Q: If step A takes 10 bits of work, and step B takes 20 bits, then how much to do both steps A then B?

Answer: 20 bits.

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8XXXXXXXXXXXXXXXXXXXXX
+   XXXXXXXXXXXXXXXXXX
----------------------
XXXXXXXXXXXXXXXXXXXXXX

Bits of Security

// Bits of security // Bits of work

26! -> 4 x 10^26 ~90 bits

— Brute force smartly

MITM Attacks

-> Replay attack
Capture the information, and retransmit it

Challenge Response :: Date / Time, ID.
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Birthday attack

Collision is higher than you think

sqrt of the hash size

ie in 52 cards, sqrt(52) ~= 7; chance of collision after 7 cards