L4 - Ciphers Flashcards
What are stream ciphers?
Stream ciphers operate on the plaintext a single bit (or
single character) at a time
for instace: Simple substitution cipher
What are substitution and transposition ciphers usually combined?
- Substitution ciphers on their own are usually not very secure
- Transposition ciphers on their own are also not very secure
- That is why they are combined with transposition ciphers
What is a simple columnar transposition cipher?
the plaintext is written horizontally onto a piece of graph
paper of fixed width
while the ciphertext is read off vertically
What is a doule columnar transposition cipher?
- A simple columnar transposition can be broken by tryingout different width/column lengths
- Putting the ciphertext through a second transposition enhances security
- In contrast to a simple substitution cipher, where a second application does not increase security
What is the Vigenère cipher?
A Polyalphabetic Substitution Cipher.
What is Playfair?
An example of an early block cipher.
What are Homophonic substitution ciphers?
Homophonic substitution ciphers try to obscure the
frequencies by mapping a character to more than one code.
For example, “A” could correspond to 5, 13, 25, or 56; while
for “B” this could be 7, 19, 32, or 42.
Why are monoalphabetic ciphers not very secure?
They can be easily broken by employing statistical means such as frequency analysys.
What is a a polyalphabetic substitution cipher?
- A polyalphabetic substitution cipher uses multiple simple substitution ciphers.
- The particular one used changes with the position of each character of the plaintext
- There are multiple one-letter keys
- The first key encrypts the first letter of the plaintext, the second key encrypts the second letter of the plaintext, and so on
- After all keys are used, you start over with the first key
- The number of keys determines the period of the cipher
How secure are polygram substitution ciphers?
Still not a very secure way of encrypting data:
This hides the frequencies of individual letters
However, natural languages also show typical frequencies
for n-grams (although the curve is flattened).
What are Rotor Machines?
- In the 1920s, mechanical encryption devices weredeveloped (to automate the process)
- Basically, these machines implemented a complexVigenère cipher
- A rotor is a mechanical wheel wired to perform a general substitution
- A rotor machine has a keyboard and a series or rotors, where the the output pins of one rotor are connected to the input of another
How secure are polyalphabetic substitution ciphers?
- not very secure
- code can be broken by analyzing the period
- Looking at the example below, we notice that KIOV is repeated after 9 letters, NU after 6 letters
- As 3 is a common divisor of 6 and 9, this is a hint that the period could be 3
- Knowing which letters were encoded with the same key allows the application of frequency methods again
What is a transposition cipher?
In a transposition cipher the symbols of the plaintext
remain the same, but their order is changed
What are polygram substitution ciphers?
Instead of encoding single characters, a polygram
substitution cipher encrypts groups of letters
For example, “ABA” could correspond to “RTQ’, while
“ABB” could correspond to “SLL”
What are block ciphers?
Block ciphers operate on groups of bits (or groups of
characters)
An example of an early block cipher is Playfair
What is a monoalphabetic cipher?
A simple substitution cipher. Each latter is mapped to another one.
What is the Ceasar cipher?
An early monoalphabetic substitution cipher.
Each letter is shifted by 3.
How does the Vigenere cipher work?
C = M + K mod 26
