CYPHER INDEX

2. CYPHER INDEX • Introduction • Background • Demo

3. INTRODUCTION • Cypher is a software toolkit designed to aid in the decryption of standard (historical) ciphers by providing statistical data and algorithmic analysis on encrypted messages. • This toolkit hopes to serve as a cryptographic learning tool and as a past time for amateur cryptographers.

4. BACKGROUND • Substitution ciphers are characterized by a substitution of a character in the original plain text message by the corresponding character in the cipher alphabet, often according to some protocol derived from a predetermined key. • Transposition ciphers retain a characters true form but instead change its position.

5. Substitution Ciphers • Monoalphabetic Substitution Ciphers • Polyalphabetic Substitution Ciphers

6. Monoalphabetic Substitution • In a monoalphabetic substitution cipher, a single character in the plain text alphabet is replaced by a single character in the cipher alphabet. • Frequency Analysis is a key to solve monoalphabetic substitution cipher.

7. Monoalphabetic Substitution • Frequency example • E - 12.3% • T - 9.6% • A - 8.0% • O - 7.94% • N - 7.19% • I - 7.18%

8. Monoalphabetic Substitution • Plain alphabetabcdefghijklmnopqrstuvwxyz • Cipher alphabetxzavoidbygerspcfhjklmnqtuw

9. Polyalphabetic Substitution • Multiple cipher alphabets are used in Polyalphabetic Substitution. • Vigenère cipher is the simplest type of polyalphabetic cipher.

10. Keyword Plaintext Ciphertext W H I T E W H I T E W H I T E W H I T E W H I D I V E R T T R O O P S T O E A S T R I D G E Z P D X V P A Z H S L Z B H I W Z B K M Z N M Vigenère Cipher

11. Transposition Ciphers • Transposition cipher rearranges the positions of the characters but not their true values, so that a frequency analysis of a message encrypted by transposition appears "normal". • Rotating columns and/or rows of the message, rotating the entire message left or right, or along diagonals.

12. Transposition Ciphers • Plaintext: The day is coming to an end, and • Ciphertext:TDIOHASMEYCI NOE,GANATNDN

13. Other Ciphers • Ceaser shift cipher • Plaintext : abcdefghijklmnopqrstuvwxyz • Ciphertext: defghijklmnopqrstuvwxyzabc • Encrypt a message is by using a key-text, such as the Declaration of Independence or a well-known poem, numbering the words in the said document and replacing letters in the plaintext by a number that represents the first character of that number word in the key-text.

14. Demo