CSCE 715: Network Systems Security

Transcription

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

2 Next Topic in Cryptographic Tools Symmetric key encryption Asymmetric key encryption Hash functions and message digest Nonce 02/13/2017 2

3 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 02/13/2017 3

4 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; use redundancy or a checksum to detect any modification 02/13/2017 4

5 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 the ability to distinguish intelligible messages from corrupted messages But at cost of two public-key uses on message 02/13/2017 5

6 Providing Msg Authentication by Asymmetric Encryption 02/13/2017 6

7 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 1) message is unaltered and 2) comes from claimed sender 02/13/2017 7

8 Uses of MAC 02/13/2017 8

9 MAC Properties Like a 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 02/13/2017 9

10 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 02/13/

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 could be too small for security 02/13/

12 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 is used to create a digital signature 02/13/

13 Uses of Hash Functions 02/13/

14 Uses of Hash Functions 02/13/

15 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 02/13/

16 Requirements for Hash Functions 02/13/

17 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 02/13/

18 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-inthe-middle attack Other variants also susceptible to attack 02/13/

19 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 /13/

20 Example with 2 37 Variations 02/13/

21 Hash Algorithm Structure 02/13/

22 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 Specified as Internet standard RFC /13/

23 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 02/13/

24 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 02/13/

25 Revised Secure Hash Standard NIST issued revision FIPS in 2002 Adds 3 additional versions of SHA SHA-256, SHA-384, SHA-512: collectively known as SHA-2 Designed for compatibility with increased security provided by the AES cipher Structure and detail are similar to SHA-1 Hence analysis should be similar But security levels are rather higher 02/13/

26 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 02/13/

27 SHA-512 Overview 02/13/

28 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 K t based on cube root of first 80 prime numbers 02/13/

29 SHA-512 Round Function 02/13/

30 SHA-512 Round Function 02/13/

31 SHA-3 SHA-2 still shares the same structure and mathematical operations as its predecessors so this is a cause for concern NIST announced in 2007 a competition for SHA-3, next generation NIST hash function Winning design was announced by NIST in October 2012 SHA-3 is a cryptographic hash function that is intended to complement SHA-2 as the approved standard for a wide range of applications 02/13/

32 The Sponge Construction Underlying structure of SHA-3 is a scheme referred to by its designers as a sponge construction Takes an input message and partitions it into fixedsize blocks Each block is processed in turn with the output of each iteration fed into the next iteration, finally producing an output block The sponge function is defined by three parameters: f : internal function used to process each input block r : size in bits of the input blocks, called the bitrate pad : the padding algorithm 02/13/

33 Sponge Function 02/13/

34 02/13/

35 SHA-3 Parameters 02/13/

36 SHA-3 Iteration Function 02/13/

37 SHA-3 Step Functions 02/13/

38 Security of Hash Functions and MAC Brute-force attacks strong collision resistance hash have cost 2 m / 2 have proposal for hardware MD5 cracker 128-bit hash is vulnerable, 160-bit is better but could get compromised soon MACs with known message-mac pairs can either attack keyspace or MAC at least 128-bit MAC is needed for security 02/13/

39 Security of Hash Functions and MAC Cryptanalytic attacks exploit structure like block ciphers, we want brute-force attacks to be the best alternative for attacker Have a number of analytic attacks on iterated hash functions CV i = f[cv i-1, M i ]; H(M)=CV N typically focus on collisions in function f like block ciphers, often composed of rounds attacks exploit properties of round functions 02/13/

40 Keyed Hash Functions as MACs Desirable to create a MAC using a hash function rather than a block cipher hash functions are generally faster not limited by export controls on block ciphers Hash includes a key along with the message Original proposal: KeyedHash = Hash(Key Message) some weaknesses were found with this proposal Eventually led to development of HMAC 02/13/

41 HMAC Specified as Internet standard RFC2104 Use hash function on the message: HMAC K = Hash[(K + XOR opad) Hash[(K + XOR ipad) M)]] K + is the key padded out to size opad, ipad are specified padding constants Overhead is just 3 more hash compression function calculations than the message alone needs Any of MD5, SHA-1, SHA-2, RIPEMD-160 can be used 02/13/

42 HMAC Structure 02/13/

43 Security of HMAC Security of HMAC relates to that of the underlying hash algorithm Attacking HMAC requires either: brute force attack on key used birthday attack (but since keyed would need to observe a very large number of messages) Choose hash function used based on speed versus security constraints 02/13/

44 Summary of Hash Functions and MAC Hash Functions condense arbitrary size message to fixed size by processing message in blocks through some compression function either custom or block cipher based Message Authentication Code (MAC) fixed sized authenticator for some message to provide authentication for message by using block cipher chaining mode or hash function 02/13/

45 Next Class Replay attacks Timestamps and nonces Anti-replay protocols 02/13/