Cryptology: intro

Question 1

What is Viginere encryption?

Answer 1

A more advanced version of Caesar cipher, where you have a key consisting of a set of multiple rotations.
The n’th letter of the message will be encrypted using the (n % KEY_LENGTH)th number in the key.

Question 2

What is Vernam encryption and one-time-pad encryption?

Answer 2

Encryption where the key is the same length as the message. Vernam cipher has a dedicated alphabetic rotation for each letter, whereas one-time-pad typically has a bitstring that is XORed with the bits of the message. This form of encryption is unbreakable without having the key, but suffers from enourmous key sizes.

Example of Vernam Cipher:

plaintext: “aba”
key: “abc” (can also be thought of as “0 1 2”)
ciphertext: “acc”

Question 3

What is transpositions?

Answer 3

Encryption where you divide a text into blocks of n letters and permute with secret permutation.

Example: 
Let n = 5 and permutation = (3,5,2,1,4)
Plaintext = "cryptographical"
Breaking into groups of 5: "crypt" "ograp" "hical"
Permute: "pyctr" "aropg" "achli"
Ciphertext: "pyctrarophachli"

Can also be done using matrices, where you use a n-by-n matrix. See Patricks example at 21:39 in recording 2 of crypto intro.