Cryptography Basics and Symmetric Ciphers

Symmetric Cipher Model

• A secret key is shared by the sender and recipient.
• Plaintext is inputted into the encryption algorithm (e.g., DES).
• Ciphertext is transmitted.
• Ciphertext is inputted into the decryption algorithm (reverse of encryption).
• Plaintext is output.

Basic Terminology

• Plaintext: The original message to be encrypted.
• Ciphertext: The encrypted message.
• Enciphering/Encryption: The process of converting plaintext into ciphertext.
• Encryption Algorithm: Performs the encryption process, taking plaintext and a secret key as input.

Deciphering/Decryption

• Decryption Algorithm: Recovers plaintext from ciphertext, using the ciphertext and the secret key as input.

Cryptography & Cryptanalysis

• Cipher/Cryptographic System: A scheme for encryption and decryption.
• Cryptography: The science of studying ciphers.
• Cryptanalysis: The science of studying attacks against cryptographic systems.
• Cryptology: The combination of cryptography and cryptanalysis.

Ciphers

• Symmetric Cipher: Uses the same key for encryption and decryption.
• Block Cipher: Encrypts a block of plaintext at a time (typically 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

• 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 prior to the invention of asymmetric-key ciphers in the 1970s.
• Widely used.

Symmetric Encryption (Mathematical)

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

Cryptanalysis

• Objective: To recover the plaintext of a ciphertext or, more commonly, the secret key.
• Kerkhoff's Principle: The adversary knows all details about a cryptosystem except the secret key.
• Brute-Force Attack: Tries every possible key until a successful decryption of the ciphertext is achieved. (This approach is computationally intensive and time-consuming, particularly with increasingly strong encryption keys)
• Non-Brute-Force Attack (Cryptanalytic Attack): Exploits the nature of the encryption algorithm to guess the secret key.

Steps of a Block Cipher (Example)

• Step 1: Get Text: Obtain the plaintext to be encrypted.
• Step 2: Convert to Binary: Convert the plaintext into its binary representation.
• Step 3: Break into 64 bit blocks: Divide the binary data into fixed-size blocks (typically 64 bits).
• Padding: Data may need to be padded according to the specific cipher to handle plaintext that doesn't neatly divide into the appropriate block size.

Ciphertext-Only Attack

• The attacker only has access to the ciphertext.
• This is generally considered the easiest attack to defend against due to the limited information available.
• An encryption scheme is considered insecure if it cannot withstand a ciphertext-only attack.

Known-Plaintext Attack

• The attacker has access to the plaintext and its corresponding ciphertext.
• The attacker uses this information to analyze the encryption scheme and potentially derive the secret key.
• A common cryptanalytic attack used in block ciphers.

Chosen-Plaintext Attack

• The attacker can choose plaintext messages to be encrypted.
• The attacker then analyzes the resulting ciphertexts to derive information about the secret key or the encryption technique employed.

Chosen-Ciphertext Attack

• The attacker chooses ciphertext messages and receives the corresponding decrypted plaintexts.
• This technique allows the attacker to infer information about the secret key used in the encryption system.

Classical Encryption Techniques

• These encryption techniques provide a foundation and introduction to basic concepts and terminology essential for understanding the processes behind modern cryptography.