Exam 5

Character Substitution

A cipher technique where each character in the plaintext is replaced with another character according to a rule or key.

ROT13

A simple substitution cipher that shifts each letter by 13 places in the alphabet.

Substitution Cipher

Replaces each letter of the plaintext with another letter based on a permutation of the alphabet.

Two-Letter Substitution – Playfair Cipher

A digraph substitution cipher that encrypts pairs of letters using a 5x5 matrix of the alphabet.

Kerckhoffs’s Principle

A foundational principle in cryptographic system design emphasizing security should rely solely on key secrecy.

Stream Cipher

Encrypts plaintext one bit or byte at a time, typically by XORing it with a keystream.

Block Cipher

Encrypts fixed-size blocks of plaintext (e.g., 64 or 128 bits) by mixing it with a key.

Ciphertext-Only Attack (COA)

The attacker has access only to ciphertexts, not the corresponding plaintext.

Known-Plaintext Attack (KPA)

The attacker has both plaintext and matching ciphertext samples.

Chosen-Plaintext Attack (CPA)

The attacker can choose arbitrary plaintexts and obtain their ciphertexts.

Chosen-Ciphertext Attack (CCA)

The attacker can choose ciphertexts and obtain their decrypted plaintexts.

Block Ciphers

A method of encrypting data in fixed-size blocks (e.g., 64, 128 bits), rather than bit-by-bit or character-by-character.

DES (Data Encryption Standard)

A symmetric block cipher developed in the 1970s using 64-bit blocks and a 56-bit key.

3DES (Triple DES)

An enhancement of DES applying it three times to increase security.

Brute Force

Trying every possible key until the correct one is found.

Birthday Attack

Targets hash functions by exploiting probability of hash collisions (two inputs with the same output).

Meet-in-the-Middle Attack

Used against encryption schemes like double DES by precomputing encryptions from one end and decryptions from the other.

Salting

A unique, random value added to passwords before hashing to prevent use of rainbow tables.

Digital Signature

A cryptographic technique to verify the authenticity and integrity of a message.

Integrity

Assurance that a message has not been changed in transit or storage.

Public-Key Encryption

Asymmetric method using a public/private key pair.

RSA (Rivest-Shamir-Adleman)

A public-key encryption algorithm based on factoring large integers.

SHA-2 / SHA-3

Secure hash functions used for file fingerprinting and integrity checking.

Diffie-Hellman (DH)

A key exchange method to securely derive a shared secret over an insecure channel.

OTR (Off-the-Record Messaging)

A cryptographic protocol for instant messaging that provides encryption, authentication, forward secrecy, and deniability.

Forward Secrecy (Perfect Forward Secrecy)

A security feature that ensures past encrypted communications remain secure even if the long-term keys are later compromised.