Computer Network Unit 1 q what are the topologies in computer n/w ?

Homophonic Substitution

An early attempt to increase the difficulty of frequency analysis attacks on substitution ciphers was to disguise plaintext letter frequencies by homophony. In these ciphers, plaintext letters map to more than one cipher text symbol. Usually, the highest-frequency plaintext symbols are given more equivalents than lower frequency letters. In this way, the frequency distribution is flattened, making analysis more difficult.

• 
  Polyalphabetic Substitution
  

In a polyalphabetic cipher, multiple cipher alphabets are used. To facilitate encryption, all the alphabets are usually written out in a large table, traditionally called a tableau. The tableau is usually 26×26, so that 26 full cipher text alphabets are available. The method of filling the tableau, and of choosing which alphabet to use next, defines the particular polyalphabetic cipher. All such ciphers are easier to break than once believed, as substitution alphabets are repeated for sufficiently large plaintexts.

• 
  Polygraphic substitution
  

In a polygraphic substitution cipher, plaintext letters are substituted in larger groups, instead of substituting letters individually. The first advantage is that the frequency distribution is much flatter than that of individual letters (though not actually flat in real languages; for example, 'TH' is much more common than 'XQ' in English). Second, the larger number of symbols requires correspondingly more ciphertext to productively analyze letter frequencies.
Q 10. Give features of public key cryptography and authentication protocol.
Ans; Public-key cryptography refers to a widely used set of methods for transforming a written message into a form that can be read only by the intended recipient. This cryptographicapproach involves the use of asymmetric key algorithms — that is, the non-message information (the public key) needed to transform the message to a secure form is different from the information needed to reverse the process (the private key). The person who anticipates receiving messages first creates both a public key and an associated private key, and publishes the public key. When someone wants to send a secure message to the creator of these keys, the sender encrypts it (transforms it to secure form) using the intended recipient's public key; to decrypt the message, the recipient uses the private key.

The primary advantage of public-key cryptography is increased security: the private keys do not ever need to be transmitted or revealed to anyone. In a secret-key system, by contrast, there is always a chance that an enemy could discover the secret key while it is being transmitted. 

Another major advantage of public-key systems is that they can provide a method for digital signatures. Authentication via secret-key systems requires the sharing of some secret and sometimes requires trust of a third party as well.
Furthermore, digitally signed messages can be proved authentic to a third party, such as a judge, thus allowing such messages to be legally binding. Secret-key authentication systems such as Kerberos were designed to authenticate access to network resources, rather than to authenticate documents, a task which is better achieved via digital signatures. 
A disadvantage of using public-key cryptography for encryption is speed: there are popular secret-key encryption methods which are significantly faster than any currently available public-key encryption method. But public-key cryptography can share the burden with secret-key cryptography to get the best of both worlds.