Introduction to Cryptography and Monoalphabetic Ciphers

Flashcards about Introduction to Cryptography and Monoalphabetic Ciphers

Cryptography use in prehistory

Military purposes, Private business, Religious affairs, Secret trades, Illegal activities, To protect privacy of individuals

Kerckhoff's Principle

A cryptosystem should be secure even if everything about the system, except the key, is public knowledge. Security should rely on the secrecy of the key only and not on the security of the algorithm.

Confidentiality (secrecy)

Insurance that a given information cannot be accessed by unauthorized parties.

Privacy

Ability for a person to control how his personal information spreads in a community

Cryptosystem

A five-tuple (P, C, K, E, D), where P is a finite set of possible plaintexts, C is a finite set of possible ciphertexts, K is a finite set of possible keys, and for each K in K, there is an encryption algorithm EK in E and a corresponding decryption algorithm DK in D such that DK(EK(X)) = X for every plaintext X in P.

Monoalphabetic Ciphers

Ciphers for which, once a key is chosen, each alphabetic character is mapped to a unique alphabetic character. Examples include Shift cipher, Substitution Cipher, and Affine cipher.

Shift Cipher

A monoalphabetic cipher where each letter of the plaintext is shifted a fixed number of positions down the alphabet.

Caesar Cipher

A specific Shift Cipher with a key of 3, used in the Roman Empire.

Cryptanalysis of Shift Cipher

Shift cipher (modulo 26) is not secure, since it can be cryptanalyzed by brute force attack. Only 25 keys to try, until a meaningful plaintext string is obtained.

Substitution Cipher

A method of encryption by which units of plaintext are replaced with ciphertext according to a fixed key; a permutation of 26 alphabetic characters.

Cryptanalysis of The Substitution Cipher

Exhaustive key search is infeasible, there are 26! = ~4 x 10^26 possible keys. Is performed using Frequency Analysis.

Frequency Analysis

A method to crack simple substitution ciphers based on the statistical distribution of letter frequencies in a language.

Common Digraphs

EN, RE, ER, NT, TH

Common Trigraphs

THE, ING, THA, ENT

Affine Cipher

A type of monoalphabetic substitution cipher where each letter in an alphabet is mapped to its numeric equivalent, encrypted using a mathematical function, and converted back to a letter.

Cipher

Mostly used for ‘secret key cryptographic systems’, Encryption or Decryption algorithms

Cryptosystem

Mostly used for ‘public key cryptographic systems’

Cleartext

Information encoded by using a public code

Plaintext

Input of an encryption algorithm

Ciphertext (Cryptogram)

Information encoded by a cryptographic system

Encryption (Encipherment), Decryption (Decipherment)

Action to transform a plaintext into a ciphertext or the opposite