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# basic encryption and decryption

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### basic encryption and decryption

1. 1. Basic Encryption and Decryption
2. 2. • Encryption: a process of encoding a message so that its meaning is not obvious • Decryption: the reverse process
3. 3. Encode(encipher) vs. Decode(decipher) • Encoding: the process of translating entire words or phrases to other words or phrases • Enciphering: translating letters or symbols individually • Encryption: the group term that covers both encoding and enciphering
4. 4. Basic operations • plaintext to ciphertext: encryption C = E(P) • ciphertext to plaintext: decryption: P = D(C) requirement: P = D(E(P))
5. 5. Classical Encryption Techniques • Symmetric key encryption • Asymmetric key encryption
6. 6. Symmetric Key Encryption • Sender and recipient share a common key • Was the only type of cryptography, prior to invention of public-key in 1970’s • All traditional schemes are symmetric / single key / private-key encryption algorithms, with a single key, used for both encryption and decryption, since both sender and receiver are equivalent, either can encrypt or decrypt messages using that common key.
7. 7. Symmetric Cipher Model
8. 8. Requirements • Two requirements for secure use of symmetric encryption: – a strong encryption algorithm – a secret key known only to sender / receiver Y = EK(X) X = DK(Y) Here, plaintext X, ciphertext Y, key K, encryption algorithm Ek, decryption algorithm Dk. • Assume encryption algorithm is known • Implies a secure channel to distribute key
9. 9. Types of Ciphers • Substitution ciphers • Permutation (or transposition) ciphers
10. 10. Classical Substitution Ciphers • A substitution cipher replaces one symbol with another. • Substitution ciphers can be categorized as either monoalphabetic ciphers or polyalphabetic ciphers.
11. 11. Monoalphabetic Ciphers • In monoalphabetic substitution, the relationship between a symbol in the plaintext to a symbol in the ciphertext is always one-to-one.
12. 12. • The simplest monoalphabetic cipher is the additive cipher. This cipher is sometimes called a shift cipher and sometimes a Caesar cipher
13. 13. Caesar Cipher • Earliest known substitution cipher • First attested use in military affairs • Replaces each letter by 3rd letter on • example: meet me after the toga party PHHW PH DIWHU WKH WRJD SDUWB
14. 14. • Note: when letters are involved, the following conventions are used in this course: Plaintext is always in lowercase; ciphertext is in uppercase
15. 15. • 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 • then have Caesar cipher as: C = E(p) = (p + k) mod (26) p = D(C) = (C – k) mod (26)
16. 16. When the cipher is additive, the plaintext, ciphertext, and key are integers in Z26
17. 17. • This mathematical description uses modulo arithmetic (ie clock arithmetic). Here, when you reach Z you go back to A and start again. Mod 26 implies that when you reach 26, you use 0 instead (ie the letter after Z, or 25 + 1 goes to A or 0). • Example: howdy (7,14,22,3,24) encrypted using key f (5) is MTBID
18. 18. Example • Use the additive cipher with key = 15 to encrypt the message “hello”.
19. 19. • Use the additive cipher with key = 15 to decrypt the message “WTAAD”.
20. 20. Cryptanalysis of Caesar Cipher • only have 26 possible keys, of which only 25 are of any use, since mapping A to A etc doesn't really obscure the message. • Advantage : easy to use • Disadvantage: simple structure and easy to break
21. 21. Polyalphabetic Ciphers • another approach to improving security is to use multiple cipher alphabets • called polyalphabetic substitution ciphers • makes cryptanalysis harder with more alphabets to guess and flatter frequency distribution • use a key to select which alphabet is used for each letter of the message
22. 22. Vigenere Cipher • Vigenere key stream does not depend on the plaintext characters; it depends only on the position of the character in the plaintext
23. 23. Example • encrypt the message She is listening using the 6-character keyword “PASCAL”. • The initial key stream is (15, 0, 18, 2, 0, 11). The key stream is the repetition of this initial key stream (as many times as needed
24. 24. TRANSPOSITION CIPHERS
25. 25. TRANSPOSITION CIPHERS • A transposition cipher does not substitute one symbol for another, instead it changes the location of the symbols.
26. 26. Keyless Transposition Ciphers • Simple transposition ciphers, which were used in the past, are keyless. • Text is written into a table column by column and then is transmitted row by row. • Text is written into a table and row by row, then is transmitted column by column.
27. 27. Example: Rail fence cipher. • The ciphertext is created reading the pattern row by row. • For example, to send the message “Meet me at the park” to Bob, Alice writes She then creates the ciphertext “MEMATEAKETETHPR
28. 28. Example • Alice and Bob can agree on the number of columns and use the second method. Alice writes the same plaintext, row by row, in a table of four columns. She then creates the ciphertext “MMTAEEHREAEKTTP”.
29. 29. Keyed Transposition Ciphers • Divide the plaintext into groups of predetermined size, called blocks, and then use a key to permute the characters in each block separately.
30. 30. Steganography • an alternative to encryption • hides existence of message – using only a subset of letters/words in a longer message marked in some way – using invisible ink – hiding in LSB in graphic image or sound file • has drawbacks – high overhead to hide relatively few info bits
31. 31. CONVENTIONAL ENCRYPTION ALGORITHMS
32. 32. CONVENTIONAL ENCRYPTION ALGORITHMS • Triple DES • International Data Encryption Algorithm (IDEA) • Blowfish • RC5 • CAST-128 • RC2 • Characteristics of Advanced Symmetric Block Ciphers