in common use, he didn't publish.
The weakness of this type of cipher is that the key, and thus the set of alphabets repeat. This means that common words will be repeatedly enciphered with the same set of cipher alphabets. Spotting repeated groups of letters in the enciphered text gives a good indication of the key length (or a multiple of it), and also the likely keyword itself by considering the frequency of common words and letter groupings in English. For example, by far the most common word in English is the, and the th pairing of characters is itself by far the most common combination of letters – these should show up regularly in any enciphered text. Given a known (or maybe assumed) key length, the problem becomes one of a set of mono-alphabetic, Caesar shift ciphers.
The following enciphered text is a well known passage from a popular English novel. The passage and the key have been chosen to show up the flaws in the Vigenère cipher and make cracking the code relatively easy. Use the Vigère Table, below right, to help decode the ciper.
BHLIU MVTJG LHDNV BATAK MKPAV ASLWT LHDNV BATAK MKPAV ASPOG HTLQU WCBQV POHBJ
XOVMQ YTDWN BGWVG LGXBY TGIPG XDDKJ HTQMN BSUQV POHBJ XSEWE ACUQP VFTLW EWIGK
MKPAV ASHMC LCCWH EWVPV BHLIU MVTAG TGDVQ YRPZM GSHAK MKPAV ASHXT BBVWH ACEMK
MKPAV ASLQP MSGWH WSHXC BFLMJ TRTDG KMIPK GUQMH HFTCU PSWIF GCIPK GUQMH HFTCU
PSLMT XOATI HWCOF BFTKV MCWMC OSCEG PSGMC EZVWK GUSQT XQIBJ XCIPG KKPGK GGWWT
MHWMR XFXWF POHAQ YOGTK DSIPG IFTAG GHEMT BCSBJ THHWO XCUQV LBDQU BSHBC NHWWT
BHXMU BBHQU MSSWP BHHJG BBVZG VSXDG WTDZI HCSWT YCGMX BZXVV ASHCR XFAIV BJTLG
ZFTMQ YQDUR TFXAQ GCCTA MVTZG PSGMC DWCOY BHWIN TFVML TKPVF TEJMG GKXBJ TDAIK
GTPKG HBIPG MVGWP XCUMP ZZPVF MVTZG PSGMC DWCOY BHWIN TFVML TKPVF TEJMG GKXBJ
TTPQT YORMQ GHWMV AFDVG HTUZC GQTQP UCIPE HICBT BSHQV POHKN XOGMT MVPVE KMHBC
EHDBJ XZDZF LCUBJ XGIIV XDGMU XFKMU HTAWC OSHIP WTXAJ XGIPC MHWQP ZGXVI XBTZC
EKTZG LSIBN XRUWT XJTZK MKPAV ASNMC KCUWW KZDZF HBTBJ HIHIP WGTDG GVJVF KSSIP
WGTDG GHNNK OSHXK KWICC EFTDG EOIQQ GGLMT XQDVE XRTLV HSCON TBSIV MVPBH TJDCT
XREMT BCSIU THIPK LAGAU HIIPE HHIPC WFTKG GHAGC MHPQP XRWMT YWKMC GRIEG GHXMV
APAMU LSSJK KHWLC RCUEJ HAPXT HDWMV BQEZK OOIMK GHWMN BTTOW TFSAJ TRWMT TZSMF
