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VIII' MESSAGE AUTHENTICATION AND HASH FUNCTIONS

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Message encryption by itself can provide a measure of authentication. E. M. M. D. K. K. EK(M) E. M. M. D. KU. KR. EKU (M) b. b. b ... – PowerPoint PPT presentation

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Title: VIII' MESSAGE AUTHENTICATION AND HASH FUNCTIONS


1
VIII. MESSAGE AUTHENTICATION AND HASH FUNCTIONS
  • Introdution to the requirements for
    authentication and digital signature and the
    types of attacks to be countered. Then the basic
    approaches are surveyed.

2
1.AUTHENTICATION REQUIREMENTS
  • Disclosure Release of message contents to any
    person or process not possessing the appropriate
    cryptographic key.
  • Traffic analysis Discovery of the pattern of
    traffic between parties.
  • Masquerade Insertion of message into the
    network from a fraudulent source.
  • Content modification Changes to the contents of
    a message, including insertion, deletion,
    transposition, and modification.

3
1.AUTHENTICATION REQUIREMENTS
  • Sequence modification Any modification to a
    sequence of messages between parties, including
    insertion, deletion, and reordering.
  • Timing modification Delay or replay of
    messages.
  • Repudiation Denial of receipt of message by
    destination or denial of transmission of message
    by source.

4
2.AUTHENTICATION FUNCTIONS
  • Authentication functions may be grouped into
    three classes
  • Message encryption The ciphertext of the entire
    message serves as its authenticator.
  • Message authentication code (MAC) A public
    function of the message and a secret key that
    produces a fixed-length value that serves as the
    authenticator.
  • Hash function A public function that maps a
    message of any length into a fixed-length hash
    value, which serves as the authenticator.

5
2.AUTHENTICATION FUNCTIONS
  • Message Encryption
  • Message encryption by itself can provide a
    measure of authentication.

6
2.AUTHENTICATION FUNCTIONS
Source
Destination
M
M
E
D
(c) Public-key encryption authentication and
signature
M
M
E
D
D
E
(d) Public-key encryption confidentiality,
authentication and signature
Basic Uses of Message Encryption
7
2.AUTHENTICATION FUNCTIONS
  • Message Authentication Code
  • generated a small fixed-size block of data by use
    of a secret key

8
2.AUTHENTICATION FUNCTIONS
9
2.AUTHENTICATION FUNCTIONS
  • Hash function
  • accepts a variable-size message M as input and
    produces a fixed-size hash code H(M)
  • The hash code is a function of all the bits of
    the message and provides an error-detection
    capability

10
2.AUTHENTICATION FUNCTIONS
11
2.AUTHENTICATION FUNCTIONS
12
2.AUTHENTICATION FUNCTIONS
13
3.MESSAGE AUTHENTICATION CODES
  • Requirements for MACs
  • If an opponent observers M and Ck(M), it should
    be computationally infeasible for the opponent to
    construct a message M such that Ck(M) Ck(M)
  • Ck(M) should be uniformly distributed In the
    sense that for randomly chosen messages, M and
    M, the probability that Ck(M) Ck(M) is 2-n,
    where n is the number of bits in the MAC.
  • Let M be equal to some known transformation on
    M. That is M f(M). For example, f may involve
    inverting one or more specific bits.In that case,
    PrCk(M) Ck(M) 2-n.

14
3.MESSAGE AUTHENTICATION CODES
  • Message Authentication Code Based on DES
  • ANSI Standard X9.17 MAC

15
4.HASH FUNCTIONS
  • Requirement for a Hash Function
  • H can be applied to a block data of any size.
  • H produces a fixed-length output.
  • H(x) is relatively easy to compute for any given
    x, making both hardware and software
    implementations practical.
  • For any given code h, it is computationally
    infeasible to find x such that H(x) h. (one-way
    property)
  • For any given block x, it is infeasible to find y
    ? x with H(y) H(x). (weak collision resistance)
  • It is infeasible to find any pair (x, y) such
    that H(x) H(y). (strong collision resistance)

16
4.HASH FUNCTIONS
  • Simple Hash Function
  • Ci Ith bit of the hash code, 1lt I ltn
  • m number of n-bit blocks in the input
  • bij ith bit in jth block
  • XOR operation

17
4.HASH FUNCTIONS
  • Birthday Attacks
  • Message M
  • Find an M such that H(M) H(M)
  • On average, the opponent would have to try about
    263 message to find one that matches the hash
    code of the intercepted message
  • By the birthday paradox, the level of effort
    required is only on the order of 232

18
4.HASH FUNCTIONS
  • Block Chaining Techniques
  • Based on using a cipher block chaining technique
    without secrete key
  • Meet in the middle attack

19
5.SECURITY OF HASH FUNCTIONS AND MACS
  • Brute-Force Attacks
  • Hash Function
  • One Way -gt 2n
  • Weak collision resistance -gt 2n
  • Strong collision resistance -gt2n/2
  • Message Authentication Codes
  • Min(2k, 2n)
  • k key length
  • n MAC length

20
5.SECURITY OF HASH FUNCTIONS AND MACS
  • Cryptanalysis
  • An ideal hash of MAC algorithm require a
    cryptanalytic effort greater than or equal to the
    brute-force effort.
  • Hash Functions
  • Focuses on the internal structure of f
  • Based on attempts to find efficient techniques
    for producing collisions for a single execution
    of f
  • Message Authentication Codes
  • Difficult to generalize
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