W1S1-2
1. Anything which can be done with a Trusted authority can be done without it through some secret protocol communication among all the parties.2. If there are 2 Random variables with uniform distribution, their XOR is also a uniform distribution.
3. Birthday paradox - 1.2 * sqrt(size(U)) samples would yield 2 distinct elements with same values where size(U) is the size of the entire set. 1.2*sqrt(365) = 24 people in a room would yield 2 people with same birthday. 2^64 samples of 128 bit numbers would yield 2 same numbers. Probability of this happening is >= 0.5
W1S3
1. Definition of perfect secrecy (E,D) over (K,M,C), Pr [ E(k,m0) = c] = Pr[E(k,m1) = c] given that |m0| = |m1|. In other words, CT only attacks are not possible. So One Time Pad (OTP) as perfect secrecy. OTP is simply m XOR k = c.
2. Perfect secrecy also requires that len(k) >= len(m) . OTP satisifies this with equality. So OTP is not practical since if you can transmit the key securely, you can as well transmit the message securely as well(they are the same length).
How to make OTP more secure with stream ciphers?
How to make OTP more secure with stream ciphers?
1. PRG but PRG must be unpredictable. Predictable means that given first few bits of PRG output I can deduce the rest of the bits. If that's so, if the attacker knows first few bits of m and sees the CT, by XORing can get first few bits of PRG output. From those first few bits, can generate rest of the bits.
2. Weak PRGs - A. glibc random() B. LCG
3. Negligible/non negligible epsilon corresponds to polynomial/exponential
No comments:
Post a Comment