Encryption - AQA A Level Computer Science

Definition of encryption

• Using an algorithm and a key to convert message data into a form that is not understandable without that key.

• A method of trying to find the plain text from the cipher text without the decryption key.

• Process of turning legible text into gobbledygook.

• Using an algorithm and a key to convert encrypted message data into its plain text equivalent.

Definition of decryption

• Using an algorithm and a key to convert message data into a form that is not understandable without that key.

• A method of trying to find the plain text from the cipher text without the decryption key.

• Process of turning legible text into gobbledygook.

• Using an algorithm and a key to convert encrypted message data into its plain text equivalent.

A Caesar cipher encrypts the plaintext Et tu, Brute into the ciphertext ix xy, fvyxi. What would the same cipher encrypt it was Greek to me into?

• mx aew kviio xs qi

• ny bfx lwjjp yt rj

• ep swo cnaag pk ia

• qb eia ozmms bw um

Why are Caesar ciphers so easy to crack?

• There are only 25 possible 'shifts', so a brute force attack will easily find the key required.

• Use of frequency analysis will be able to find common letters and words.

• Each key is only used once.

• The number of shifts is calculated from the number of characters in the plaintext.

The Caesar cipher is an example of what type of cipher?

• Substitution cipher

• Transposition cipher

• Public key encryption

• Vernam cipher

What are the conditions required for a Vernam cipher to be 100% mathematically secure?

• The key is used only once, and then destroyed/

• The key is made up of truly random characters/

• The key must be made up of fewer characters than the plaintext message.

• Different keys must be used to encrypt and decrypt a message.

Which Boolean operator is used to produce the cipher text from the plaintext and the key?

• XOR

• NOR

• OR

• NAND

The letter m has an ascii code of 1101101. As part of a Vernam cipher, the key 0101001 is applied to that letter. What is the resulting ciphertext?

• 1000100

• 0111011

• 1100101

• 1101001

Which cipher is considered to be 100% mathematically secure?

• Caesar cipher

• Vernam cipher

• Transposition cipher

• Railfence cipher

If a cipher is theoretically possible to break, but not when using current technology in a timeframe that would be useful is known as what?

• Computationally secure

• Intractably resistant

• Algorithmically impossible

• Cryptographically unbreakable