Symmetric Cipher Model Quiz

Symmetric Cipher Model

• Symmetric ciphers use the same key for encryption and decryption.
• Plaintext is the original message.
• Ciphertext is the encrypted message.
• Encryption is the process of converting plaintext to ciphertext.
• Encryption algorithms take plaintext and a secret key as input.
• Decryption is the process of converting ciphertext back to plaintext.
• Decryption algorithms take ciphertext and the same secret key as input.

Basic Terminology

• Plaintext: the original message
• Ciphertext: the encrypted message
• Encryption/enciphering: converting plaintext to ciphertext
• Encryption algorithm: performs encryption; takes plaintext and a secret key as input
• Decryption/deciphering: converting ciphertext to plaintext
• Decryption algorithm: performs decryption; takes ciphertext and a secret key as input
• Secret key: the same key used for encryption and decryption

Ciphers

• Symmetric cipher: uses the same key for encryption and decryption
• Block cipher: encrypts data in blocks (typically 64 or 128 bits)
• Stream cipher: encrypts data one bit or byte at a time
• Asymmetric cipher: uses different keys for encryption and decryption

Symmetric Encryption

• Also called conventional, secret-key, or single-key encryption
• The same key is used for encryption and decryption
• All classical encryption algorithms are symmetric
• The only type of cipher before asymmetric-key ciphers (1970s)
• Most widely used

Mathematically

• Y = E(X) or Y = E(K, X) (encrypt plaintext X with key K to get Y)
• X = D(Y) or X = D(K, Y) (decrypt ciphertext Y with key K to get X)
•  X = plaintext
• Y = ciphertext
• K = secret key
• E = encryption algorithm
• D = decryption algorithm
• E and D are known to the public

Cryptanalysis

• Objective: recover plaintext from ciphertext or the secret key
• Kerkhoff's principle: adversary knows all details of a cryptosystem except the secret key
• Brute-force attack: tries every possible key
• Cryptanalytic attack: exploits characteristics of the algorithm

Input, Conversion, Breaking into Blocks

• Input: "Hello World!"
• Convert to binary: binary representation of each character
• Break into 64-bit blocks: split the binary data into 64-bit chunks. Padding may be required to fill out the last block.

Cryptanalysis and Brute-Force Attack

• Objective: recovering the key used to encrypt ciphertext
• Two approaches
  • Cryptanalysis: analyzing the algorithm and possible plaintext characteristics
  • Brute-Force: trying every possible key
• If either attack succeeds, all future and past messages are compromised.

Cryptanalytic Attacks

• Categorized by the amount of information the attacker has:
  • Ciphertext-only attack: only ciphertext available
  • Known-plaintext attack: plaintext and its corresponding ciphertext available
  • Chosen-plaintext attack: attacker chooses plaintext and obtains corresponding ciphertext
  • Chosen-ciphertext attack: attacker chooses ciphertext and obtains corresponding plaintext

Brute-Force Attack Time

• Time needed for brute-force depends on the size of the key space.
• Larger key spaces require exponentially more time to try all possible keys. (e.g., 56-bit key is much quicker to break than a 128-bit key)

Ciphertext-only Attack

• Attacker only has ciphertext
• Attempts to decrypt ciphertext to plaintext, or determine the key
• Can sometimes work; an encryption system is insecure if it is vulnerable to this. (Often impractical)

Known-Plaintext Attack

• Attacker has some known plaintext and its corresponding ciphertext.
• Used to deduce the key or recover plaintext
• Helps deduce some characteristics of the encryption scheme

Chosen-Ciphertext Attack

• Attacker can choose ciphertexts and obtain the corresponding plaintexts.
• Used to discover vulnerabilities in encryption schemes