Comparing cryptographic strength

Overview

Brute force attack and balanced conclusion about cryptographic strength.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What determines the strength of a modern cipher?

   Model answer

   The size of the key space and the absence of any attack faster than trying keys.

2. 2

   What is a brute force attack?

   Model answer

   Trying every possible key until the plaintext is recovered.

3. 3

   How does adding one bit to a key affect brute force?

   Model answer

   It doubles the number of keys, so each extra bit doubles the time required.

4. 4

   How many keys does a 128-bit key allow?

   Model answer

   Two to the power of 128, which is beyond exhaustive search by any foreseeable computer.

5. 5

   Why is a long key no help if the algorithm is flawed?

   Model answer

   A structural weakness lets an attacker recover the key without searching, so the key space is never explored.

6. 6

   Rank the Caesar, Vigenère and modern symmetric ciphers by strength, with a reason.

   Model answer

   The Caesar cipher has 25 keys and falls instantly, the Vigenère cipher resists simple frequency analysis but falls once the key length is found, and a modern cipher has no known attack faster than brute force over an infeasible key space.

Other ways to work this

The same content, four other ways.

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Six ways to work the same material.