# Definitions These are definitions found in course material (mostly exercises) which may prove useful for the exam ##Chapter 1 Confidentiality : preventing unauthorised disclosure of information Integrity : preventing unauthorised (accidental or deliberate) modification or destruction of information Availability : ensuring resources are accessible when required by an authorised user Entity authentication : provides confirmation of the claimed identity of an entity Data origin authentication : provides confirmation of the claimed source (origin) of a data unit (message) Non-repudiation : Placeholder Cipher : See own section Kerckhoffs’ principle : the cryptanalyst has complete knowledge of the cipher, i.e. only unknown thing is the decryption key Transposition : the characters in the plaintext are mixed up with each other (permuted) Substitution : each character (or set of characters) is replaced by a different character (or set of characters) ## Chapter 2 Confusion : This involves substitution to make the relationship between the key and ciphertext as complex as possible. Diffusion : This involves transformations that dissipate the statistical properties of the plaintext across the ciphertext. Product cipher : A product cipher is a cryptosystem in which the encryption function is formed by applying (or composing) several sub-encryption functions. Iterated cipher : Most modern ciphers in this category, read more below. Feistel cipher : An iterated cipher in which the round function swaps the two halves of the block and forms a new right hand half Substitution-permutation network : An iterated cipher. Takes a block of the plaintext and the key as inputs, and applies several rounds of permutations, known as substitution boxes(S-box) and permutation boxes (P-box). A __S-box__ substitutes sub-blocks of size l bits (its input) by another block of bits (its output). It can be thought of as a substitution cipher. A __P-box__ takes the output from the S-boxes of one round, permutes the bits and feed them into the S-boxes in the next round. It can be thought of as a transposition cipher. At each round, the __round key__ is combined with some operation such as XOR. Group generator : placeholder Finite field : placeholder TODO: Scrape definitions from all exercises (currently they are just from exercise 1) # Ciphers ## Symmetric cipher (secret key cipher) encryption and decryption keys known only to sender and receiver. (DES) ## Asymmetric cipher (public key cipher) each participant has a public key and a private key, may allow both encryption and signatures. (RSA) ## Attacks ### Ciphertext only attack the cryptanalyst has available only the intercepted cipher text. ### Known plaintext attack the cryptanalyst knows a small amount of plaintext and its cipher text equivalent ### Chosen plaintext attack The cryptanalyst can obtain the cipher text equivalent of some plaintext which can be selected by the attacker, i.e the attacker has an "inside encryptor” available ### Chosen cipher text attack The cryptanalyst can obtain the plaintext equivalent of some cipher text which can be selected by the attacker, i.e. the attacker has an “inside decryptor” available. ## Block ciphers ### Block cipher modes ## Stream ciphers ## Historical ciphers - Caesar - Substitution - Vigenère. Caesar, but also uses a key in order to choose how many steps to shift the alphabet for each letter ### DES

### RSA # Iterated ciphers - Encryption process divided into _r_ similar rounds - the sub encryption functions _g_ are the same for all rounds - Each key $K_i$ is derived from the overall master key K. The keys $K_i$ are called round keys or subkeys and are derived from K using a process called the key schedule. ## Encryption Given a plaintext block, P, a round function g and round keys $K_1, K_2, . . . ,K_r$ , the ciphertext block, C, is derived through r rounds as follows. $$W_{0} = P$$ $$W_{1} = g(W_{0}, K_{1})$$ $$W_{2} = g(W_{1}, K_{2})$$ $$.$$ $$.$$ $$.$$ $$.$$ $$.$$ $$W_{r} = g(W_{r−1}, K_{r})$$ $$C = W_{r}$$