Classical Systems Flashcards
What is kerckhoff’s principle?
“The cipher method must not be required to be secret, and it must be able to fall into the hands of the enemy without inconvenience.”
Explain how Ceasar’s cipher works
To encrypt a message, every letter is shifted (to the right) by 3 places.
A -> D
B -> E
How does a shift cipher work?
The letters in the message are shifted by a value of 0 < k < 26. Key space is very small, so it is easy to solve.
How are shift ciphers solved?
They compare the letter frequency of the language to that of the cipher’s. For each value between 1 - 26 you can compute the likelyhood of that being the key.
Explain what a substitution cipher is
Each letter in the alphabet is replaced by a different letter. The mapping for each letter is random, but consistent. this allows for 26! = 4 * 10e26 keys.
How is the substitution cipher solved?
Again letter frequency analysis. It is not necesarily harder to compute than shift cipher, especially when the cipher text is large enough.
Exlpain Vignère cipher
It chooses a word as a key, and then every letter in the plaintext is shifted by the individual difference of the letter (modulo) in the key-word.
How is Vignère cipher solved?
You find repeating sequences in the cipher text. These are most likely to be the same word(s) encrypted by the same part of the key word. Hence, using GCD and CRT, the length of the key word can be determined. For each letter in the key word, letter frequency analysis can be performed.
Explain the permutation cipher.
the letters per block (k) are consistently scrambled. 1-2-3-4-5 is mapped to 2-4-1-5-3. Most often the words can be guessed by letters in- and the structure of the sentence.
