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**Information Security and Management 11**

Information Security and Management Message Authentication and Hash Functions Chih-Hung Wang Sep. 2008

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**Message Authentication**

Authentication Requirement Possible attacks on the network Disclosure Traffic analysis Masquerade Content modification Sequence modification Timing modification Source repudiation Destination 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 (A) Conventional encryption: confidentiality**

and authentication

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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, authentication And 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 Segment The receiver can be assured of the proper sequence because an attacker cannot successfully alter the sequence 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: Knowing a message and MAC, is infeasible to find another message with same MAC 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. 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 developed by Ron Rivist at MIT Secure Hash Algorithm (SHA) FIPS PUB 180 in 1993 (NIST) 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 Logic Append padding bits: pad message so its length is 448 mod 512 Append length: append a 64-bit length value to message Initialize MD buffer: initialise 5-word (160-bit) buffer (A,B,C,D,E) to ( ,efcdab89,98badcfe, ,c3d2e1f0) 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 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 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**

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