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Information Security and Management

11. Message Authentication and Hash Functions

Chih-Hung Wang

Sep. 2008

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Message Authentication

Authentication Requirement Possible attacks on the network

DisclosureTraffic analysisMasqueradeContent modificationSequence modificationTiming modificationSource repudiationDestination repudiation

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Authentication Functions

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 fix-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

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Message Encryption

Conventional encryption: confidentiality and authentication

(A)

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Message Encryption

(B)

Public-key encryption: confidentiality

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Message Encryption

(C)

Public-key encryption: authentication and signature

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Message Encryption

(D)

Public-key encryption: confidentiality, authenticationAnd signature

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Table 11.1 (1)

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Table 11.1 (2)

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Error Control

Append an error-detecting code (frame check sequence, FCS) or checksum to each message before encryption

Internal error control

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Error Control

External error control

An opponent can construct messages with valid error-control codes

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Example of TCP SegmentThe receiver can be assured of the proper sequencebecause an attacker cannot successfully alter thesequence number

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TCP-level Encryption

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MAC (1)

The use of a secret key to generate a small fixed-size block of data

That is appended to the message A MAC function is similar to encryption. One

difference is that MAC algorithm need not be reversible

It is less vulnerable to being broken than encryption

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MAC (2)

Three situations in which a message authentication code is used The same message is broadcast to a number of destinations

It is cheaper and more reliable to have only one destination responsible for monitoring authenticity

An exchange: one side has a heavy load and cannot afford the time to decrypt all incoming message.

Message being chosen at random for checking Authentication of a computer program in plaintext is an

attractive service The computer program can be executed without having to

decrypt it every time

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MAC (3)

Other rationales For some applications, it may not be concern to keep

message secret, but it is important to authenticate message

SNMPv3:separates the functions of confidentiality and authentication

Separation of authentication and confidentiality functions affords architectural flexibility

Perform authentication at the application level but to provide confidentiality at a lower level

A user may wish to prolong the period of protection beyond the time of reception and yet allow processing the message content

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MAC (4)

Message authentication

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MAC (5)

Message authentication and confidentiality;Authentication tied to plaintext

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MAC (6)

Message authentication and confidentiality;

Authentication tied to ciphertext

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Basic Uses of MAC (Table 11.2)

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MAC Function

A MAC function is similar to encryption. One difference is that the MAC algorithm need not be reversible, as it must for decryption.

In general, the MAC function is a many-to-one function. If an n-bit MAC is used, then there are 2n possible MACs, whereas there are N possible messages with N>>2n.

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Requirements for MACs (1)

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Requirements for MACs (2) Taking into account the types of attacks Need the MAC to satisfy the following:

1. Knowing a message and MAC, is infeasible to find another message with same MAC

2. If we assume that the opponent does not know k but does have access to the MAC function and can present messages for MAC generation, then the opponent could try various messages until finding one that matches a given MAC. MACs should be uniformly distributed. A brute-force method would require, on average, 2(n-1) attempts.

3. The MAC should not be weaker with respect to certain parts or bits of the message than others.

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Using Symmetric Ciphers for MACs

Can use any block cipher chaining mode and use final block as a MAC

Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC using IV=0 and zero-pad of final block encrypt message using DES in CBC mode and send just the final block as the MAC

or the leftmost M bits (16≤M≤64) of final block

but final MAC is now too small for security

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DAC

Data Authentication Code (FIPS PUB 113 and ANSI standard X9.17)

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Hash Function

Definition A hash function accepts a variable-size message M as

input and produces a fixed-size hash code H(M) Sometime called a message digest Hash Algorithm

MD5 RFC 1321 developed by Ron Rivist at MIT

Secure Hash Algorithm (SHA) FIPS PUB 180 in 1993 (NIST) 180-1 in 1995 FISP: Federal Information Processing Standard

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Hash Function

PlaintextM

Message Digest

Hash value H(M)

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Requirements of Hash H can be applied to a block of 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. This is sometimes referred to in the literature as the one-way property

For any given block x, it is computationally infeasible to find yx with H(y)=H(x). This is sometimes referred to as weak collision resistance

It is computationally infeasible to find any pair (x,y) such that H(x)=H(y). This is sometimes referred to as strong collision resistance.

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Requirements of Hash

m1

m2

H(m1)

H(m2)

It is difficult to find m1 and m2 (m1 m2) such that H(m1)=H(m2)

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Basic Use of Hash (A)

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Basic Use of Hash (B)

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Basic Use of Hash (C)

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Security of Hash Functions

For a code of length n One-way: 2n

Weak collision resistance: 2n

Strong collision resistance: 2n/2

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The Famous Hash Functions

MD5 SHA

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SHA-1 Logic1. Append padding bits: pad message so its length is 448 mod

512 2. Append length: append a 64-bit length value to message3. Initialize MD buffer: initialise 5-word (160-bit) buffer

(A,B,C,D,E) to (67452301,efcdab89,98badcfe,10325476,c3d2e1f0)

4. Process message in 512-bit (16-word) blocks: expand 16 words into 80 words by mixing & shifting use 4 rounds of 20 bit operations on message block & buffer add output to input to form new buffer value

5. Output: output hash value is the final buffer value

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SHA-1 Compression Function

Each round has 20 steps which replaces the 5 buffer words thus:(A,B,C,D,E) <-(E+f(t,B,C,D)+S5(A)+Wt+Kt),A,S30(B),C,D)

A,B,C,D,E refer to the 5 words of the buffer t is the step number, 0 t 79 f(t,B,C,D) is nonlinear function for round Wt is derived from the message block Kt is an additive constant value Sk is circular left shift by k bits

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SHA-1 Compression Function

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SHA-1 Compression Function

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Function Summarized

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80-word Input Sequence Wt=S1(Wt-16Wt-14 Wt-8 Wt-3)

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Comparison of SHA-1 and MD5

Brute force attack for SHA-1 is harder (160 vs 128 bits for MD5)

SHA-1 is not vulnerable to any known attacks (compared to MD4/5) ??

(Speed) SHA-1 is a little slower than MD5 (80 vs 64 steps)

Both designed is simple and compact SHA-1 uses big endian scheme (MD5 uses

little endian scheme)

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Revised Secure Hash Standard

NIST have issued a revision FIPS 180-2 and adds 3 additional hash algorithms: SHA-256, SHA-384, SHA-512.

Designed for compatibility with increased security provided by the AES cipher

Structure & detail are similar to SHA-1 and hence analysis should be similar.

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Comparison of SHA Properties