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