MESSAGE AUTHENTICATION and HASH FUNCTIONS - Chapter 11

• Masquerade – message insertion, fraud, ACK • Content Modification • Sequence Modification – insertion, deletion, re-ordering • Timing Modification – delay, replay MESSAGE AUTHENTICATION and HASH FUNCTIONS - Chapter 11

• Message Encryption – EK (M) • Message Authentication Code (MAC) – CK(M) • Hash Function – H(M) AUTHENTICATION

BASIC USES OF MESSAGE ENCRYPTION

INTERNAL AND EXTERNAL ERROR CONTROL

Fig 11.1a : Legitimacy test at B (intelligible) - small subset of plaintext legitimate - structured Fig 11.2a : Structured redundancy via FCS - internal ECC - authentication Fig 11.2b : External ECC – opponent can construct code words - authentication Any ’structure’ will do e.g. Fig 11.3 STRUCTURE

BASIC USES OF MESSAGE ENCRYPTION

Fig 11.1b : Confidentiality Fig 11.1c : Authentication - plaintext needs structure Signature - only A could have sent, not even B Fig 11.1 : Confidentality / Authentication Table 11.1 PUBLIC-KEY

TCP SEGMENT

BASIC USES of MESSAGE AUTHENTICATION CODE (MAC)

A, B share key, K MAC =CK(M) Transmit message + MAC (Fig 11.4a) MAC not necessarily reversible - less vulnerable than encryption MAC

BASIC USES of MESSAGE AUTHENTICATION CODE (MAC)

Figs 11.4b and 11.4c - Two separate keys (Table 11.2) - Fig 11.4b preferred Use MAC, not conventional Encryption - MAC gives no signature - sender/receiver share key Authentication + Confidentiality

Broadcast message – one destination monitors authenticity • 2. Heavy load – selective authentication • 3. SporadicAuthentication of computer program • 4. Secrecy Unimportant • 5. Separation of authentication and confidentiality • - flexible • 6. Prolong protection against modification Authentication + ConfidentialitySCENARIOS

BASIC USES OF HASH FUNCTION

HASH FUNCTIONS variable size  fixed size M  H(M)  M|H(M) (error detection) Fig 11.5 – Table 11-3 (b) and (c) require less computation (e) - no encryption

FOR AUTHENTICATION: COMPARE HASH WITH ENCRYPTION • Encryption is: • Slow • Costly in hardware • Optimised for large data blocks • Patented • Export control

MAC MAC = CK(M) many-to-one, domain is arbitrary length Attack: MAC collisions : 2k keys, 2n MACs, 2n < 2k Many keys for one MAC : opponent cannot choose Opponent must iterate attack for many MACs: Round 1 : 2k-n keys Round 2 : 2k-2n keys .. .. .. Round r : 1 key

MAC PROPERTIES Given M and CK(M), too much work to construct M’ such that, CK(M’) = CK(M) 2. CK(M) uniformly distributed: pr(CK(M) = CK(M’)) = 2-n

DATA AUTHENTICATION ALGORITHM (CBC Mode)

HASH FUNCTIONS h = H(x) - file fingerprint Properties: 1. Any size input 2. Fixed-size output 3. H(x) easy to compute 4. Infeasible to compute x given h – (one-way) – 2n 5. (Weak Collision Resistance) – 2n Given x, infeasible to compute y not equal to x such that, H(y) = H(x) - prevents forgery 6. (Strong Collision Resistance) – 2n/2 Infeasible to find (x,y) such that H(x) = H(y) - Birthday Attack

BIRTHDAY ATTACK Given M , find M’ such that H(M’) = H(M) ~ 2n-1 hashes But (Fig 11.5c), • Prepare 2n/2 variations of M • Prepare 2n/2 variations of M’ • Search for H(M) = H(M’) • Pr(success) > 0.5 using 2n/2 hashes • A signs M  H(M) • Opponent substitutes M’ for M • A encrypts M’|H(M)

MEET-IN-THE-MIDDLE ATTACK • Block Chaining Given M = M1 | M2 | ………| MN H0 = init Hi = EMi[Hi-1] G = HN Opponent has M and encrypted signature, G • Construct arbitrary message Q1 | Q2 | …….| QN-2 • Compute Hi = EQi[Hi-1] up to HN-2 • Find X,Y such that EX[HN-2] = DY[G] (prob 2n/2) • Construct Q1 | Q2 | ….| QN-2 | X | Y = M’ • Substitute M’ for M

BRUTE-FORCE ATTACKS Hash : 2n/2 MAC : min(2k,2n) - like symmetric encryp.

SECURE HASH CODE If compression function collision-resistant then so is iterated hash function

THE BIRTHDAY PARADOX

MESSAGE AUTHENTICATION and HASH FUNCTIONS - Chapter 11

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