2 - Cypto Key Concepts Flashcards
What is a cipher?
A method to encrypt text.
Usually translates text into symbols, or a function of the alphabet.
What is the problem with sending an unprotected message?
It can be read by anyone who can intercept it
How can you protect a message?
Encypt its contents
What two types of cyptography are used today?
Symmetric and Asymmetric
What is symmetric cryptography?
Using a pair of keys to encrypt and then decrypt a message
plaintext -> encypt(plaintext, key) -> cipher text -> decypt(ciphertext, key) -> plaintext
w -> * -> w
What are some examples of symmetric cryptography?
Substitution and transposition ciphers
What are some substitution ciphers?
Monoalphabetic e.g. Caesar cipher
Polyalphabetic e.g. Vingere cipher
What is a transposition cipher?
Rail fence cipher
What is a Caesar cipher?
Letters are shifted along in the alphabet to encrypt
Shifted back the same amount to decrypt
Plain: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher: DEFGHIJKLMNOPQRSTUVWXYZABC
What is a Vigenere cipher?
Encypts text by using a series of different Caesar ciphers based on the letters of a keyword.
In a grid, same letter won’t be used for same subsitution
What is a rail fence cipher?
Places plaintext in a zigzag along imaginary rails.
What is a column cipher?
Message is written out in rows of a fixed length
Then read out again column by column, and the columns are chosen in some scrambled order (cipher text)
Both the width of the rows and the permutation of the columns are usually defined by a keyword.
What is a stream?
Caesar and Vigenere are streams
What is a block?
Used for DES, AES
0110111001010011
0110 1110 0101 0011
Block ciphers
Translate between plain and cipher text using a key and applying a logic gate to key and block, e.g. XOR
What are the main ideas in cryptography?
Substitution
Transposition
Bitwise operations e.g. XOR
What is the problem with cryptography?
If you know how e.g. Caesar, rail road works, you can decrypt a message
What is a solution to a problem in cryptography?
Use blocks with a key
What is DES?
64 bit blocks (56 bit key + 8 bit check)
Check bits not used by DES
Used to detect errors
What is AES?
128bit plain text
128, 192 or 256 bit key
What is a One Time Pad (OTP)?
Plaintext combined with truly random key
Key is used only once, destroyed after
What are some rules for One Time Pad?
Key is as long as plain text
Key is truly random
Each key used only once
Should be two copies of the key: one for sender and one for receiver
What does pseudo random mean?
Not truly random
Calculated based on a seed
