Practical Approaches

Theoretical Security vs. Practical Security

Expected use != practical exploitation.

Defective practices can introduce vulnerabilities.

• Example: reuse of keys.

Computational Security

Computational complexity of break-in attacks

• Using brute force.

Security bounds:

• Cost of cryptanalysis.

• Availability of cryptanalysis infra-structure.

• Lifetime of ciphertext.

5 Shannon Criteria

• The amount of offered secrecy.

  • e.g. key length.

• Complexity of key selection.

  • e.g. key generation, and detection of weak keys.

• Implementation simplicity.

• Error propagation.

  • Relevant in error-prone environments.

  • e.g. noisy communication channels.

• Dimension of ciphertexts.

  • Regarding the related plaintexts.

Confusion

Complex relationship between the key, plaintext, and ciphertext.

• Output bits (ciphertext) should depend on the input bits (plaintext + key) in a very complex way.

Diffusion

Plaintext statistics are dissipated in the ciphertext.

• If one plaintext bit toggles, then the ciphertext changes substantially, in an unpredictable or pseudorandom manner.

Avalanche effect.

Always assume the worst-case

Cryptanalysts know the algorithm.

• Security lies in the key.

Cryptanalysts know/have many ciphertext samples produced with the same algorithm & key.

• Ciphertext is not secret!

Cryptanalysts partially know original plaintexts.

• As they have some idea of what they are looking for.

• Know-plaintext attacks.

• chosen-plaintext attacks.