CSCE 715: Network Systems Security Chin-Tser Huang University of South Carolina

02/11/20092 Next Topic in Cryptographic Tools Symmetric key encryption Asymmetric key encryption Hash functions and message digest Nonce

02/11/20093 Message Authentication Message authentication is concerned with protecting the integrity of a message validating identity of originator non-repudiation of origin (dispute resolution) Three alternative functions to provide message authentication message encryption message authentication code (MAC) hash function

02/11/20094 Providing Msg Authentication by Symmetric Encryption Receiver knows sender must have created it because only sender and receiver know secret key Can verify integrity of content if message has suitable structure, redundancy or a checksum to detect any modification

02/11/20095 Providing Msg Authentication by Asymmetric Encryption Encryption provides no confidence of sender because anyone potentially knows public key However if sender encrypts with receiver’s public key and then signs using its private key, we have both confidentiality and authentication Again need to recognize corrupted messages But at cost of two public-key uses on message

02/11/20096 Providing Msg Authentication by Asymmetric Encryption

02/11/20097 Message Authentication Code (MAC) Generated by an algorithm that creates a small fixed-sized block depending on both message and some key like encryption though need not to be reversible Appended to message as a signature Receiver performs same computation on message and checks if it matches the MAC Provide assurance that message is unaltered and comes from claimed sender

02/11/20098 Uses of MAC

02/11/20099 MAC Properties Cryptographic checksum MAC = C K (M) condenses a variable-length message M using a secret key K to a fixed-sized authenticator Many-to-one function potentially many messages have same MAC make sure finding collisions is very difficult

02/11/ Requirements for MACs Should take into account the types of attacks Need the MAC to satisfy the following: 1. knowing a message and MAC, it is infeasible to find another message with same MAC 2. MACs should be uniformly distributed 3. MAC should depend equally on all bits of the message

02/11/ Using Symmetric Ciphers for MAC 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

02/11/ Hash Functions Condense arbitrary message to fixed size Usually assume that the hash function is public and not keyed Hash value is used to detect changes to message Can use in various ways with message Most often to create a digital signature

02/11/ Uses of Hash Functions

02/11/ Uses of Hash Functions

02/11/ Hash Function Properties Hash function produces a fingerprint of some file/message/data h = H(M) condenses a variable-length message M to a fixed-sized fingerprint Assumed to be public

02/11/ Requirements for Hash Functions 1. can be applied to any sized message M 2. produce fixed-length output h 3. easy to compute h=H(M) for any message M 4. one-way property: given h, is infeasible to find x s.t. H(x)=h 5. weak collision resistance: given x, is infeasible to find y s.t. H(y)=H(x) 6. strong collision resistance: infeasible to find any x,y s.t. H(y)=H(x)

02/11/ Simple Hash Functions Several proposals for simple functions Based on XOR of message blocks Not secure since can manipulate any message and either not change hash or change hash also Need a stronger cryptographic function

02/11/ Block Ciphers as Hash Functions Can use block ciphers as hash functions use H 0 =0 and zero-pad of final block compute H i = E M i [H i-1 ] use final block as the hash value similar to CBC but without a key Resulting hash is too small (64-bit) both due to direct birthday attack and to “meet-in- the-middle” attack Other variants also susceptible to attack

02/11/ Birthday Attacks Might think a 64-bit hash is secure However by Birthday Paradox is not Birthday attack works as follows given hash code length is m, adversary generates 2 m / 2 variations of a valid message all with essentially the same meaning adversary also generates 2 m / 2 variations of a desired fraudulent message two sets of messages are compared to find pair with same hash (probability > 0.5 by birthday paradox) have user sign the valid message, then substitute the forgery which will have a valid signature If 64-bit hash code is used, level of attack effort is only on the order of 2 32

02/11/ Example with 2 37 Variations

02/11/ Hash Algorithm Structure

02/11/ MD5 Designed by Ronald Rivest (the R in RSA) Latest in a series of MD2, MD4 Produce a hash value of 128 bits (16 bytes) Was the most widely used hash algorithm in recent times have both brute-force and cryptanalytic concerns Specified as Internet standard RFC1321

02/11/ Security of MD5 MD5 hash is dependent on all message bits Rivest claims security is good as can be However known attacks include Berson in 1992 attacked any 1 round using differential cryptanalysis (but can’t extend) Boer & Bosselaers in 1993 found a pseudo collision (again unable to extend) Dobbertin in 1996 created collisions on MD compression function (but initial constants prevent exploit) Wang et al announced cracking MD5 on Aug 17, 2004 (paper available on Useful Links) Thus MD5 has become vulnerable

02/11/ Secure Hash Algorithm SHA originally designed by NIST & NSA in 1993 Was revised in 1995 as SHA-1 US standard for use with DSA signature scheme standard is FIPS , also Internet RFC3174 Based on design of MD4 but with key differences Produces 160-bit hash values Recent 2005 results (Wang et al) on security of SHA- 1 have raised concerns on its use in future applications

02/11/ Revised Secure Hash Standard NIST issued revision FIPS in 2002 Adds 3 additional versions of SHA SHA-256, SHA-384, SHA-512 Designed for compatibility with increased security provided by the AES cipher Structure and detail similar to SHA-1 Hence analysis should be similar But security levels are rather higher

02/11/ SHA-512 Overview 1. pad message so its length is 896 mod 1024 padding length between 1 and append a 128-bit length value to message 3. initialize 8 64-bit registers (A,B,C,D,E,F,G,H) 4. process message in 1024-bit blocks: expand bit words into 80 words by mixing & shifting 80 rounds of operations on message block & buffer add output to input to form new buffer value 5. output hash value is the final buffer value

02/11/ SHA-512 Overview

02/11/ SHA-512 Compression Function Heart of the algorithm Processing message in 1024-bit blocks Consists of 80 rounds updating a 512-bit buffer using a 64-bit value W t derived from the current message block and a round constant based on cube root of first 80 prime numbers

02/11/ SHA-512 Round Function

02/11/ SHA-512 Round Function

02/11/ Whirlpool Endorsed by European NESSIE project Uses modified AES internals as compression function Addressing concerns on use of block ciphers seen previously With performance comparable to dedicated algorithms like SHA

02/11/ Whirlpool Overview

02/11/ Whirlpool Block Cipher W Designed specifically for hash function use With security and efficiency of AES But with 512-bit block size and hence hash Similar structure & functions as AES but input is mapped row wise has 10 rounds a different primitive polynomial for GF(2^8) uses different S-box design & values

02/11/ Whirlpool Block Cipher W

02/11/ Whirlpool Performance & Security Whirlpool is a very new proposal Hence little experience with use But many AES findings should apply Does seem to need more h/w than SHA, but with better resulting performance in terms of throughput

02/11/ Next Class Replay attacks Timestamps and nonces Anti-replay protocols