Define Cryptography

Science of discovering weaknesses in existing encryption/decryption systems so, that the plaintext can be recovered without foreknowledge of the key.

Type of encryption that substitutes words or phrases for other words or phrases. According to a specific code book. The code book is the key.

Is an encryption method where symbols in the plaintext are replaced by other symbols in the Ciphertext according to some rule. The rule is the key and the key defines the rule. Replacing isn't always substitution.

List the five rules to minimally secure information

1. Systematically Communicate
2. Hide information content of a message  (during transmission)
3. Recover the information content at reception
4. Process must be fast & easy to use
5. Neither the process nor the "message" should reveal original information to an eavesdropper.

What do we need to assume about the eavesdropper.

Eve is intellegent, motivated and that eve has the same resources as Alice + Bob.

Name the six criteria of a good cipher according to Auguste Kerckoff

1. System should be unbreakable in practice if not absolutely unbreakable. (no one can break it in the time it is useful) (long enough and expensive enough that it's not worth doing.)
2. Compromise of the system should not inconvenience the correspondents. (if the system is compromised than the correspondents can still use the system)
3. The key should be easy to remember.
4. The cryptograms( Ciphertext) should be transmittable over telegraph.
5. Any apparatus or documents should be portable & operable by one person
6. The system should be easy to use neither requiring knowledge of a long list of rules nor involve use of mental strain.

What were Claude Shannon's characteristics of a good cryptosystem.

Confusion: cipher should hide any local patterns or characteristics of the plaintext language.

Diffusion: Cipher should mix segments so that no character remains in its original position. (transposing)

Name the three general attacks of Cryptanalysis

1. Cipher text only attack (everything but the key & most difficult attack)
2. Known plaintext - all or part of the original plaintext is known & can be located in ciphertext. (matching plaintext and ciphertext)
3. Chosen (seeded) plaintext - get an opponent to encrypt and send a message of which you know the content. Easiest method.

Alice needs to send a message (plaintext) to bob without eve reading the message. Alice devises a cipher to encrypt the message. Alice shares cipher with bob (via plaintext). Alice encrypts the plaintext and sends results (ciphertext) to bob. Bob receives and decrypts the ciphertext to extract plaintext.

Government sponsored organization responsible for cryptography & cryptanalysis for the government.