Page 221 - DCAP207_NETWORKS_DCAP406_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.

14.4.2 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.
Authentication refers to keep a secret of two persons secure from the third person. However, the
non-repudiation requires to prove that even the sender could not have generated the message.
To implement security issues as given above, a technique called cryptography is applied.
Encryption is of two types:

1. Asymmetric Encryption: Two mathematically related keys namely public key and private
keys are generated to encrypt and decrypt the message. Asymmetric encryption is
considered more secure than symmetric encryption. Asymmetric key encryption that
involves a key pair as public and private keys involves six major steps:

(a) Plaintext: Plaintext is the text message to which an algorithm is applied.
(b) Encryption Algorithm: It provides mathematical operations to conduct substitutions
and transformations to the plaintext.

(c) Public and Private Keys: They constitute a pair of keys which are used for encryption
and decryption of the message.
(d) Ciphertext: Application of algorithm on plaintext produces the encrypted or
scrambled message.
(e) Decryption Algorithm: This algorithm is applied to generate the ciphertext and the
matching key to produce the plaintext.

The encryption process converts the text message to a hash code by using a mathematical
formula. This hash code is then encrypted with the help of the sender’s private key. The
private key is generated with the help of the algorithm.
The encrypted hash code and the message are encrypted again using the sender’s private
key. Subsequent to this, the sender encrypts the secret key with the recipient’s public key,
so only the recipient can decrypt it with his or her private key.
In the decryption process, the recipient using his or her private key long with the secret
key to decipher the encrypted hash code and the encrypted message. The recipient then
uses the sender’s public key to decrypt the hash code and to verify the sender’s identity.
The recipient generates a hash code from the message. If thus generated hash code equals
the hash code forwarded by sender, then this verifies that the message has not been
changed on the way.
2. Symmetric Encryption: Symmetric encryption also referred to as conventional or single-
key encryption is based on a secret key, which is shared by both communicating parties.
The sending party encrypts the plain text to cipher text message using the secret key. The
receiving party on receipt of the cipher text message uses the same secret key to decrypt it

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