Exchanging data

Question 1

run length encoding

Answer 1

Records the value of a pixel and the number of time it repeats, saving bytes and so memory

Question 2

dictionary based compression

Answer 2

The compression algorithm searches through the text to find suitable entries in its own dictionary (or a known dictionary) and translates accordingly, reassembling perfectly. Dictionary must also be stored with text- but insignificant size compared with original.

Question 3

encryption

Answer 3

Transformation of data from one form to another to prevent unauthorised third party from understanding it.

Question 4

encryption: original data/message

Answer 4

plaintext

Question 5

encryption: encrypted data

Answer 5

ciphertext

Question 6

encryption method/algorithm

Answer 6

cipher

Question 7

encryption: secret info to unlock the message

Answer 7

key

Question 8

Caesar/shift cipher

Answer 8

Substitution cipher works by shifting letters of alphabet a given number of characters- the parameter being the key

Question 9

Vernam cipher invented by

Answer 9

American scientist Gilbert Vernam in 1917

Question 10

Vernam cipher is

Answer 10

unbreakable. others are breakable with enough comput. power, ciphertext and time as they are based on computational security

Question 11

private key/symmetrical encryption

Answer 11

same key to encrypt and decrypt data. key has to be transferred (key exchange) to the same destination= security issues as it can be intercepted as easily as the ciphertext

Question 12

one time pad

Answer 12

encryption key: must be the same length/longer than the number of char in plaintext, must only be used once, must be random (so distrib of char are random, and cryptanalysis results = meaningless)

Question 13

one time pad exchange

Answer 13

used in pairs: sender and recipient are both party to key. meet in person, exchange securely, use once then destroy after encryption/decryption