Flashcards in Lecture 1: 18th September 2019 Deck (34)
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)
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.
How can you break the Caesar cipher?
Find the frequencies of each letter, and try to find a grammatically valid mapping in which the frequencies of both letters are equal in both the ciphertext and language as a whole.
What is a substitution cipher?
An algorithm for encryption and/or decryption in which letters are swapped to generate a mapping which can be used to switch (substitute) ciphertext and plaintext letters.
What is a one time pad?
An encryption algorithm that uses one-time use keys, traditionally on paper "pads", to encrypt plaintext information. Each key can only be used once and must be equal to or greater than the information in length.
How do one time pads work?
Have plaintext input and current key. Sum the ith characters in the plaintext and key to produce the ith character in the ciphertext. Do this for all the characters in the plaintext.
Why are one time pads hard to break?
Pseudo-randomness of key (and use of one at all) breaks normal frequencies of letters in ciphertext and removes any direct mapping between plaintext and ciphertext letters. Different key each time makes it even more difficult to decrypt.
What is the Vernam Cipher?
OTP with numbers; key a sequence of pseudo-random numbers. Input text characters (all letters) converted to numeric equivalent (a - 1, b - 2, c - 3, etc) and then added to corresponding (ith) key number. The resulting ciphertext number is this sum modulo 26.
What is a Vigenere Tableau?
A matrix of the letters a-z as the rows and columns that stand in for the value of key and plaintext letters. To find a ciphertext, find the row corresponding to the current plaintext letter, and then the column corresponding to the key letter within that row.
What is a Vigenere Cipher?
An algorithm for encryption and/or decryption which uses Vigenere Tableaus. These are a matrix of the letters a-z as the rows and columns that stand in for the value of key and plaintext letters. To find a ciphertext, find the row corresponding to the current plaintext letter, and then the column corresponding to the key letter within that row.
This cipher cannot be broken by frequency analysis as there is no direct mapping from plaintext to ciphertext characters but can be broken with transposition, but can be by deducing the key length from repeated mappings of words, by chance, if the key is shorter than the ciphertext, and the ciphertext is long enough. Once the key length is known it is straightforward to obtain the plaintext, and afterwards the key.
What is a Book Cipher?
An algorithm for encryption and/or decryption which uses a book or a certain section thereof as the encryption key. They will be decided prior to communications and accessible by both parties. OTPs often uses books for keys in this manner.
What is a message transposition?
Methods of shifting the position of characters in plaintext by amounts determined by some regular system or algorithm. This produces some permutation (different ordering) of the message.
What is a message permutation?
Methods of rearranging the characters in plaintext by some regular system or algorithm.
What is pattern analysis?
The study of patterns in one or more letters in repeated sequences of ciphertext in the process of cryptanalysis.
How can pattern analysis extend past lone letters?
Natural languages has sequences of commonly repeated groups of multiple letters (n-grams) that can also be analysed in a manner similar to the frequency analysis on sole letters. For example, "rh" and "ent" are common digrams and trigrams in English.
What is a column transposition?
When you scramble text by putting it into a grid of n columns and reading output as letters top to bottom and then left to right - i.e. vertically then horizontally rather than horizontally then vertically as in the original.