Introduction to Cryptography – Key Vocabulary

These vocabulary flashcards cover foundational terms, concepts, algorithms, attack models, and key management issues discussed in the lecture on computer security and cryptography (CSC662). They provide concise definitions to aid revision and exam preparation.

Cryptography

The study and practice of securing information through techniques such as encryption, authentication, digital signatures, one-way functions, hash algorithms, and key management.

Encryption

The process of transforming readable data (plaintext) into an unreadable form (ciphertext) using an algorithm and a key.

Plaintext

The original, readable message or data that is fed into an encryption algorithm.

Ciphertext

The unintelligible output of an encryption algorithm; the encrypted version of the plaintext.

Decryption

The process of converting ciphertext back into its original plaintext using a decryption algorithm and key.

Encryption Key (e)

A secret value used with an encryption algorithm to convert plaintext into ciphertext.

Decryption Key (d)

A secret value used with a decryption algorithm to convert ciphertext back to plaintext; may be identical to or derivable from the encryption key in symmetric systems.

Encryption Algorithm (f)

A mathematical procedure that combines plaintext with an encryption key to produce ciphertext.

Decryption Algorithm (g)

A mathematical procedure that combines ciphertext with a decryption key to recover the original plaintext.

Cryptosystem

The complete specification of an encryption scheme, including message and ciphertext alphabets, keyspace, algorithms, blocking method, and key selection rules.

Keyspace (K)

The set of all possible keys for a given cryptosystem.

Caesar Cipher

A simple substitution cipher that shifts each letter in the plaintext by a fixed number (k) positions in the alphabet.

Substitution Cipher

An encryption method where each symbol in the plaintext alphabet is replaced by a unique symbol in the ciphertext alphabet according to a fixed substitution key.

Permutation Cipher

A cipher in which symbols are permuted; modern block ciphers that treat blocks as permutations belong to this class.

Cryptanalyst (Charles)

An adversary who intercepts encrypted messages and attempts to uncover the plaintext or the keys.

Cryptographer

A person who studies, designs, or analyzes cryptographic systems.

Ciphertext-only Attack

An attack model where the cryptanalyst has only the ciphertexts to work with.

Known-plaintext Attack

An attack in which the cryptanalyst possesses one or more pairs of plaintext and corresponding ciphertext.

Probable-plaintext Attack

An attack where the analyst guesses likely portions of the plaintext and checks them against ciphertext.

Chosen-plaintext Attack

An attack where the cryptanalyst can obtain ciphertexts for plaintexts of their choosing.

Chosen-ciphertext Attack

An attack in which the cryptanalyst can obtain plaintexts corresponding to ciphertexts of their choice.

Exhaustive Key Search (Brute Force)

Trying every possible key in the keyspace until the correct one is found.

Statistical Analysis Attack

Using frequency counts and other statistics of ciphertext symbols to deduce information about the key or plaintext.

Differential Cryptanalysis

A sophisticated attack that examines how small differences in plaintext affect the differences in ciphertext to recover keys.

Diffusion

A property of block ciphers where a small change in plaintext causes widespread, unpredictable changes in ciphertext.

Confusion

A property ensuring that the relationship between key and ciphertext is highly complex, so near-correct keys reveal no information.

Completeness

A block cipher property where every bit of ciphertext depends on every bit of the key, preventing divide-and-conquer attacks.

Symmetric Cryptosystem

A cryptosystem in which the encryption and decryption keys are identical or easily derived from each other.

Session Key

A key with a limited lifetime, often used for a single session or message exchange.

Chaining Keys

A method where a new session key is sent encrypted under the previous session key; vulnerable because compromise of one key reveals subsequent keys.

Random Number Generator (RNG)

A function or device that produces numbers that appear random; critical for generating cryptographic keys.

Pseudo-random Number Generator (PRNG)

An algorithmic RNG whose output is deterministic given its internal state; security relies on keeping the function or seed secret.

One-Time Pad (OTP)

A stream cipher using a random key as long as the message, used only once; offers perfect secrecy.

Perfect Secrecy

A property where ciphertext reveals no information about plaintext; achievable with a properly used one-time pad.

Stream Cipher

An encryption method that combines plaintext with a keystream bit-by-bit, often generated from a short secret key.

Block Cipher

An encryption algorithm that operates on fixed-size blocks of plaintext, producing equal-size blocks of ciphertext.

XOR (Exclusive OR)

A binary operation (⊕) used extensively in cryptography; combining a bit with a key bit: 0⊕k = k, 1⊕k = ¬k.

ASCII

American Standard Code for Information Interchange; maps characters to numerical codes, often represented in binary.

DES (Data Encryption Standard)

A 56-bit key symmetric block cipher with 64-bit blocks, once a U.S. government standard; now considered insecure due to short key length.

Triple DES (3DES)

An enhancement of DES that encrypts, decrypts, and re-encrypts data with three different 56-bit keys, giving an effective key length of 168 bits.

AES (Advanced Encryption Standard)

The current U.S. standard block cipher, based on the Rijndael algorithm, supporting 128-, 192-, or 256-bit keys.

Rijndael

The block cipher algorithm selected as AES; treats data as polynomials and uses 10–14 rounds of substitutions and permutations.

Electronic Codebook (ECB)

A block-cipher mode where each block is encrypted independently; identical plaintext blocks produce identical ciphertext.

Cipher Block Chaining (CBC)

A mode that XORs each plaintext block with the previous ciphertext block before encryption, adding dependency between blocks.

Output Feedback (OFB)

A mode that turns a block cipher into a stream cipher by encrypting iterative feedback blocks to generate a keystream.

Key Generation

The process of creating cryptographic keys, either by the communicating parties or a trusted third party (TTP).

Key Distribution

The secure delivery of cryptographic keys to participants who need them.

Key Storage

The method and location used to keep keys safe after generation.

Key Replacement

The policy and practice of changing keys periodically; frequent changes limit the damage if a key is compromised.