chapter-5 - CS549 Cryptography and Network Security by...

Cryptography and Network Security
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CS549:
Cryptography and Network
Security
© by Xiang-Yang Li
Department of Computer Science,
IIT

Cryptography and Network Security
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Notice©
This lecture note (Cryptography and Network Security) is prepared by
Xiang-Yang Li. This lecture note has benefited from numerous
textbooks and online materials. Especially the “Cryptography and
Network Security” 2 nd edition by William Stallings and the
“Cryptography: Theory and Practice” by Douglas Stinson.
You may not modify, publish, or sell, reproduce, create derivative
works from, distribute, perform, display, or in any way exploit any
of the content, in whole or in part, except as otherwise expressly
permitted by the author.
The author has used his best efforts in preparing this lecture note.
The author makes no warranty of any kind, expressed or implied,
with regard to the programs, protocols contained in this lecture
note. The author shall not be liable in any event for incidental or
consequential damages in connection with, or arising out of, the
furnishing, performance, or use of these.

Cryptography and Network Security
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Cryptography and Network
Key Management and generation
Xiang-Yang Li

Cryptography and Network Security
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Key Exchange

Public key systems are much slower than
private key system

Public key system is then often for short data
 Signature, key distribution

Key distribution

One party chooses the key and transmits it to other user

Key agreement

Protocol such two parties jointly establish secret key
over public communication channel

Key is the function of inputs of two users

Cryptography and Network Security
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Distribution of Public Keys

can be considered as using one of:

Public announcement

Publicly available directory

Public-key authority

Public-key certificates

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Public Key Management

Simple one: publish the public key

Such as newsgroups, yellow-book, etc.

But it is not secure, although it is convenient
 Anyone can forge such a announcement
 Ex: user B pretends to be A, and publish a key for A
 Then all messages sent to A, readable by B!

Let trusted authority maintain the keys

Need to verify the identity, when register keys

User can replace old keys, or void old keys

Cryptography and Network Security
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Possible Attacks

Observe all messages over the channel

So assume that all plaintext messages are available to
all

Save messages for reuse later

So have to avoid replay attack

Masquerade various users in the network

So have to be able to verify the source of the message

Cryptography and Network Security
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Public Announcement

users distribute public keys to recipients
or broadcast to community at large

eg. append PGP keys to email messages or post to news
groups or email list

major weakness is forgery

anyone can create a key claiming to be someone else
and broadcast it

until forgery is discovered can masquerade as claimed
user