Week 2 - History of Cryptography

symmetric key algorithm

An encryption method where the same key is used for both encryption and decryption, ensuring that both parties have access to the same secret key.

key

a string of characters (determined randomly or mathematically) that’s used with a cryptographic algorithm to secure data

keyspace

a set of all possible keys an algorithm can use. For example AES-128 has 2^128

message space

a set of all possible messages that can be sent using a cryptographic scheme. It is also known as plaintext

cyphertext space

a set of all possible cyphertext from a cryptographic scheme

correctness

Required in symmetric encryption schemes and can be written as Dec(Enc(m, k), k) = m. It means that the algorithm encrypts and decrypts correctly with the same key

kerckhoffs’ principle

The idea that if an attacker knows everything about a cryptosystem, it is still secure as long as the attacker doesn’t know the key. Therefore secrecy of the key is…key (benefits include better transparency, algorithms can be brute-forced, keys are easier)

substitution cypher

this cypher substitutes a unit of plaintext (such as a letter) for a unit of cyphertext, based on a key

Caesar cypher

A historical substitution cypher where each letter in the plaintext is substituted a fixed number of places to get the cyphertext

permutation

all the possible rearrangements of a set (e.g. the alphabet has 26! possible arrangements)

frequency analysis

Using the frequency of letters, digraphs/trigraphs and doubles to break a substitution cypher

monoalphabetic

substitution cyphers where a plaintext letter always maps to the same cyphertext letter such as with the Caesar cypher

polyalphabetic

substitution cyphers where a plaintext letter maps to a different cyphertext letter such as Enigma changing with each key stroke

one-time pad

An unbreakable cypher where each message has it’s own key with the same or bigger length, meaning every possible key has an equal chance to be the correct one

perfect secrecy

when being able to see the cyphertext reveals nothing about the plaintext - as long as the key is only used once and is randomly generated

vernam cipher

A version of the one-time pad that uses a sequence of bits XOR’d with a random bit key to encrypt/decrypt text

10010110

The result of a Vernam Cipher encryption with the message 00101101 and the key 10111011

00101101

The result of a Vernam Cipher encryption with the message 10010110 and the key 10111011

stream cypher

encrypts bit(or byte)-by-bit: is faster but can be less secure if not implemented properly (e.g. if the key is reused)

block cypher

encrypts in fixed sized blocks: is slower and more complex but more secure

128 192 256

___, ___ and ___ bits are the key lengths used in the Advanced Encryption Standard

128

The size of the blocks used in the Advanced Encryption Standard