Page 222 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 14: Network Security

to plain text. Examples of symmetric encryption are the RSA algorithm. Symmetric Notes
encryption method has the following five major parts:
(a) Plaintext: It is the text message to be transmitted on which an algorithm is applied.

(b) Encryption Algorithm: It enables mathematical operations to conduct substitutions
and transformations to the plaintext.
(c) Secret Key: They constitute a part of algorithm for encryption and decryption of the
message.
(d) Ciphertext: This is the encrypted message generated by applying the algorithm to
the plaintext message using the secret key.
(e) Decryption Algorithm: This is the encryption algorithm that decrypts the cipher text
into plain text by using the ciphertext and the secret key.

In the application of the symmetric encryption, the sender and receiver are required to exchange
secret keys in a secure manner with the aid of a strong encryption algorithm.

14.4.3 Authentication

It verifies the antecedents of the remote party being the real party rather than an impostor.
It means that the message is received from an authentic person not from an impostor. Digital
signature is one of the several authentication approaches that use public key encryption method.

14.4.4 Data Compression

It compresses data to reduce the amount of transmitted data thus saving in bandwidth and
money. There are three general methods to data compression. Each method considers that the
data stream may be transformed into a more compact representation. This compact data stream
is reconstructed back into the original data at the destination machine.

Finite Set of Symbols

It is considered that a library with many branch offices in which the previous days transactions
are transmitted to every other branch after closing. Transactions are comprised of checked out
and returned books. The information could be exchanged in the following manners:
1. The name of the book, its author, the copy number, etc. together with the type of transaction
are sent.
2. The library needs to maintain a officewise table assigning a unique ID number to every
book in every branch. Transactions then refer to the book’s ID number instead of its title.
As the book IDs are small and contain few bytes, so less data will be transmitted.

Notes The above descriptions that the above technique is used throughout programming
and pointers and array subscripts are frequently exchanged to avoid the cost of transferring
large amounts of data between subroutines. It is also assumed that all objects occur with
equal frequency and that the set of objects, books in this case, is finite. When text is
examined, it is immediately noticed that some words appear more often than others.
Taking cue from this, the number of bits could be reduced that are required to represent a
document by using a coding scheme that employs small code words to represent common
words and longer code words to represent words that appear infrequently.

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