Crash Course Computer Science 33
THE CAESAR CIPHER
IS ONE EXAMPLE OF A LARGER CLASS OF TECHNIQUES CALLED SUBSTITUTION CIPHERS. THESE
REPLACE EVERY LETTER IN A MESSAGE WITH SOMETHING ELSE ACCORDING TO A TRANSLATION
ONE-WAY FUNCTIONS
Mathematical operations that are very easy to do in one direction, but hard to reverse
AES
chops data up . into 16-byte blocks, and then applies a series of substitutions and permutations, based on the key
value, plus some other operations to obscure the message, and this process is repeated ten or more times for
each block.
Stallings Reading 2.1
![[Module 1 Notes Figure 1.png]] A symmetric encryption scheme has five ingredients
Plaintext:
This is the original message or data that is fed into the algorithm as input.
Encryption algorithm:
The encryption algorithm performs various substitutions and transformations on the plaintext.
Secret key:
The secret key is also input to the algorithm. The exact substitutions and transformations performed by the
algorithm depend on the key.
Ciphertext:
This is the scrambled message produced as output. It depends on the plaintext and the secret key. For a
given message, two different keys will produce two different ciphertexts.
Decryption algorithm:
This is essentially the encryption algorithm run in reverse. It takes the ciphertext and the same secret key
and produces the orig- inal plaintext.
![[Module 1 Notes .png]] ![[Module 1 Notes -1.png]]
Research Feistel Ciphers
Important Questions to
Answer
Q: What are the essential ingredients of a
symmetric cipher?
Q: What are the two basic functions used
in encryption algorithms?
Q: How many keys are required for two
people to communicate via a asymmetric
cipher?
Q: Describe the difference between a
block cipher and a stream cipher.
Q: Why is "symmetric encryption" called
symmetric encryption?
Q: Compare and contrast cryptanalysis
with a brute force attack.