MVTAW UZXUG TDEMC KOCKG UMPVP HICKK GUIPC MOGZC GUTUG GHHEG KSBIF XTDZV ASHEC
EZDEK GUJXQ YZDVF HBPVF PSHBO BBHBG KSKMP MVTKQ VYAIP XUWWU MVPLD XSCTC BRDVN
ROGWW GRSWB XBDNA XOGAC YHTZT TDEQP ZCJBK MGBMU LOVMU TGIPG LDXZK MGDNV AWHDG
KMNMC KZPAV IOHBU NDTZP THJZC EZNLG YWRQG GHXVQ KWVQP TZXBA KOEXG WCJBV ASXZU
FSGMO XGHII XGXVV ASTIT MVAGQ KRTZQ YSKMP MGWIF EOIMN RQDUG MCIPG XBVTK LVRZQ
PBPVF ISDXN XTGWO TQDVI KSHAQ YPGQV BGWAW UXTKV LWCIO XFXKC PVXKJ LHGIP ZSIWT  
XZPBG AOKMR KCKMF FCGMK FDDZV TBIBQ MVTPW FOCZC VSIPC GOCGE HABCP BQPBK HBHGG
MFTKG BJTLV AFDCI AOCGQ YHWME AWRSG GGDNV ASRWE DZPVG UFDWF YFPVE XZTAU YOKWW
KSSWP MVTEJ HZTIU MCBIV MSGAU IWGQV NOABJ TBWMT LWHBG KCUBJ XGWQG ERPVF MFXLG
GHGWN ESSEK MVTFE XSSQP ZGBWQ MVCMU LRDEP AWATO TYXVI IOEMT FCCMA TBSAR XBSQP
ZWICP WSGBJ XUJQF TBRMQ YVTZE AFXAV BOCXC LHDZU LVTMP MSGBC BBTLJ XFHMN YPTAK
WSHEK MVHCE AVJUC GSPKJ BSKMO XBIAC LGTVV XBRQP ZONWW MVIWJ TJTPK LVPVF LQJBQ
YTWQU MCCOW XHDZP HIIEK MVEQP VSGAC GRWQU UCSGD NFCMF TZXDG USRIW LSWMJ TRCWV
DBTMN XRSWY GWCBJ XFPQP MCSWJ HBDCT MCPLK KHNXT HQTAU BCCWH FCCSU PVXKJ IOHAG
WKXBJ BBWQU OWTEC MOSQU MOCKG HTHWO XTXNV RCGAK QHNGC KRHQV BGAQM XZNMP HIVPV
AOIZQ HHTLK GHWMY HCSAQ YTGIP VSPVF GCGEC RHWMT XKTZG ZFDEK GUIZG XGLPG GHWIV
LIUNG KSGEC LDJBV HRTIV AOAZG TRNUC KYTLD RHWMY HCSUC GTPBG MCRWO XRDEP TBSJG
LOLVK GHDJQ TFSAV HAPSG TQTZV TWCUQ OOQTG YFPUG PCGSY BHWIU TQZIP WOZVK YSXVK
MHTZT BPAMK GVXAV HFNQV BGAQM XZNMP HIVPV AOIQP MVTZQ NUWWW MVDCU XGDNU HATBK
EZTZU HTIPG ASPDA EOCLU TRYIE XBIBQ IOGQU MVTZG PSGMU ASABG KSSNT HAIPG PSPBJ
XFIPC MJTZA WONZW WSRIT MGQMU IOIBG KSSEK MVGCU MWRUK KSHVW YTTLC UCJBD RDXOU
TBSZQ HGIMF BBQGR HIABT RKWQE AHWMH TFBMT WSPBJ AOSIN KSPLA LSIIR TFIBQ USWQU
MIBJT BZHWH MVTZG OCACV BCCJW MHWIV PCDLO TBPVF MVPBH TFBMT MVDCI AHWMA PCGSW
GQTIU BBVTA PCGSU BZTVV EMPVF GCDVG ASPZF MVTUC LHWMA PSCBC UCJBY BHWUW YTAMF
MFTIF MVTZC MVTZH HFPAO NQWIU MCTVV XFIIK GOCGU NGEQE BCCBJ THIPG RKTZG TKPSG
POHBQ USPBJ XWHBK VOAIP WHGIK MCGWW LWCMP ZZPVF MVTZG POHAE TFRMN ROCIO HICBQ
YCGLG KOCLR KCIME MWDVV HXJAV BTNUW VVCIV BCCIN UCPAV BBVLC KWCOD NFVTC KWTAD
ROGUG WATVC GRWQI AKPGT HPQMT BSHBQ HYETC VSXVV ASRIR BHPTK MGTTH XJTZA GWVPV
