Establishing Public and secret keys

1. 18CSE354T – NETWORK SECURITY
Unit – III : Session – 2 : SLO -2
Establishing Public and secret keys
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2. SRM Institute of Science and Technology 2
Establishing Public keys
Establishing Public Keys If Alice wants to send a signed message to Bob, she can just
sign the message and send it, either hoping Bob will already have her certificate, can
obtain it if necessary, or she can include her certificate in the email message. However,
if she wants to send an encrypted message to Bob, she needs to know his public key
before she can construct the message. There are various methods by which she may
discover Bob’s public key
• she might have received Bob’s public key through some secure out-of-band
mechanism, and installed it on her workstation
• she might obtain it through a PKI (e.g., looking it up in a directory) (see Chapter 13
PKI (Public Key Infrastructure))
• The email system could allow piggybacking of certificates (and perhaps CRLs) on
email messages. Alice can send her certificates by sending Bob a signed message. If
she doesn’t already know Bob’s public key, she can request that he send her a signed
email message with his certificates (and perhaps relevant CRLs) attached.

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How can Alice and Bob establish a shared secret key for email? The simplest way
is with some other means of private communication, for instance by meeting in
person in a private place, or by talking on the phone (if they aren’t paranoid about
the phone being tapped). This strategy is OK for a few scattered private parties,
but doesn’t scale well at all.
A more scalable strategy would be for Alice to obtain a ticket for Bob from a KDC (
(with KDC), and include that ticket with her first message to Bob
Establishing Secret Keys

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Reference
1. Charlie Kaufman, Radia Perlman, Mike Speciner, Network
Security, Prentice Hall of India, 2002.