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SUBSTITUTION CIPHERS

SUBSTITUTION CIPHERS. A substitution technique is one in which the letters/number/symbols of plain text are replaced by other letters/numbers/symbols. Visit for more Learning Resources. Substitution cipher techniques. Caesar's Cipher – right shift by 3

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SUBSTITUTION CIPHERS

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  1. SUBSTITUTION CIPHERS • A substitution technique is one in which the letters/number/symbols of plain text are replaced by other letters/numbers/symbols. Visit for more Learning Resources

  2. Substitution cipher techniques • Caesar's Cipher –right shift by 3 • Mono-Alphabetic Cipher –random substitution • A Homophonic Substitution Cipher-one to many substitution • A Polygram Substitution Cipher-many to many substitution • A Polyalphabetic Substitution Cipher-key to each plain text • e g. Vigenere Cipher • One time Pad( Vernam Cipher) [Note: write plain text in lower letter and cipher text in capital letters]

  3. 1.Caesar Cipher • It is earliest known substitution cipher • by Julius Caesar • first attested use in military affairs • replaces each letter by 3rd letter after it • Example: meet me after the party PHHW PH DIWHU WKH SDUWB

  4. Caesar Cipher • can define transformation as: 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 D E F G H I J K L M N O P Q R S T U V W X Y Z A B C • mathematically give each letter a number a b c d e f g h i j k l m 0 1 2 3 4 5 6 7 8 9 10 11 12 n o p q r s t u v w x y Z 13 14 15 16 17 18 19 20 21 22 23 24 25 • then have Caesar cipher as: C = E(p) = (p + k) mod (26) p = D(C) = (C – k) mod (26)

  5. 2.Mono alphabetic Cipher • The week ness of Caesar's cipher is it’s predictability. • Instead of using a unique form for all alphabets in a plain text message use a random substitution. • Rather than just shifting the alphabet we could shuffle (jumble) the letters arbitrarily • Each plaintext letter maps to a different random cipher text letter hence key is 26 letters long

  6. Plain text: Hello word Cipher text: ACDER PQSAK

  7. 3.A Homophonic Substitution Cipher • In this technique one plain text is mapped with more than one cipher text alphabet. • h-[ABXY] • I –[MNQR] • Plain text: hi • Cipher text: ABXYMNQR

  8. 4. A Polygram Substitution Cipher • Rather than replacing one plain text with another cipher text at time block of alphabet is replaced with another. • Plain text: hi hello • Cipher text: JH KNHPL

  9. 5. Polyalphabetic Ciphers • It uses multiple one character key. • Each of the key encrypts one plain text character. • First key encrypts 1st key , second key encrypts 2nd key, so on.. • E.g • Vigenere cipher is example of poly alphabetic cipher

  10. Vigenere Cipher • It is based on vigenere table. • It is made up of 27 rows. • 1st row of lower case letters represents plain text character. • Each subsequent row represent cipher alphabet.

  11. Vigenere Cipher Example • Key (K) : FRINGE • Plain text(P): get all soldier a meal. • Cipher text(C): • See key in vertical column and plain text in 1st row and write down intersection letter as cipher text.

  12. 6.One time pad ( Vernam Cipher) • It is implemented using random set of non repeating characters as the input cipher text. • Once the key is used to encrypt and decrypt a single message it will never used again for any other message so it is called one time pad • The length of key must equal to length of original plain text. • Plain text: how are you • One time pad: NCBTZQARX

  13. 6.One time pad ( Vernam Cipher) For more detail contact us

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