Cryptography and Security

Cybersecurity

The management of attacks and risks by adversarial and malicious elements on computers and networks that support modern businesses and economy involving business, government and community.

Information security

Focuses on the secure exchange and processing of information on modern computers and networks.

Cryptography

A method of ensuring information security that refers to techniques used to ensure data is protected from potentially malicious elements.

Diffie-Hellman Key Exchange

A method of secure key exchange that uses exponentiation under a modulus to share a key.

Discrete Logarithm Problem

Given (g, h, p). Find x such that g^x=h (mod p)

Computational Diffie-Hellman Problem

Given (g^a, g^b, g) for a random generator g and random a, b ∈ {0, …, q-1}. Find g^ab

Passive

Type of attack in which a unauthorized user monitors, intercepts, or observes network traffic and communications

Active

Type of attack where an attacker directly interacts with, alters, disrupts, or destroys a system, network, or data

Kerckhoff’s Principle

Security through obscurity is not recommended. It is unwise to make the security of information reliant on how the encryption algorithm works. Instead, we rely on the secrecy of the shared key

Unconditional security

The security of the cipher is independent of the computing resource available to the adversaries

Computational security

Adversaries have limited computational resources and the security is reliant on the size of the computations needed to break the cipher.

Substitution

Principle of encryption algorithm that maps plaintext characters to different values

Transposition

Principle of encryption algorithm that maps plaintext characters to different positions in the ciphertext

Caeser cipher

Classical cipher in which each character is shifted k places

Affine Cipher

Classical cipher in which each character mapped using ap + b mod n

Monoalphabetic cipher

Classical cipher in which each character mapped to a different character

Row transposition cipher

Classical cipher in which the plaintext is written in rows and the ciphertext is built from the columns arranged in a particular order.

Polyalphabetic Cipher

Complex classical cipher that involves a set of monoalphabetic ciphers being applied

Vigenere Cipher

Complex classical cipher that involves rotating set of caeser ciphers being applied, changing each letter of the plaintext

Hill cipher

Complex classical cipher that involves converting the plaintext into a vector and performing matrix multiplication with the key (matrix)

Product cipher

Complex classical cipher that involves combining several substitution and transposition ciphers

Block cipher

Cipher that breaks message down into fixed length blocks before encrypting

Stream cipher

Cipher that generates a key stream and applies it to an incoming plaintext stream.

Vernam Cipher

A Vigenère Cipher where the length of the key is equal to the length of the message

One time pad

An extension of the Vernam cipher that uses a truely random key to provide perfect secrecy

Diffusion

Principle of encryption that dissipates statistical structure of plaintext over ciphertext

Confusion

Principle of encryption that makes relationships between ciphertext and key as complex as possible

Iterated block cipher

A block cipher that involved several repetitions of an iterated round function

Fiestel Cipher

Iterated cipher that involves breaking the plaintext into two parts R_i and L_i, moving R_i to the left and XORing L_{i with the encrypted version of} R_i

Data Encryption Standard

A type of Fiestel cipher that has a block size of 64 and key size of 56 bits

DESX

Extension of DES that involves XORing additional keys before and after applying the DES algorithm.

Electronic Codebook

Cipher Block Chaining

Cipher Feedback Mode

Output Feedback Mode

Counter Mode

Advanced Encryption Standard

Symmetric Encryption algorithm commonly used today that is more secure than DES

SubBytes

1st AES operation: each byte is replaced with a different value from a lookup table

ShiftRows

2nd AES operation: each row in the grid moves by a different amount

MixColumns

3rd AES operation: finite field arithmetic used to transform bytes in each column

AddRoundKey

4th AES operation: result is XORed with round key

Symmetric

Type of cryptography that uses a single, shared key for each pair of communicators

Asymmetric

Type of cryptography that uses a public/private key pair per communicator

RSA

Asymmetric encryption/signature algorithm that is based on the assumption that factoring an integer that is the product of two primes is hard.

Factorisation Problem

Given n (where n is the product of two primes p, q), find p and q 

RSA problem

Given (n, e, C = M^e) determine the e-th root of C mod n

Chosen Plaintext

Type of attack on encryption in which the adversary can produce a ciphertext for any given plaintext

Chosen Ciphertext

Type of attack on encryption in which the adversary can produce a plaintext for any given ciphertext (ciphertexts must all be chosen in advance)

Adaptive Chosen Ciphertext

Type of attack on encryption in which the adversary can produce a plaintext for any given ciphertext (ciphertexts can be chosen in response to previous plaintexts)

Optimal Asymmetric Encryption Padding

Way of formatting RSA messages to prevent against chosen plaintext attacks

Timing Attack

Type of attack in which the attacker observes the behaviour of the cryptographic algorithms and use the experience to break the secret directly.

Constant time

Counter measure against timing attack that ensures all operations take the same amount of time

Random delay

Counter measure against timing attack that adds in random amounts of extra time to computations

Blinding

Counter measure against timing attack that involves multiplying the input and output of RSA by a random number to change timing of algorithm,

Digital Signature

An electronic tag used to ensure the integrity and authentication of a message

Existential forgery

An attack where an adversary successfully creates a valid digital signature for at least one new message

Redundancy function

Tool applied to input of RSA function to prevent blinding attacks on digital signatures

Mathematical Attack

Type of attack that uses an exception/edge-case to a hard problem to help break an encryption/signature algorithm

Broadcast problem

Flaw in RSA when the same e value is used for the same message (can be broken using CRT)

Elementary attack

Attack that is possible when two users use the same values for e/d in RSA and so can use their own information to break each other’s encryption.