Symmetric Encryption and Block Ciphers

Vocabulary terms covering symmetric cryptography, block cipher structures like the Feistel cipher, common attack models, and modes of operation such as ECB, CBC, and Counter Mode.

Symmetric Encryption

An encryption scheme where both the sender and the receiver share the same key for encryption and decryption.

Stream Cipher

A type of symmetric encryption that uses a shared secret key to generate an arbitrarily long sequence of random-looking bits, which are then XORed with the message.

Block Cipher

An encryption scheme that takes a fixed-size message (e.g., $64$ bits) and outputs a ciphertext of the same size based on a key.

Feistel Structure

A high-level design used in block ciphers where the message is split into two halves; the right half is processed through a function $f$ with a key and XORed with the left half to create a new state, which is then repeated over multiple rounds.

DES (Data Encryption Standard)

An ancient block cipher from the 1970s that uses a $64$-bit block size, a $56$-bit key, and $16$ rounds of the Feistel structure.

Kirchhoff's Principle

The cryptographic principle assuming that the attacker knows exactly how the cryptosystem works (the algorithm, functions, and key schedule) and that the only secret is the key.

Ciphertext-only Attack

A type of attack where the attacker only has access to ciphertexts and does not know the underlying messages or the secret key.

Brute Force Attack

An exhaustive search for the secret key by trying every possible key until the correct one is found, typically requiring a known message-ciphertext pair.

Malleability

A property of a cryptosystem where an attacker can modify a ciphertext to produce a predictable change in the underlying message without knowing the secret key.

Distinguishability

A high-level security property where an attacker, given a ciphertext, cannot determine if it corresponds to message $m_0$ or message $m_1$ with a probability higher than random guessing.

Electronic Code Book (ECB)

The simplest mode of operation for block ciphers where a document is chopped into blocks of fixed length and each block is encrypted separately using the same key.

Initial Value (IV)

A random, non-secret value chosen by the sender and sent with the ciphertext to introduce freshness and ensure that identical messages encrypt to different ciphertexts.

Cipher Block Chaining (CBC)

A mode of operation where each message block is XORed with the previous ciphertext block before being encrypted, using an IV for the first block.

Counter Mode

A mode of operation that turns a block cipher into a stream cipher by encrypting a combination of an initial value and an incrementing counter to generate a key stream.

Mode of Operation

A technique for using a fixed-sized block cipher to encrypt an arbitrarily large document.