3. Now Bob send Alice a message using the public key and Alice decrypt it using her private key. Step 3: Alice publish a public key to send her a message. And has a private key to decrypt it. Step 2: The most common ones have the property that Alice and Bob each own two keys, one for encryption and one for decryption Step 1:

5. A GENERAL APPROACH Contd .

6. When user1 receives the message, he decrypts it using his private key. No other recipient can decrypt the message because only user1 knows his private key Step 4 : If user1 wishes to send a confidential message to user2, user1 encrypts the message using user2's public key. Step 3 : Each user places public key in a public register or other accessible file. As encryption figure suggests, each user maintains a collection of public keys obtained from others. Step 2 : Each user generates a pair of keys to be used for the encryption and decryption of messages. Step 1 :

8. Authentication & security: Secrecy Contd .

9. Secrecy in a public key encryption : B decrypt the massage using it’s private key PRb. Using it’s private key PRb and the cipher text Y it obtain the original massage X Step 2: A encrypt the massage using B’s public key PUb and send it to B. With the message X and the encryption key PUb as input, A forms the cipher text Y = [Y1, Y2,..., YN]: Step 1:

10. Authentication & security: Authentication Contd.

11. Authentication in a public key encryption : B can decrypt the message using A's public key. Because the message was encrypted using A's private key, only A could have prepared the message. Step 2: A prepares a message to B and encrypts it using A's private key before transmitting it. Step 1:

13. Authentication & security: Contd.

19. Weaknesses Public-key cryptography also has vulnerabilities to attacks such as the man in the middle attack. In this situation, a malicious third party intercepts a public key on its way to one of the parties involved Vulnerable to the man in the middle attack Keys in asymmetric cryptography are more vulnerable to brute force attacks than in secret-key cryptography. Vulnerable to brute force attacks Keys in public-key cryptography, due to their unique nature, are more computationally costly than their counterparts in secret-key cryptography. Computati-onal cost