Security_Part2_Public Key Cryptography - Oregon State University

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21 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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Network Security

Part 2

Public Key Cryptography

V.T. Raja, Ph.D.,

Oregon State University

Network Security

Part 2


Public Key Cryptography

Public keys and Private keys

RSA Algorithm


Authentication Protocol (

ap 1.0, 2.0, 3.0, 3.1, 4.0, 5.0

Exchanging Public Keys

Man (Woman) in the middle


Public Key Cryptography

Disadvantage of symmetric key cryptography?

Until 1970s encryption involved symmetric key

Is it possible for two parties to communicate using
encryption/decryption without using a shared secret key?

A radically different and marvelously elegant approach
towards encryption/decryption

Public key cryptography is useful not only for
encryption/decryption, but also for authentication and
digital signatures as well.

Basic Idea of Public Key Cryptography

Each participant has a private key (known only to the participant)
and a public key.

The public key is created with one’s private key.

Public key is made available to others and could be posted even on
a website which is accessible by the rest of the world.

Public key of recipient is used by sender to encrypt message.

Recipient decrypts message using recipient’s private key.


Alice wishes to send a message to Bob.

Alice fetches Bob’s public key.

Alice uses Bob’s public key to encrypt message

Alice sends encrypted message to Bob.

Bob decrypts cipher text with Bob’s private key.

Notation and Choice of Keys

Assume Alice’s plain text message, (which
has to be encrypted and then sent to Bob)
is denoted as m.

Assume Bob’s public key is denoted as

and his private key is denoted as K

These keys are chosen such that:



(m)) = K


(m)) = m

RSA Algorithm

How does RSA work?

Class Participation Exercise on RSA application

Why does RSA work?

(See MS Word handout for answers to above

RSA Algorithm

Even for small p and q, as seen in the
participation exercise, we had to deal with
extremely large numbers.

If we follow the suggestion of RSA labs and
select p and q to be several hundred bits long,
then the following practical issues come to mind:

How to choose large prime numbers p and q?

How to choose e and d?

How to perform exponentiation with large numbers?

(For those who are interested in this area, refer to
Kaufman 1995 for answers to the above
mentioned questions).


DES is at least 100 times faster than RSA.

In practice, RSA is often used in
combination with DES or AES. How?

(Alice encrypts DES key with Bob’s public
key. Bob decrypts and obtains DES key
with his private key. The data is encrypted
using DES key, which now both Alice and
Bob have access to in order to
encrypt/decrypt data).

Security of RSA

The security of RSA relies on the fact that
there are no known algorithms for quickly
factoring a number (n), into the primes p
and q. If one knew p and q, then given e,
one could then easily compute the secret
key d. It is not known whether or not there
exist fast algorithms for factoring a
number, and in this sense the security of
RSA is not guaranteed.


ap 1.0

ap 2.0

ap 3.0

ap 3.1

ap 4.0

ap 5.0

Exchanging Public Keys

Why should public key be publicly

Wouldn’t it be better for Alice and Bob to
exchange their respective public keys via
mail, after authenticating each other?

Due to possibility of “
man (woman) in the
middle attack

Man (Woman) in the Middle Attack

Man (Woman) in the Middle Attack