Categories of Ciphers Substitution ciphers - replace letters in the plaintext with other letters or symbols, keeping the order in which the symbols fall the same. Transposition ciphers - keep all of the original letters intact, but mix up their order. Use both methods, one after the other, to further confuse an unintended receiver as well.

Substitution Ciphers Substitution ciphers could properly be called codes in most cases Standard Public Codes Morse code, shorthand, ASCII code Secret Codes Captain Midnight Decoder Ring Julius Caesar’s secret code

"encoder" ring/ Caesar’s code Uses two concentric wheels of letters, A to Z. Rotate outside ring and substitute the letters in your message found on the outside ring with the letters directly below on the inside ring. Algorithm - offset the alphabet using a key (number of characters to offset it). Julius Caesar used this simple scheme, offsetting by 3 characters Example: "EXPLORATORIUM" Answer: "HASORUDWRULXP"

Another Substitution Code Little tougher to decode: Randomly assign a code to each letter. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Q W E R T Y U I O P A S D F G H J K L Z X C V B N M Example: “MEET ME AFTER SCHOOL BEHIND THE GYM.” Answer: “DTTZ DT QYZTK LEIGGS WTIOFR ZIT UND.”

Tricks to breaking code Word lengths - especially the short words - give great clues as to the nature of the code (see frequency charts). Example: Cryptoquip puzzles To help conceal your message, ignore the spaces and break the message up into equal-sized chunks. Five letters are customary in the spy biz Example: DTTZD TQYZT KLEIG GSWTI OFRZI TUNDM

diagrammatic cipher Used by many children in school, substitutes symbols for letters instead of other letters. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z abcdef ghijklmnopqr stuvwxyz Easier to remember than 26 randomly picked letters. Example: Meet me after school behind the gym

Frequencies An arm-chair cryptologist would take less than 10 minutes to figure out the outer-space alien text. Why? Given enough ciphertext, certain patterns become obvious. Notice how often eappears: 6 times out of 29 characters (about 20% of the time). The eis almost certainly the symbol for "E," the most frequently used letter in English.

Francis Bacon’s secret code Uses two different type faces slightly differing in weight (boldness). Break up ciphertext into 5 character groups, each of which would represent one character in plaintext. Depending on which characters of the group were bold, one could determine the plaintext character using the following table (* stands for a plain character and B for a bold character)

Francis Bacon’s code Secret Code A=***** G=**BB* M=*BB** S=B**B* Y=BB*** B=****B H=**BBB N=*BB*B T=B**BB Z=BB**B C=***B* I=*B*** O=*BBB* U=B*B** D=***BB J=*B**B P=*BBBB V=B*B*B E=**B** K=*B*B* Q=B**** W=B*BB* F=**B*B L=*B*BB R=B***B X=B*BBB

Francis Bacon’s code Secret Message: To be or not to be that is the question. Whether 'tis nobler in the mind to suffer the slings and arrows of outrageous fortune or to take arms against a sea of troubles and by opposing end them? Decoded: M E E T M E B E H I N Tobeo rnott obeth atist heque stion Wheth ertis noble rinth emind D T H E G Y M A F T E tosuf ferth eslin gsand arrow sofou trage ousfo rtune ortot akear R S C H O O L msaga insta seaof troub lesan dbyop posin gendt hem

Transposition ciphers Original letters are kept intact, but rearranged in some way. oo-day oo-yay ow-knay ig-pay atin-lay? Scytale : Spartans in 5th century B.C Uses a cylinder with a ribbon wrapped helically around it from one end to the other. The message was written across the ribbons, and then unwrapped from the cylinder. Someone with an identical diameter cylinder could re-wrap and read the message.

The scytale depended on a piece of hardware, the cylinder, which could be stolen or broken. It would be better if the method were completely "intellectual" and could be remembered and used without resorting to a physical device. Both the sender and receiver must use this algorithm for enciphering and deciphering. Geometrical figures serve as the basis for a whole class of transposition ciphers.

Transcription Code M E E T M E MACEH EFHHE ETOIG TEONY MRLDM ESBTO We can now transcribe the message by moving down the columns instead of across the rows. MACEH EFHHE ETOIG TEONY MRLDM ESBTO You don't have to write your plaintext into the box row by row Let's put our message into the shape of a box. Since there are 29 characters, we'll add a dummy ("O") to make 30 and write the message in a six by five box. M E E T M E A F T E R S C H O O L B E H I N D T H E G Y M O

columnar transposition Think of a secret key word. Example: “SECRET”. Write it above the columns of letters in the square. Number the letters of the key word in alphabetical order. (If there are duplicate letters, like the "E", they are numbered from left to right.) 5 2 1 4 3 6 S E C R E T M E E T M E A F T E R S C H O O L B E H I N D T H E G Y M O Now write the columns down in the order indicated by the numbers. The result: ETOIG EFHHE MRLDM TEONY MACEH ESBTO