This document appears to be notes from an information security course. It discusses several methods for generating and sharing secret keys securely, including using an online trusted third party, Merkle puzzles, and the Diffie-Hellman key exchange protocol. The notes cover how each method works at a high level and their advantages and disadvantages. The document focuses on generating mutual keys between two parties without needing an online trusted third party.

1. Hoang Nguyen Van
Mail: hoangnvvnua@gmail.com
Department of Computer Science – FITA – HUA
Information Security Course --------------------------------------------- Fall 2013
Dept. of Computer Science – FITA – HUA

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Hoang Nguyen Van
How I can share with my boyfriend, a secret key.

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Hoang Nguyen Van
How to manage mutual keys

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Hoang Nguyen Van
I hate my keys

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Hoang Nguyen Van

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Online Trusted 3rd Party (TTP)
O(n) keys per user

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Hoang Nguyen Van
Online Trusted 3rd Party (TTP)
O(1) one key per user
kAB

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Online Trusted 3rd Party (TTP)
Generating keys: a toy protocol
“Alice wants key with Bob”
(kA)
choose
random kABE(kA,”AB”||kAB)
ticket = E(kA,”AB”||kAB)
(kB)
ticket
kAB kAB
(Eavesdropping security only)

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Can we generate shared keys without
an online trusted 3rd party?

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Ralph Merkle (1952)

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Merkle Puzzles (1974)

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Merkle Puzzles (1974)
“Puzzle # xi” ll ki

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Merkle Puzzles (1974)
BobAlice
puzzle1 , … , puzzlen
xj
kj kj
O(n)
O(n)
O( n2 )
But Infeasible

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The Diffie-Hellman protocol

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2
5
Very easy
Very hard
One-way function

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10 10
510 310
10 5
3
10 5
3
310 510
KEY
Alice Bob
The Diffie-Hellman protocol

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Hoang Nguyen Van
The Diffie-Hellman protocol
Alice Bob
choose random a in {1,…,p-1} choose random b in {1,…,p-1}
kAB = gab (mod p) = (ga)
b
= Ab (mod p)Ba (mod p) = (gb)
a
=

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