YOBQN BSHEG KSECD EWRTA VOJBK HBTLP HHIWI HCJBQ YHDEP PWIPQ NHGMO HJXVI MVTQT
YIGVK MIGMV HIEPQ EGIMT XFHEC KSWWW LSHNQ KGTKW KWIGV ASWQI AKPGO TBXVV ASSIT
DKPAC VWIGV KOSMU FOCQP MVTTK ZVIIP WPTQP ZFTKQ ZBXAG WOCLE AOATG GUTLD RVXAH
XZAWY MFPLG LAPVY ACBPG LHDXR XRXVJ BGRPC KORBG KCUBJ XQPXV TWCOC EZPVV EMHPQ
MVXUV AFDCI AHWMJ XOSIP WFDLG TKPGV ASBIK EKPAY TMAIK WPNAG OSCZQ UPTZU TBSBJ
XUJIT WGWWV MVGMG WSPLC GRIPG GUDBU ACILG TRWQO LSAND RHWMQ MVTZH HIGQP VCCAG
JITVE XCUBJ XTPQN NFTWH AWHIO FICQV BCCIH MSGEJ BQWBJ XAPQN POHZQ UPTLK GDTIE
XHWIV FOVVK YWRMP MDDBG GHPBG MVTTQ KRBIA HFDNN HBSWP POHUC WSIWU MOCLC GRSMN
BJTZQ GHJZP AOBOT XSCJA HBTPK ZVLIA FOCEJ HRTAR HWAMF MVTQN EIHBT BCJAE KSPBW
KSXVU BUWBQ YOATJ BGGMV BBJMR KWHWP XFHQP ECCLQ GUPWN LTDCI AHQIV MZTAY BHWBJ
XWGBW KBZMA LOCLV ASBIL XGIGQ YHWMN TKUQT XRQTW GRTZD NGHMU BBPUQ GUIPG FZDIF
XRLQV AFDCP WGDNU ACIIP WPPTN MVXMX XGHVK IDTLQ YTSQC FCCLE KCHAG LTGWO MVTVG
VYHWH GCQTG ECGLU THRWW KHSZC PWCOT HCBAO NGZMV XSGAY XBIQP MCHBI BZTAU MCHMC
KQWNQ KQDVV KOQIP WUDWF LOCLV ASBWD YWGMF HBIPG FIHSG MSTZU TBSBJ XAJAM XHTMT
LTXZG WCCBJ XADJC GRCWD HRNBJ HIVPV TBNWH MVTAG HQRCT KSCKG LAJKJ HIIWH MVTKQ
FADVY TMXVV ASBQF LHDNV ASBBJ XVPVI FOCMX XFQCU ROCLG OSGEQ KGTBJ TBJAG ESHAY
TGXVE HBHBC GHGMS NWHQV BCCVQ PGIZK GUXVI NDAWP ZFDEU HTBQU VSATC GSDCU VFXUK
GOAAP HKWIP ZWCOC ACJAG UFTIM XFDVU THJZF TMLPQ AOSJG XBIIM XBDVV NSHLC RBDED
NFCQP ZDTWR ESXVV ASWIP WOIVG PUPBG UMIPG WCOMP TBSVQ PPJZP BBVXC FDWTG MGPBV
ASSWQ KCUEG LHBQP LHTZJ TZABQ WONBC DWCOV ASAQH XCUIP THGWE BCJAO NFSMT XFPVF
MCBWT KCLWH TKGMV VVTLR BZUMT XFLPQ AOSZQ UPTLC YOGUG KGQWA HTHQZ ISCKG TZABJ
XGTBJ BBVAC GRPBJ HIHIP WZXSG MVTUE TATBQ IOHAK GOCLE ECHMW ICCBJ XRTIT HZSGG
TFDVG MVDCU TBSAG OSCPW GRGMF TBSAG OSCBA YWKMG GJXZQ GSSJA MVTUY AWAMV ASLWQ
WAPVC GRIPG YOGUG KKDZM XRJVJ XSSMF MVDAG MKDWH MVTTC KUTRC PGPVF MVDAG HHWMT
MKDWH MVTXN TWCIP WHWMH TWGNC VSHBT HRLQV AGIQT XBDCI AOCLE TFGQG WHWMK KRXDK
GSGQI AHHEK MVPPK ZVWIP WHWCU WWSBJ XMTIT HBTBJ HIHIP WGTDG GVJVF KSSIP WGTDG
GHNNK OSRWP WIRBV ASXZI KSPBP XGHMU TBSUA KWPLU HTHUC EZRZG THJZG LHWME KSPBW
KSHWH MVXAE AFDVK VZTIO HBVBJ XFTAV TZDVI MVTZQ TRHBJ THAIA USUWT XHWMO LQDBV


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