Classical Encryption Techniques Quiz

Classical Encryption Techniques

• Classical encryption differs from modern cryptography, focusing on fundamental concepts and terminology.
• Its purpose is to prepare students for studying modern cryptography.

Symmetric Cipher Model

• A symmetric cipher uses a single key for both encryption and decryption.
• The sender and recipient share this secret key.
• Plaintext is input into an encryption algorithm (like DES) to produce ciphertext.
• Ciphertext is input into the decryption algorithm (reverse of encryption) to recover plaintext.

Basic Terminology

• Plaintext: The original message before encryption.
• Ciphertext: The encrypted message.
• Encryption/enciphering: The process of converting plaintext to ciphertext.
• Encryption algorithm: The process used to encrypt plaintext. It takes plaintext and a secret key as input.
• Decryption/decipherment: The process of converting ciphertext back to plaintext.
• Decryption algorithm: The process used to decrypt ciphertext. It takes ciphertext and a secret key as input.
• Secret key: The shared key used for both encryption and decryption. Also referred to as a symmetric key.

Ciphers

• Symmetric cipher: Uses the same secret key for both encryption and decryption.
• Block cipher: Encrypts data in fixed-size blocks (e.g., 64 or 128 bits).
• Stream cipher: Encrypts data one bit or one byte at a time.
• Asymmetric cipher: Uses different keys for encryption and decryption.

Symmetric Encryption

• Uses a single shared key for encryption and decryption.
• All classical ciphers before the invention of asymmetric methods used this.
• Most widely used approach.

Mathematical Representation

• Y = E(X) or Y = E(K, X) : Encryption equation.
• X = D(Y) or X = D(K, Y): Decryption equation.

Cryptanalysis

• Aims to recover the plaintext or the secret key from ciphertext.
• Kerckhoffs' principle: The attacker knows all details of the cryptosystem except the key.
• Brute-force attack: Tries every possible key.
• Cryptanalytic attack: Exploits the characteristics of the algorithm.

Input and 64-bit Blocks

• Steps for working with 64 bit blocks:
  • Get input text.
  • Convert the text into binary format.
  • Break the binary data into 64-bit blocks.

Cryptanalytic Attacks

• Ciphertext-only attack: Only ciphertext is available.
• Known-plaintext attack: Plaintext/ciphertext pairs are known.
• Chosen-plaintext attack: Attacker chooses plaintext to be encrypted.
• Chosen-ciphertext attack: Attacker chooses ciphertext to be decrypted.

Brute-Force Attack

• Requires trying every possible key.
• The required time is proportional to the size of the keyspace.
• The time taken drastically increases as the size of the key increases.

Attacks Continued

• Ciphertext-only attack: The easiest attack to defend against as attackers have limited information.
• Other attacks (known-plaintext, chosen-plaintext, chosen-ciphertext) may require more knowledge about the system (i.e. pairs of plaintext/ciphertext).
• Often, patterns in the plaintext are known.

Known-Plaintext Attack

• The attacker knows plaintext-ciphertext pairs.
• This information can be used to deduce potential encryption keys.

Chosen-Ciphertext Attack

• Attacker chooses ciphertext and receives corresponding decrypted plaintext.
• This allows for the deduction of details about the encryption algorithm and secret key.

Additional Notes

• Using a known-plaintext attack is related to a probable-word attack.
• If there is specific information needed, parts of a message can likely be known (example: a file header from an accounting database).
• Examples of attacks include known ciphertext only, known pairs, chosen plaintexts, and chosen ciphertexts.

Test your knowledge on classical encryption techniques and their basic terminology. This quiz covers concepts of symmetric ciphers, plaintext, ciphertext, and the encryption/decryption process. Prepare yourself for a deeper understanding of modern cryptography.