CS 2302 Computer Networks Unit 5

11. How is a secret key different from public key?

In secret key, the same key is used by both parties. The sender uses this key and an encryption algorithm to encrypt data; the receiver uses the same key and the corresponding decryption algorithm to decrypt the data. In public key, there are two keys: a private key and a public key. The private key is kept by the receiver. The public key is announced to the public.

12. What is a digital signature?

Digital signature is a method to authenticate the sender of a message. It is similar to that of signing transactions documents when you do business with a bank. In network transactions, you can create an equivalent of an electronic or digital signature by the way you send data.

13. What are the advantages & disadvantages of public key encryption?

Advantages:

a) Remove the restriction of a shared secret key between two entities. Here each entity can create a pair of keys, keep the private one, and publicly distribute the other one.

 b) The no. of keys needed is reduced tremendously. For one million users to communicate, only two million keys are needed.

Disadvantage:

If you use large numbers the method to be effective. Calculating the cipher text using the long keys takes a lot of time. So it is not recommended for large amounts of text.

14. What are the advantages & disadvantages of secret key encryption?

Advantage:

Secret Key algorithms are efficient: it takes less time to encrypt a message. The reason is that the key is usually smaller. So it is used to encrypt or decrypt long messages.

Disadvantages:

a) Each pair of users must have a secret key. If N people in world want to use this method, there needs to be N (N-1)/2 secret keys. For one million people to communicate, a half-billion secret keys are needed.

b) The distribution of the keys between two parties can be difficult.

15. Define permutation.

            Permutation is transposition in bit level. 

Straight permutation: The no. of bits in the input and output are preserved.

Compressed permutation: The no. of bits is reduced (some of the bits are dropped).

Expanded permutation: The no. of bits is increased (some bits are repeated).

CS 2302 Computer Networks Unit 5

1. What is the function of SMTP?

The TCP/IP protocol supports electronic mail on the Internet is called Simple Mail Transfer (SMTP). It is a system for sending messages to other computer users based

on e-mail addresses. SMTP provides mail exchange between users on the same or different computers.

2. What is the difference between a user agent (UA) and a mail transfer agent (MTA)?

            The UA prepares the message, creates the envelope, and puts the message in the envelope. The MTA transfers the mail across the Internet.

3. How does MIME enhance SMTP?

            MIME is a supplementary protocol that allows non-ASCII data to be sent through SMTP. MIME transforms non-ASCII data at the sender site to NVT ASCII data and deliverers it to the client SMTP to be sent through the Internet. The server SMTP at the receiving side receives the NVT ASCII data and delivers it to MIME to be transformed back to the original data.

4. Why is an application such as POP needed for electronic messaging?

Workstations interact with the SMTP host, which receives the mail on behalf of every host in the organization, to retrieve messages by using a client-server protocol such as Post Office Protocol, version 3(POP3). Although POP3 is used to download messages from the server, the SMTP client still needed on the desktop to forward messages from the workstation user to its SMTP mail server. 

5. What is the purpose of Domain Name System?

Domain Name System can map a name to an address and conversely an address to name.

CS 2302 Computer Networks Unit 5

1. Discuss the basic model of FTP. (NOV/DEC 2007 & MAY/JUNE  2009)

The client has three components: the user interface, the client control process, and the client data transfer process. The server has two components: the server control process and the server data transfer process. The control connection is made between the control processes. The data connection is made between the data transfer processes.

2.What is a digital signature?

Digital signature is a method to authenticate the sender of a message. It is similar to that of signing transactions documents when you do business with a bank. In network transactions, you can create an equivalent of an electronic or digital signature by the way you send data.

3. Define Cryptography

n  Cryptography refers to the science and art of transforming messages to make them secure and immune to attacks.

n  Original message before being transformed is called plaintext.

n  After the message is transformed, is called ciphertext.

n  An encryption algorithm transforms the plaintext to ciphertext; a decryption algorithm transforms the ciphertext back to plaintext.

n  The term cipher is used to refer to encryption and decryption algorithms.

4. What are the types of DNS Message

n  Two types of messages

                Query: header and question records

n  Response: Header, question records, answer records, authoritative records, and additional records.

5. What is TELNET PROTOCOL?

   A TELNET connection is a Transmission Control Protocol (TCP)    connection used to transmit data with interspersed TELNET control information.

   The TELNET Protocol is built upon three main ideas:  first, the    concept of a "Network Virtual Terminal"; second, the principle of    negotiated options; and third, a symmetric view of terminals and    processes.