CS计算机代考程序代写 CM30173/50210 Cryptography

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Useful information

Course outline

Useful information

Course outline

CM30173/50210 Cryptography

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Useful information

Course outline

Useful information

Course outline

Major topics

Introduction to the problem

Private-key cryptography: block ciphers

Unkeyed and keyed cryptographic hash functions

The key distribution problem

Public-key cryptography: ciphers

Public-key cryptography: digital signatures

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Part I

Introduction to the problem

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

1 Key ideas

2 Classical cryptography

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Cryptography and Security

Cryptography supports secure exchange (and
storage) of information.

We also need secure protocols in place for
exchanging messages, etc (cf. “man-in-the-middle”
attack on Needham-Schroeder protocol).

Typically, such protocols assume the cryptographic
properties they need, and we need to analyze more
closely how far these are satisfied.

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Cryptographic Techniques

Encryption Only the intended recipient can
understand the message.

Steganography: Only the intended recipient is
aware that there is a message.

Cha�ng and Winnowing: Only the intended
recipient can find the real message.

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Goals

Confidentiality: Keeping information a secret
from those not authorised to have it.

Data integrity: Ensuring information has not
been altered by those not authorised to do so.

Authentication: Confirmation of the identity of
an entity.

Message authentication: Confirmation of the
source of information.

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Goals

Confidentiality: Keeping information a secret
from those not authorised to have it.

Data integrity: Ensuring information has not
been altered by those not authorised to do so.

Authentication: Confirmation of the identity of
an entity.

Message authentication: Confirmation of the
source of information.

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Signature: A way of binding information to an
entity.

Certification: Endorsement of information by a
trusted entity.

Non-repudiation: Preventing an entity from
denying previous actions or commitments.

Revocation: Retracting certification or
authorisation.

CM30173/50210 Cryptography

CM30173/50210
Cryptography

Key ideas

Classical
cryptography

Secure communication

Alice Bob

Oscar

PlaintextPlaintext

Encryption Decryption

Unsecured channel
ek(x) = y dk(y) = x

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Cryptosystem

Definition

A cryptosystem is a five-tuple (P , C,K, E ,D), where
1 P is a finite set of possible plaintexts
2 C is a finite set of possible ciphertexts
3 K is a finite set of possible keys called the keyspace
4 For each key k 2 K there is an encryption rule

ek 2 E , ek : P ! C and a corresponding decryption
rule dk 2 D, dk : C ! P such that

dk(ek(x)) = x

for all plaintext elements x 2 P .

CM30173/50210 Cryptography

CM30173/50210

Cryptography

Key ideas

Classical

cryptography

Key ideas

Classical cryptography

Important properties

For a cryptosystem to be useful in practice, we need:

1 to be able to e�ciently compute the encryption
and the decryption functions

2 that an unauthorised party should not be able to
determine the key or the plaintext

CM30173/50210 Cryptography

Key ideas
Classical cryptography