Symmetric Cryptography Flashcards Preview

Protocols and Algorithms for Network Security [pme] > Symmetric Cryptography > Flashcards

Flashcards in Symmetric Cryptography Deck (28):

How many keys are used in symmetric cryptography?

Only one, since the same key KA,B is used to encrypt and to decrypt the message.


If you have the key and the encrypted message, how can you decrypt?

D=(key, E(key, P)) = P, where P is the plain text.


Cite some symmetric cryptographic protocols.



What is a stream cipher?

In stream ciphers, one bit or byte is encrypted at a time. It is based on pseudorandom bit sequence (emulating one-time pad).


Cite examples of stream ciphers.



What is a block cipher?

In block ciphers, blocks of equal length are encrypted at a time. Typically size are power of 2. Usually they have more diffusion than stream ciphers.


Cite examples of block ciphers.

DES, 3DES, AES, IDEA, Blowfish, CAST-n


What is more common: stream or block ciphers?

Block ciphers, because they are commonly considered more efficient.


Which standard is currently being used for block ciphers?

Advanced Encryption Standard (AES)


Which algorithm is used for Advanced Encryption Standard (AES)?



What is shannon's substitution-permutation network? In which cryptographic primitives is it based on?

Shannon's substitution-permutation network is modern substitution-transposition product cipher. It is based on two cryptographic operations: substitution (S-box) and permutation (P-box)


Which important characteristics does Shannon's S-P Networks provide?

They provide confusion and diffusion of message.


What does diffusion in the context of cipher means?

Diffusion is the propriety to dissipate statistical structure of a plaintext over bulk of ciphertext.


What does confusion in the context of cipher means?

Confusion is the propriety in which makes the relationship between ciphertext and key as complex as possible.


What is cornerstone of modern symmetric cryptography?

Shannon's S-P networks.


What is the main advantage of the Feistel model compared to a substitution-permutation network?

One advantage of the Feistel model compared to a substitution-permutation network is that the round function "F" does not have to be invertible.


How to Encrypt using a Feistel model?

Split the plaintext block into two equal pieces, (L_0, R_0)
For each round i =0,1,...,n, compute
L_{i+1} = R_i,
R_{i+1}= L_i XOR F(R_i, K_i).

Then the ciphertext is (R_{n+1}, L_{n+1}).


How to decrypt using a Feistel model?

Decryption of a ciphertext (R_{n+1}, L_{n+1}) is accomplished by computing for i=n,n-1,...,0

R_{i} = L_{i+1},
L_{i} = R_{i+1}XOR F(L_{i+1}, K_{i}).

Then (L_0,R_0) is the plaintext again.


What happens if one or more of those parameters are increased using Feistel cipher: Block size, Key size, number of rounds, subkey generation. round function?

Improved security but they may slow cipher.


Is it secure to use only substitution to generate a cipher?

No, because of language characteristics.


Is it secure to use only transposition to generate a cipher?

No, because of language characteristics.


Explain what happens if the following operations are being used:
1. two substitutions
2. two transpositions
3. substitution followed by a transposition

1. make a more complex substitution
2. make a more complex transposition
3. makes a new much harder cipher


What is the bridge between classical to modern ciphers?

The use of transposition associated with substitution.


What is the avalanche effect?

It is the key desirable property of encryption algorithm in which a change of one input or key bit results in changing approximately half of the output bits - highly non-linear, chaotic behavior.


Which symmetric cryptographic protocol exhibits a strong avalanche effect?

Data Encryption Standard (DES).


Which kind of key is considered weak for Data Encryption Standard (DES) security?

- Keys that generate subkeys with either all 0's or all 1's (four in total)
- Keys that generate only two different subkeys (six pair of keys) -> those are considered semiweak keys.
- Keys that generate only four different subkeys (48 keys) -> those are considered possible weak keys.

As whole, 64 keys out of ~7.2 x10^16 are considered weak.


Algebraic structure: is DES closed?

No, DES is not closed. It means that it is possible to use double encryption, since it is not true that for every K1 and K2 there is a K3 such as E(K2, E(K1,M)) = E(K3,M). Since this is not true, double encryption might be used.


Algebraic structure: is DES pure?

No, DES is not pure. By not being pure, DES allows to use triple encryption to increase the key length. To be pure, for every K1, K2, K3 there would be a K4 such that E(K3, E(K2, E(K1, M))) = E(K4, M). Since DES is not pure, it is possible to use 3-DES.