16: Basic Ciphers Flashcards
1
Q
What is the basic idea of cryptography ?
A
A sender sends a plain text input message which is encrypted using a secret key and an encryption algorithm (cypher), and then decrypted to the plain text using the key.
2
Q
Who has the key ?
A
Both the sender and the recipient.
3
Q
Which way do we turn for Caesar’s cypher ?
A
Left (unless mentioned otherwise).
4
Q
For letter A, what is the encrypted letter for
1) k = 25 ?
2) k = 26 ?
A
1) B
2) A
5
Q
What is the key to a strong encryption algorithm ?
A
The secret key.
6
Q
How can we break Caesar’s cipher ?
A
- We can calculate the offset k
- We can try all the 25 offset possibilities until text decodes
- We can look at language’s letter frequency
7
Q
What is Playfair’s cipher ?
A
Each pair of successive letter by another pair letter.
Key is 5x5 table.
8
Q
What is a mono-alphabetic cipher ?
A
It uses an arbitrary permutations of letters.
A bijective key (1-1 map): A->B and B->A
