Basic Encryption Terminology

• Encryption
• process of encoding information (encode, encipher)
• Decryption
• process of transforming encrypted information back to its original form (decode, decipher)
• Cleartext
• easily human-readable without cryptographic mechanisms or procedures

More Terminology

• Plain text
• Cryptography
• Cryptosystem
• Ciphertext
• Cryptology
• Cryptanalyst

Encryption and Decryption Algorithms

• Encryption is denoted by functions E
• where E(cleartext) = ciphertext
• Decryption is denoted by functions D
• where D(ciphertext) = cleartext
• Usually D is the inverse of E so that
• the following is true D(E(clear)) = clear
• A key is an additional piece of information that must be known in order to encrypt or decrypt data

Use of Keys

• Use of a key is denoted as
• E(clear, K) = cipher
• D(cipher, K) = clear
• Different keys for encryption and decryption are denoted as KE (for encryption) and KD (for decryption)
• Another denotation is
• cipher = [clear]^KE or clear = [cipher]^KD

"Secret" Algorithms

• Algorithm should not be easily breakable
• even when the algorithm and the cipher text are known
• The ideal algorithm is difficult to break even if it is known
• Most algorithms are breakable, but practicality is a big issue
• Ideal algorithms are openly known and impractical to break

Cryptanalysis

• Tools (main tool is computer)
• encrypted messages
• encryption algorithms
• mathematical tools
• known properties of language and the cleartext
• Techniques
• recognizing patterns
• finding weaknesses in encryption algorithms

Two Strategies for Encryption

• Substitution
• each cleartext character is replaced with some other character
• famous example: Caesar Cipher
• Transposition
• sequence of cleartext characters are rearranged
• example: the matrix approach

Advantages and Disadvantages of Simple Strategies

• Strengths
• Easy to encode
• Easy to remember the algorithm
• practical for short periods of time
• Weaknesses
• Easy to decode
• impractical for data that needs protection for long periods

Monoalphabetic Vs. Polyalphabetic Substitution

• Monoalphabetic ciphertext is easily attacked by statistical techniques
• studying the frequency distribution of the characters in the ciphertext
• certain characters appear more often in the realtext
• Using more than one alphabet for substitution flattens the distribution curve

Polyalphabetic Substitution Ciphers

• Two Distributions using two different substitution tables
• Use one table for even positioned characters and the other for characters in odd positions
• Vigenere Tableaux
• A collection of 26 permutations allows for up to 26 systemized substitutions in the same cipher
• Substitutions depend on a key word

Other Good Polyalphabetic Substitution Ciphers

• One-Time Pads
• provides a large non-repeating set of keys
• sender and receiver need identical pads
• Long Random Number Sequences
• computerized one-time pads
• Vernam Cipher and Binary Vernam Cipher
• Books as sources of one-time pads

Cryptanalysis of Polyalphabetic Substitutions

• Kasiski Method
• helps find the number of different substitutions
• brings frequency distribution back into play
• Index of Coincidence
• Measure the variation of frequencies in a distribution
• Function of observed frequencies of characters

Stream Vs. Block Ciphers

• Substitutions are stream ciphers
• Individual characters of plaintext are encrypted immediately into characters of ciphertext
• Transpositions are block ciphers
• A group of plaintext characters are encrypted as a block
• the block can be the entire message

Shannon Characteristics of a Good Cipher

• The amount of protection needed should determine the costs
• Complexity is not a virtue
• Simple implementation is a virtue
• Errors should not cause further errors
• The cipher text should not be longer than the original real text

Summary of Characteristics of Good Ciphers

• Confusion
• Diffusion
• Effectively secure, i.e. p(h(C) = P) < e when C = E(P)