Page 226 - DCAP406_DCAP_207_Computer Networks

Unit 14: Network Security

Self Assessment Notes

State whether the following statements are true or false:
6. Modern cryptanalysis makes simple substitution and transposition ciphers obsolete.
7. According to the unicity distance of English, 20 letters of ciphertext are required to crack
a mixed alphabet simple substitution.
8. In substitution cipher, a bijective function is used on the characters’ positions to encrypt
and an inverse function to decrypt.

9. Traditionally, the ciphertext is written out in blocks of fixed length, omitting punctuation
and spaces.
10. ROT13 is a Caesar cipher, a type of transposition cipher.

14.6 Summary

Data on the network is not secret and therefore requires to be kept secure from undesirable
persons sitting behind the machines attached to the network.

The malicious intentions may include bringing down of servers attached to the network,
using people’s private information like credit card numbers for fraudulent activities and
sabotaging of major organizations by accessing their websites. It is therefore aimed to
secure data and prevent from eavesdroppers from listening to and stealing data. The user
data on a computer is also protected by providing password restricted access to the data
and resources so that only authorized people get to use these. Security aspects also involve
identifying miscreants and thwarting their attempts to cause damage to the network
among other resources.

Authentication involves verifying of the antecedents of the person who has requested for
services from a remote machine or access to the remote machine either through physically
or by sending an e-mail before allowing him or her to do so. Authentication involves a
process to authenticate person’s identity to a remote machine.
Integrity involves the veracity of the message which is received by a remote machine. In
other words, it is indeed the same message without any alteration which was sent by the
source machine. In this case, cyclic redundancy code method will not be enough as intruders
in the system or communication channel may deliberately alter the message. Security
should ensure that nobody along the entire route should be able to alter the message.
Confidentiality: It ensures that no person should be able to read the message on the way.
This necessitates implementation of the encryption techniques down the line.
The message is encrypted at the sender end and decrypted at the receiving end to maintain
privacy with the help of the encryption and decryption techniques. The secret key and
public key techniques are the available techniques with their advantages and disadvantages.

Substitution and transposition ciphers are two categories of ciphers used in classical
cryptography. Substitution and transposition differ in how chunks of the message are
handled by the encryption process.

14.7 Keywords

Ciphertext: This is the encrypted message generated by applying the algorithm to the plaintext
message using the secret key.

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