CNT4403 Cryptography

Cryptosystem

A 5-tuple (E, D, M, K, C) consisting of an encryption algorithm (E), a decryption algorithm (D), a set of plaintext messages (M), a set of keys (K), and a set of ciphertexts (C).

Key

An input to a cryptographic algorithm used to obtain a property like confidentiality, integrity, or authenticity over some data.

Keyspace

The set of all possible keys for a given cryptosystem.

Entropy

A measure of the variance or randomness in a set of keys, typically measured in bits. Higher entropy corresponds to better security.

Symmetric Key Cryptography

A form of cryptography where a single, shared key is used for both encryption (E) and decryption (D), such that D(k, E(k, p)) = p.

Data Encryption Standard (DES)

A symmetric-key block cipher introduced by the US NBS in 1972. It uses a 56-bit key and operates on 8-byte blocks, which is vulnerable to brute-force attacks.

Advanced Encryption Standard (AES)

The successor to DES, selected by NIST through an international competition. It is currently implemented in many modern devices and software.

Cipher Block Chaining (CBC)

A block cipher mode where each plaintext block is XOR'd with the previous ciphertext block before being encrypted. It requires an initialization vector (IV) for the first block.

Counter Mode (CTR)

A block cipher mode that encrypts a counter value and then XORs the result with the plaintext block to generate ciphertext.

Electronic Code Book (ECB)

The most naïve and insecure block cipher mode, where the cipher is applied individually to each block of plaintext.

Hash Algorithm

An algorithm that compresses data into a fixed-size hash value. In cryptography, these must be one-way and collision-resistant, making it difficult to reverse back to the original text. It is primarily used to store passwords on servers; MD5, SHA-512, Bcrypt.

Message Authentication Code (MAC)

Is a cryptographic tool used within protocols to authenticate content and guarantee the integrity of a piece of data (d). It utilizes a secret key (k) and a hash function (h()) to create a tag that verifies the data has not been altered. Simple MACs fail because hash functions let attackers extend a message and compute a valid MAC without knowing the secret key.

Hash-Based Message Authentication Code (HMAC)

Is a specific construction of a MAC designed to address the vulnerabilities of simple MACs while ensuring collision resistance. It utilizes a nested hashing approach to prevent attackers from extending the content. The formula is defined as HMAC(K,d)=H(K+H(K+d)). By hashing the key and the result of the inner hash, the attacker cannot extend the MAC as they could with a simple concatenation.

Cryptography

The art of writing secretly, used for hiding plaintext (encryption) and guaranteeing other properties like authenticity.