Introduction to Cryptography

Cryptography

• Cryptography is the science of encrypting and decrypting written communication
• It is derived from the Greek words "kryptos" (hidden) and "graphia" (writing)
• It protects sensitive information during storage or transmission across networks
• Modern cryptography is often associated with electronic communications like passwords, logins, and ATM security

Goals of Cryptography

• Privacy: Protecting sensitive information from unauthorized access
• Data Integrity: Ensuring that data has not been altered during transmission
• Authentication: Verifying the identity of the sender or receiver

History of Cryptography

• Cryptography's history spans thousands of years
• The Enigma machines, electro-mechanical rotor cipher machines, were used in the 20th century for diplomatic and military communication
• Early forms of cryptography include the Scytale, a method used by the Spartans, involving wrapping a message around a wooden rod

Encryption and Decryption

• Encryption: Transforming data using an algorithm to make it unreadable without a key
• Decryption: Converting encrypted data back to its original form using the corresponding key

Ciphers

• Cipher: An algorithm for performing encryption or decryption, often depending on a key
• Substitution Cipher: Replaces units of plaintext with ciphertext based on a systematic rule
• Monoalphabetic Cipher: A substitution cipher where each letter is replaced by a different letter
  • Caesar Cipher: A simple substitution cipher shifting each letter a fixed number of positions in the alphabet
  • Atbash Cipher: A substitution cipher replacing each letter with its opposite letter in the alphabet
  • Affine Cipher: A substitution cipher which depends on two parameters for the letter substitutions
  • ROT13 Cipher: A specific Affine Cipher

Transposition Ciphers

• Replaces the order of characters in a message
• Rail Fence Cipher: Arranges characters in a series of rows and reads them down the columns
• Route Cipher: Uses a pattern to determine which character to read in the transposition
• Columnar Transposition: Arranges the plaintext into columns and reads in a specific sequence, sometimes using a keyword to create the column order
• Keyword Columnar Cipher: A more complex method where the keyword determines the order of columns

Modular Codes

• Based on modular arithmetic
• Encodes and decodes a message using a defined key and modular math to create a code
• The decoding key is the inverse of the encoding key.