Information Security and Management 11 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 Timing modification Source repudiation Destination repudiation

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

Message Encryption (A) Conventional encryption: confidentiality and authentication

Message Encryption (B) Public-key encryption: confidentiality

Message Encryption (C) Public-key encryption: authentication and signature

Message Encryption (D) Public-key encryption: confidentiality, authentication And signature

Table 11.1 (1)

Table 11.1 (2)

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

Error Control External error control An opponent can construct messages with valid error-control codes

Example of TCP Segment The receiver can be assured of the proper sequence because an attacker cannot successfully alter the sequence number

TCP-level Encryption

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

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

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

MAC (4) Message authentication

MAC (5) Message authentication and confidentiality; Authentication tied to plaintext

MAC (6) Message authentication and confidentiality; Authentication tied to ciphertext

Basic Uses of MAC (Table 11.2)

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.

Requirements for MACs (1)

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.

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

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

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

Hash Function PlaintextM Message Digest Hash value H(M)

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 yx 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.

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)

Basic Use of Hash (A)

Basic Use of Hash (B)

Basic Use of Hash (C)

Security of Hash Functions For a code of length n One-way: 2n Weak collision resistance: 2n Strong collision resistance: 2n/2

The Famous Hash Functions MD5 SHA

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 (67452301,efcdab89,98badcfe,10325476,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

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

SHA-1 Compression Function

SHA-1 Compression Function

Function Summarized

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

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)

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.

Comparison of SHA Properties