Page 211 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes includes Abstract Syntax Notation 1 (ASN.1) is recommended by OSI. This method takes care of
formatting, diverse nature of data such as text, programs, etc. and the diversity in data storage
format.

Abstract Syntax Notation

Abstract Syntax Notation (ASN.1) is an OSI standard dealing with the issue of representing,
encoding, transmitting, and decoding data structures. It has two parts as given below:
1. An abstract syntax describing the data structures in an unambiguous manner. The syntax
enables users to use integers, character strings, and structures instead of bits and bytes.
2. A transfer syntax describing the bit stream encoding of ASN.1 data objects. Data and
additional fields are sent to describe the type of data. At the destination machine, the
reverse operation is applied to convert from ASN.1 format to the internal representation
of the destination machine.
There are alternative approaches to the data representation but they have disadvantages. In one
approach, the source machine converts data into the format expected by the destination machine
so that the destination machine does not need to perform any decoding. The disadvantage to this
approach is that every source machine needs to know how to encode data for every possible
destination machine. In another approach, ASN.1 converts everything into a common form
similarly to the network standard representation of TCP/IP. However, the disadvantage of this
method is that communication between two identical machines results in needless conversions.
ASN.1’s abstract syntax is much like in form to that of any high level programming language.
ASN.1 comprises of primitive types and complex types building on primitive types.
Encryption/Decryption: It deals with security and privacy issues. Encryption is used to scramble
the data so that only authorized persons can unscramble the data of a conversation. Decryption
reverses the encryption process to translate the message back into its original form. To encrypt
the data, the sender in the source machine uses an encryption algorithm and a key to transform
the plaintext (original message) into a ciphertext (encrypted message). At the destination machine,
the reverse process takes place. The receiver has a key and decryption algorithm to translate
back the ciphertext into the original plaintext.
Encryption and decryption methods are of two types. They are conventional and public key
methods. In the conventional method, the encryption and decryption keys are the same and
secret. The disadvantage of the conventional method is that the decryption algorithm is always
the inverse of the encryption algorithm and therefore whoever knows the encryption algorithm
will be able to deduce the decryption algorithm and thus the secrecy and privacy of message is
threatened.
In the public key encryption approach, every user has the same key and algorithm for encryption
of the message. However, the decryption algorithm and key are kept secret. Thus, the message
could be encrypted by anyone; however, it could be decrypted by an authorized person. The
decryption algorithm is designed in such a way that it could not be deduced from the inverse of
the encryption algorithm. Also, different encryption and decryption keys make it difficult to
decrypt the message by an unauthorized person.

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.
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

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