# Lecture 1: 18th September 2019 Flashcards

## Cryptography, lecture 0

What is cryptography?

Cryptography refers to secure information and communication techniques derived from mathematical concepts and a set of rule-based calculations called algorithms to transform messages in ways that are hard to decipher.

What is plaintext?

Unencrypted information prior to encryption

What is ciphertext?

The result of encrypting a plaintext input

What is encryption?

Methods of encoding messages or information in such a way that only authorized parties can access it and those who are not authorized cannot. Encryption turns plaintext into ciphertext.

What is decryption?

The methods by which encrypted ciphertext is converted back to its original source plaintext.

What is a cryptosystem?

A group of cryptographic algorithms needed to implement a particular security service, usually encryption: key generation, encryption, decryption. They usually share a common key.

What is an encyption algorithm?

A mathematical procedure in order to apply a predefined code to information to make it illegible to parties unauthorised to read it.

What is an encryption key?

A pseudo-random set of data used with encryption algorithms to encrypt data. They are designed to be unique and unpredictable. Only parties with the necessary key can read encrypted data, so their distribution can be used to dictate who is authorised to read certain information.

How do you notate an encryption operation? Include ciphertext, the encryption algorithm, key, and plaintext.

C = E(K,P)

where C = ciphertext E = encryption algorithm/s K = encryption key P = plaintext

What is a ciphertext only attack? What are its aims?

Attempts to circumvent and surpass the security of a cryptographic system in which the malevolent party only has access to one or more ciphertexts.

Successful attempts manage to deduce the corresponding plaintexts and/or the encryption key/s being used.

What is a known-plaintext attack? What are its aims?

Attempts to circumvent and surpass the security of a cryptographic system in which the malevolent party has one or more pairs of corresponding plaintexts and ciphertexts.

Attackers aim to deduce the cryptographic key/s being used.

What is a chosen plaintext attack?

Attempts to circumvent and surpass the security of a cryptographic system in which the malevolent party can obtain an arbitrary number of plaintexts corresponding to chosen plaintext inputs.

Attackers aim to gain any information that allows them to deduce more about the encryption scheme.

What is cryptanalysis?

The analysis of cryptographic systems in an attempt to breach their confidentiality and authorisation mechanisms. They aim to gain an algorithm to decrypt all encrypted messages and information.

Why are people often the weak points of encryption systems?

Encryption algorithms are often too strong for cryptanalysts to break in reasonable amounts of time with brute force methods. The way people use them often allows them to do so more easily. Pattern recognition and frequency analysis can be performed on frequently recurring words, patterns, or letters, to allow information regarding the encryption mechanisms to be deduced.

What is frequency analysis?

The study of letters or groups of letters contained in a ciphertext in an attempt to reveal the message. Letters and groups of letters are not evenly distributed in natural human languages. Mappings can be built from ciphertext frequencies to the language in use to reveal its contents.