19: Symmetric Key Encryption

Question 1

What is a symmetric key algorithm ?

Answer 1

An algorithm which uses the SAME key for encryption and for decryption of a cipher text.

Question 2

What a symmetric key algorithm alternatively called ?

Answer 2

Shared-key, single-key, secret-key, private-key or one-key encryption.

Question 3

Which algorithms are symmetrical ?

Answer 3

• Caesar
• Vignère
• Rotor machines

Question 4

Give 2 types of symmetric key algorithms ?

Answer 4

• Stream ciphers: one at a time (bytes)

- Block ciphers: blocks at a time (64 bits)

Question 5

What is confusion ?

Answer 5

The act of making the relationship between the key and the cipher text as complex as possible

Question 6

What is diffusion ?

Answer 6

The act of making output should depend on the input in a complex way, eg by having a strong encryption algorithm.

Question 7

What is End-to-End encryption ?

Where should it be done ?

What is it best for ?

Answer 7

messages are encrypted by the sender and only decrypted by the intended receiver.

But headers with a source and destination remain unencrypted, meaning traffic analysis is possible.

Encryption should be done by a user at the software level

Best for actual data.

Question 8

What is Link encryption ?

Where should it be done ?

What is it best for ?

Answer 8

Messages are encrypted and decrypted at each end of a communication link.

All traffic can be secure at the hardware level, not only isolated messages.

But original input text is revealed at every node on its way, making it less secure.

Best for headers data.

Question 9

Strengths of symmetric keys ?

Answer 9

• Simple
• Low resources required
• Can create a key per channel. If one channel is compromised, only messages between those 2 are affected. Others channels are not.

Question 10

Weaknesses of symmetric keys ?

Answer 10

• Key distribution/sharing.
• New key per channel.
• Keys management and security.
• Origin and authenticity aren’t guaranteed since both parties use same key.