Introduction to Cryptography

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Presentation transcript:

Introduction to Cryptography Lecture 6

Substitution Cipher Definition: A monoalphabetic or simple substitution cipher is one where correspondents agree on a rearrangement (permutation) of the alphabet in which messages will be written. The sender encrypts a message by replacing each letter in it with the corresponding letter in the cipher alphabet.

Substitution Cipher Shift Cipher Decimation Cipher Affine Cipher

Mixed Alphabet with the Key Words Definition: Decide on the key words. To construct a substitution cipher using the key words first write the key worlds in order they appear (without repetitions), then write the rest of alphabet in original order (without repetition).

Mixed Alphabet with the Key Words Examples: Let the key worlds be: honors seminar. Than the table is: Encryption of STOP will be QTJK. 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

Mixed Alphabet with the Key Words Problem: Many letters encrypted to themselves Need to be careful when choose the key words

Keyword Columnar Transposition Substitution Definition: Decide on the key words. To construct a substitution cipher using key words first write the key worlds in order they appear (without repetitions) in one line, then put the rest of alphabet in the original order (without repetition) on successive lines beneath. Then write the letters of this array column by column.

Keyword Columnar Transposition Substitution Examples: Let the key worlds be: honors seminar. Than the table is: Now we build an encryption table using the columns of the table above. H O N R S E M I A B C D F G J K L P Q T U V W X Y Z

Keyword Columnar Transposition Substitution We get the table: Encryption of STOP will be MKWE. 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

Probabilities of Occurrence of the English Letters .082 N .067 B .015 O .075 C .028 P .019 D .043 Q .001 E .127 R .060 F .022 S .063 G .020 T .091 H .061 U I .070 V .010 J .002 W .023 K .008 X L .040 Y M .024 Z

Homework Read pg.83-91. Exercises: 1, 2, 5 on pg.92- 93. Those questions will be a part of your collected homework.