Upload
lobo
View
40
Download
2
Tags:
Embed Size (px)
DESCRIPTION
Information Security and Management 11. Message Authentication and Hash Functions. Chih-Hung Wang Sep. 2008. Message Authentication. Authentication Requirement Possible attacks on the network Disclosure Traffic analysis Masquerade Content modification Sequence modification - PowerPoint PPT Presentation
Citation preview
1
Information Security and Management
11. Message Authentication and Hash Functions
Chih-Hung Wang
Sep. 2008
2
Message Authentication
Authentication Requirement Possible attacks on the network
DisclosureTraffic analysisMasqueradeContent modificationSequence modificationTiming modificationSource repudiationDestination repudiation
3
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
4
Message Encryption
Conventional encryption: confidentiality and authentication
(A)
5
Message Encryption
(B)
Public-key encryption: confidentiality
6
Message Encryption
(C)
Public-key encryption: authentication and signature
7
Message Encryption
(D)
Public-key encryption: confidentiality, authenticationAnd signature
8
Table 11.1 (1)
9
Table 11.1 (2)
10
Error Control
Append an error-detecting code (frame check sequence, FCS) or checksum to each message before encryption
Internal error control
11
Error Control
External error control
An opponent can construct messages with valid error-control codes
12
Example of TCP SegmentThe receiver can be assured of the proper sequencebecause an attacker cannot successfully alter thesequence number
13
TCP-level Encryption
14
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
15
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
16
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
17
MAC (4)
Message authentication
18
MAC (5)
Message authentication and confidentiality;Authentication tied to plaintext
19
MAC (6)
Message authentication and confidentiality;
Authentication tied to ciphertext
20
Basic Uses of MAC (Table 11.2)
21
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.
22
Requirements for MACs (1)
23
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.
24
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
25
DAC
Data Authentication Code (FIPS PUB 113 and ANSI standard X9.17)
26
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
27
Hash Function
PlaintextM
Message Digest
Hash value H(M)
28
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.
29
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)
30
Basic Use of Hash (A)
31
Basic Use of Hash (B)
32
Basic Use of Hash (C)
33
Security of Hash Functions
For a code of length n One-way: 2n
Weak collision resistance: 2n
Strong collision resistance: 2n/2
34
The Famous Hash Functions
MD5 SHA
35
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
36
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
37
SHA-1 Compression Function
38
SHA-1 Compression Function
39
Function Summarized
40
80-word Input Sequence Wt=S1(Wt-16Wt-14 Wt-8 Wt-3)
41
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)
42
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.
43
Comparison of SHA Properties