Cryptanalysis

Goals

• Discover original plaintext.

  • Which originated a given ciphertext.

• Discover a cipher key.

  • Allows the decryption of ciphertexts created with the same key.

• Discover the cipher algorithm.

  • Or and equivalent algorithm.

  • Usually algorithms are not secret, but there are exceptions.

    • Lorenz, A5 (GSM), RC4 (WEP), Crypto-1 (Mifare)

    • Algorithms for DRM (Digital Rights Management)

  • Reverse engineering.

Attacks

Approaches

Brute force

Exhaustive search along the key space until finding a sutable key.

Usually infeasible for a large key space.

• e.g. 2¹²⁸ random keys (or keys with 128 bits).

• Randomness is fundamental!

Cleaver attacks

Reduce the search space to a smaller set of potential candidates.