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3 if bob wishes to send a private message to alice

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3. If Bob wishes to send a private message to Alice, Bob encrypts the message using Alice’s private key. 4. When Alice receives the message, she decrypts it using her private keys. Digital Signature Bob wants to send a message to Alice and it is not important that the message be kept secret, he wants Alice to be certain that the message is indeed from him. Bob uses his own private key to encrypt the message, When Alice receives the ciphertext, she finds that she can decrypt it with Bob’s public key, thus proving that the message must have been encrypted by Bob. No one else has Bob’s private key and therefore on one else could have created a ciphertext that could be decrypted with Bob’s public key. The entire encrypted message serves as a digital signature. It is impossible to alter the message without access to Bob’s private key, so message is authenticated both in terms of source and in terms of data integrity. Figure 18.11 Public-key encryption 19. Describe the IPSec Application. (10 marks) Solution Application of IPSec IPSec provides the capability to secure communications across a LAN, across private and public WANs, and across Internet. Examples of its use include the following: ± Secure branch office connectivity over the internet : A company can build a secure virtual private network over Internet or over a public WAN. This enables a business to rely heavily on lnternet and reduce its need for private networks, saving costs and network management overhead. ± Secure remote access over the Internet : An end user whose system is equipped with IP security protocols can make a local call to an Internet service
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provider(ISP) and gain secure access to a company network. This reduces the cost of toll chares for traveling employees and telecommuters. ± Establishing extranet and intranet connectivity with partners : LIPSec can be used to secure communication with other organization, ensuring aulthenticatin and confidentiality and providing a key exchange mechanism. ± Enhancing electronic commerce security : Even though some Web and electronic commerce applications have built-in security protocols, the use of IPSec enhances that security. 20. Write the short note of Transport Mode and Tunnel Mode. (10 marks) Solution Transport Mode Transport mode provides protection primarily for upper-layer protocols. Transport mode protection extends to the payload of an IP packet. Transport mode is used for end-to-end communication between two hosts. ESP in transport mode encrypts and authenticates the IP payload but not the IP header AH in transport mode authenticates the IP payload and selected portions of IP header. Tunnel Mode Tunnel mode provides protection to entire IP packet. To achieve this, after AH or ESP fields are added to IP packet, the entire packet plus security fields is treated as the payload of new "outer" IP packet with a new router IP header. The entire original, or inner, packet travels through a "tunnel" from one point of an IP network to another; no routers along the way are able to examine the inner IP header. Because the original
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3 If Bob wishes to send a private message to Alice Bob...

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