s-prg - STREAM CIPHERS and PRGs 1 / 16 Stateful Generators...

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Unformatted text preview: STREAM CIPHERS and PRGs 1 / 16 Stateful Generators Initially, St is a random seed G St St X Operation: G G G St[1] St[2] St[3] St[0] X[1] X[2] X[3] X [1] X [2] X [3] ... is the output sequence and should be “pseudorandom”. 2 / 16 Notation ( X [1] . . . X [ m ] , St ) ← G ( St , m ) means we • Run G with starting state St for m steps • Let X [1] . . . X [ m ] be the output blocks produced • Let St be the updated state G X[1] St G St X[m] ... 3 / 16 Usage for Encryption Alice maintains a state St A and Bob maintains a state St B . Initially: St A = St B is a random seed. E ( M [1] . . . M [ m ]) ( X [1] . . . X [ m ] , St A ) ← G ( St A , m ) for i = 1 , . . . , m do C [ i ] ← X [ i ] ⊕ M [ i ] D ( C [1] . . . C [ m ]) ( X [1] . . . X [ m ] , St B ) ← G ( St B , m ) for i = 1 , . . . , m do M [ i ] ← X [ i ] ⊕ C [ i ] Note that the states must be synchronized! 4 / 16 Usage for Pseudorandom Bit Generation G is initialized with a random seed and its outputs are then used coins for any purpose needing randomness, including: • Keys • IVs for block-cipher based encryption • Nonces • Simulations 5 / 16 Methods...
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This note was uploaded on 08/31/2011 for the course CSE 207 taught by Professor Daniele during the Winter '08 term at UCSD.

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s-prg - STREAM CIPHERS and PRGs 1 / 16 Stateful Generators...

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