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. English language Frequency tables English language Word frequency

Sample Ciphers

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

Caesar Shift Cipher code wheel 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

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.