Security

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Security
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Security Needs
Security policy is to ensure that

• Computers and data are used by the authorized
  persons
• Computers and their accessories, data, and
  information are available to the genuine users

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Security Services

• Authentication
• Access control
• Data confidentiality
• Data integrity
• Non-repudiation

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Security Services
Authentication

• A user proves its identity to another party
• A data sender proves that the data is actually
  sent by him/her

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Security Services
Access control

• Guard against unauthorized use of resources

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Security Services
Data confidentially

• Data and its meanings are only available to those
  who are the genuine receivers
• For other parties, the data would appear to be
  rubbish

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Security Services
Data integrity

• Guards against active attack modification,
  insertion, deletion, replay
• If a piece of data is changed, such a change can
  be detected

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Security Services
Non-repudiation

• When a party sends a piece of information, it can
  be proved that the sender is actually that party
• The sender cannot subsequently deny the act of
  having sent a piece of information

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Security Mechanisms
To provide security services, some specific
security mechanisms may be implemented

• Encipherment
• Digital signature
• Access control

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DES

• The Data Encryption Standard (DES) is a private
  key encryption system developed by the U.S.
  government in the 1970s
• It was based on a previous IBM encryption system
  called Lucifer
• It was adopted as a U.S. federal standard in
  1976, and then as an international standard

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DES Overview

• Plaintext size 64 bits
• Key size 64 bits input, only 56 bits are used
• Ciphertext size 64 bits

64 bit ciphertext
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Strength of DES

• DES has been cryptanalyzed for many years by many
  people, no serious flaws have been revealed up to
  now
• The 56-bit key size there are 2567.2x1016
  different possible keys
• May not be sufficient to resist brute-force key
  search attack

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Strength of DES

• If it takes 1 sec to test 1 key then 228 million
  years are needed to test all keys
• If it takes 1 µsec to test 1 key then 2,280 years
  to test all keys
• If there are 1 million machines working in
  parallel then the key can be found in a day!

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Triple DES

• Triple DES employs the Encrypt-Decrypt-Encrypt
  (EDE) mode of operation with two different keys
  equivalent to a key of 112 bits

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Triple DES

• The decryption process is

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Triple DES

• Triple DES can use the existing DES block
• When K2K1, the triple DES system falls back to
  the single DES system
• It is backward compatible with single key DES

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AES

• AES stands for Advanced Encryption System
• NIST (National Institute of Standards and
  Technology) of USA announced AES in 1997, and
  then called for algorithms from the public on 12
  Sept 1997

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AES

• Researchers from 12 different countries submitted
  15 algorithms for the AES
• As at Aug 1999, 5 algorithms have been chosen by
  NIST for further consideration
• On 3-Oct-2000, the proposal by Rijdael pro.
  Rhine doll Joan Daemen and Vincent Rijmen of
  Belgium was selected

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Public Key Encryption
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Public Key Encryption

• Each user will have a pair of keys K1 K2
• Use keys K1 to encrypt and K2 to decrypt
• Keep K1 private and top secret
• Gives out K2 to anybody who needs it
• K1 is called the private key
• K2 is called the public key

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Two Keys

• In a public key encryption system, the encryption
  key and the decryption key are different

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Proof of Identity

• Alice sends a message to Bob
• Bob can prove that the message could only have
  been created by Alice

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Confidentiality Identity

• Alice sends an encrypted message to Bob so that
  only Bob can decrypt the message and Bob can
  later prove that the creator was Alice

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RSA Algorithm

• The most widely used public key algorithm
• Proposed by Rivest, Shamir, and Adleman
• Security is based on the difficulty in
  factorizing a large integer that is the product
  of two large prime numbers
• E.g. 437 ? x ?
• 437 19 x 23
• Reference web page http//www.rsa.comhttp//www
  .orst.edu/dept/honors/makmur/

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Hash Function

• A Hash Functionis a one-way function yH(x),
  designed to produced a fixed length message
  digest or a fingerprint of a variable-length
  message

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MD5

• MD5 Message Digest 5
• Designed by Prof. R. Rivest of MIT
• Internet standard RFC1321
• Thought to be a strong hash function
• The message digest is 128 bits
• Message is processed in 512-bit blocks

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Secure Hash Algorithm (SHA)

• SHA was FIPS PUB 180-1, designed by the U.S.
  National Security Agency (NSA)
• To be used in the Digital Signature Algorithm
  (DSA) part of the Digital Signature Standard
  (DSS)
• Input data length is less than 264 bits
• Message digest is 160 bits

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Digital Signature

• A digital signature has functions similar to
  those of conventional signature
• Support authentic messages
• Signer of document can be confirmed
• Contents of a signed document can be verified

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Digital Signature Generation

• A widely adopted scheme is based on hash function
  and public key encryption

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Digital Signature Verification
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Public Key Infrastructure

• How to give your public key to your friend?
• How can you be sure that the public key you
  obtain is indeed your friends public key?
• For a small number of mutually trusted users, a
  web of trust system is O.K.

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Web of Trust
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Certification Authority

• For a large population of users, a central
  trusted party can act as a Certification
  Authority (CA)
• Users may deposit their public keys in a CA who
  they trust
• The CA may pass out the public keys to any user
  who need them in certificates

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A CA Supporting Many Users
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Certificate

• A certificate for a user (also called a
  subscriber) contains the users particulars and
  the users public key
• The certificate is an electronic document signed
  by the CA who issue it

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Certificate
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Revocation

• A user may revoke the validity of his/her
  certificate before the actual expiry date
• Revocation information about a CAs subscribers
  are published in a Certificate Revocation List
  (CRL)

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Public Key Infrastructure

• When there are many CAs and many subscribers, a
  hierarchy can be formed linking all the CAs and
  the subscribers
• This form a public key infrastructure
• The subscribers can communicate securely by using
  digital signature techniques

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Public Key Infrastructure
CA 3
CA 2
CA 4
CA 1
user 6
user 1
user 2
user 3
user 4
user 5