Categories of Ciphers

1. 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.

2. 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

3. "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"

4. 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.”

5. 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

6. 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

7. 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.

8. 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)

9. 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

10. Francis Bacon’s code • Secret Message: To be or not to be that isthequestion. Whether 'tis nobler in the mind to suffer the slings and arrowsof outrageous fortuneor totake arms against a sea of troublesand byopposing 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 rtuneortot akear R S C H O O L msaga insta seaof troub lesan dbyop posin gendt hem

11. 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.

12. 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.

13. Transcription Code • 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

14. 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