Actual Intro Flashcards
What is symmetric encryption?
When a message is encrypted and decrypted using the same key.
What are the limitations of symmetric encryption?
Security is inherently reliant on the secrecy/privacy of the key, so if the key is leaked, the system will no longer be secure.
In addition, both parties need to receive the key first to be able to use it, which can risk exposing the value of the key if not done in an encrypted manner.
What are the requirements for a good cypher?
FOR SENDER AND RECEIVER:
1. That, given the possession of the key, it’s easy for the sender or receiver to encrypt/decrypt the message.
FOR ADVERSARY:
1. That the adversary will find it difficult to encrypt/decrypt a message without the necessary key. (Confusion)
That the ciphertext must perform better than brute force.
That the relationship created by the key between the plaintext and ciphertext doesn’t reveal any information about the ciphertext/plaintext. (Confusion, Diffusion)
What resources can an adversary be provided with to assist in an attack?
- Cache of random ciphertexts.
- Cache or random plaintext/ciphertext pairs.
- Chosen plaintexts (the ability to find the ciphertext of certain plaintext strings)
- Chosen ciphertext (the ability to find the ciphertext of certain plaintext strings as well as the ability to choose the plaintext of a certain ciphertext)
What security designations can be given to a symmetric encryption algorithm?
IND-CPA (Resistance to a chosen plaintext attack)
IND-CCA (Resistance to a chosen ciphertext attack)
What is “keyspace”?
The amount of possible variables a key can be. For example, the keyspace of “K” if K can only be a single digit number would be 10 (0 to 9).
How does the Caeser Cipher work?
c = (m + k) mod 26
m = (c - k) mod 26
- where k is the amount of shit.
Or in non-autistic terms:
For k, draw an alphabet with a shifted alphabet underneath it. Caeser 3 would be:
ABCDEFG
DEFGHIJK
So “ABE” would be “DEH”.
Is the Caeser Cipher secure?
No, as its keyspace is very low. With a size of 26, it would be trivially rapid to brute force the key.
Given the requirement that brute-force is easy, this fails the requirement for this algorithm to be secure.
What is the Substitution Cipher?
Given a permutation key, replace letters with the given permutation key for the alphabet.
Example:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
DABCZHWYGOQXSVTRNMSKJIPFEU
HELLO -> YZXXT
There is no easy order to this unlike what is found in the Caeser cipher, which is due to the keyspace being larger.
Is the Substitution Cipher secure?
No, as although the keyspace is very large at 26 factorial (more than 1 trillion trillion combinations), information from the ciphertext can help in figuring out what the plaintext might be.
For example, e is the most common letter in the English language, so the most common letter in the ciphertext can likely be substituted with e to help find the plaintext.
What is the Vignere Cipher?
The Vignere cypher effectively uses multiple shift cyphers to encrypt the plaintext into cipher text.
This can be done by making a grid of the alphabet where x and y are letters and then using a word as a key.
@ A B C D E
A a b c d e
B b c d e f
C c d e f g
D d e f g h
E e f g h i
If we wanted to encrypt plaintext “ABE” using “CAB”, that would give “CBF”
Is the Vignere Cipher secure?
Still not secure, though frequency analysis is more difficult for this algorithm than it is for the Caeser or Substitution cypher. Words would still be able to be inferred through frequency analysis, such as “the”, which is the most common word in the English language.
