Cryptography & Network Security Basics

Cryptography & Network Security

• Cryptography is crucial for communications and computer security
• The use of computers has increased the risk of information theft
• Encryption is a primary method to protect electronic information
• Common encryption types include:
  • Conventional (symmetric) encryption
  • Public-key (asymmetric) encryption

Basic Terminologies

• Plaintext is the original message
• Ciphertext is the coded message
• Cipher is the algorithm used to transform plaintext to ciphertext
• Key is the information used in the cipher, known only to the sender/receiver
• Encipher (encrypt) means to convert plaintext to ciphertext
• Decipher (decrypt) means to recover plaintext from ciphertext
• Cryptography studies encryption principles and methods
• Cryptanalysis (code breaking) focuses on deciphering ciphertext without knowing the key
• Cryptology encompasses both cryptography and cryptanalysis
• Steganography hides messages within other media like images or graphics

Cryptographic Systems

• Cryptographic systems are defined by three aspects:
  • Operations used to transform plaintext
  • Number of keys used
  • Plaintext processing method

Types of Operations

• Encryption algorithms are based on substitution and transposition principles
• Substitution maps plaintext elements to other elements (like letters or bits)
• Transposition rearranges elements in the plaintext
• Product systems combine multiple substitution and transposition stages

Number of Keys

• Symmetric encryption uses the same key for both sender and receiver
• Asymmetric encryption uses separate keys for sender and receiver

Plaintext Processing Method

• Block ciphers process input in blocks, producing an output block for each input block
• Stream ciphers process input elements continuously, producing output one element at a time

Ciphers

• Symmetric ciphers use the same key for both encryption and decryption
• Asymmetric ciphers use different keys for encryption and decryption

Symmetric Encryption

• Also known as conventional, secret-key, or single-key encryption
• Sender and recipient share a common key
• All classical encryption algorithms are symmetric
• The only type of cipher before asymmetric-key ciphers (invented in the 1970s)
• Mathematically represented as:
  • Y = EK(X) or Y = E(K, X) (encryption)
  • X = DK(Y) or X = D(K, Y) (decryption)
  • X = plaintext
  • Y = ciphertext
  • K = secret key
  • E = encryption algorithm
  • D = decryption algorithm
  • Both E and D are publicly known

Symmetric Cipher Model

• Depicts the process of encryption and decryption using a shared secret key

Symmetric Encryption Scheme

• Consists of five components:
  • Plaintext: Original intelligible message or data
  • Encryption algorithm: Performs transformations on the plaintext
  • Secret key: Independent value input to the encryption algorithm
  • Ciphertext: Scrambled message produced as output
  • Decryption algorithm: The encryption algorithm run in reverse, using the secret key to recover plaintext

Secure Use of Conventional Encryption

• Requires two things:
  • Strong encryption algorithm
  • Secret key known only to sender and receiver
• Assumes the encryption algorithm is publicly known
• Implies the need for a secure channel to distribute the key

Unconditional and Computational Security

• An unconditionally secure scheme's ciphertext doesn't provide enough information to determine the plaintext
• Breaking the cipher is more expensive than the information's value
• The time to break the cipher exceeds the information's useful lifetime
• A computationally secure scheme meets one of the above criteria
• Estimating effort needed for cryptanalysis is difficult

Attacking Conventional Encryption

• Often aims to recover the key instead of just decrypting a single message
• Two main approaches:
  • Cryptanalysis: Exploits algorithm characteristics to deduce plaintext or the key
  • Brute-force attack: Tries every possible key until an intelligible plaintext is found

Classical Ciphers

• View plaintext as a sequence of elements (bits, characters)
• Two main types:
  • Substitution cipher: Replaces elements with other elements
  • Transposition (permutation) cipher: Rearranges elements in the plaintext

Substitution Ciphers

• Replace plaintext letters with other letters, numbers, or symbols
• Applies to bits by replacing plaintext bit patterns with ciphertext bit patterns

Caesar Cipher

• The earliest known substitution cipher, invented by Julius Caesar
• Each letter is replaced by the letter three positions further down the alphabet (wrapping around from Z to A)
• Can be described by a transformation table mapping plaintext letters to ciphertext letters
• Defined by the algorithm:
  • Plaintext: 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
  • Ciphertext: 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

Explore the fundamental concepts of cryptography and network security in this quiz. Learn about key terms such as plaintext, ciphertext, and various encryption methods, including symmetric and asymmetric encryption. Test your knowledge on the principles and applications that secure electronic communications.