Revised Network Security

7/31/2019 Revised Network Security

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Cryptography and NetworkSecurity

EVANGELINE PRICILLA


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Benjamin Franklin once said

Three people can keep a secret

if two of them are dead!


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Security is Not Easy to Achieve Human tendency

Problems of storage and communication

Trust in all the parties


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Network Security ModelTrusted ThirdParty

principal principal

Securitytransformation

Securitytransformation

attacke

r


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Cryptography and NetworkSecurity 5

Attacks, Services and MechanismsSecurity Attacks

Action compromises the information security

Could be passive or active attacks

Security Services Actions that can prevent, detect such attacks.

Such as authentication, identification, encryption, signature,secret sharing and so on.

Security mechanism The ways to provide such services

Detect, prevent and recover from a security attack


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Attacks Passive attacks Interception

Release of message contents

Traffic analysis

Active attacks

Interruption, modification, fabrication

Masquerade Replay

Modification

Denial of service


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Information Transferring


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Attack: Interruption

Cut wire

lines,Jam wirelesssignals,Droppackets,


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Attack: Interception

Wiring,eavesdrop


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Attack: Modification

intercept

Replaced info


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Information Transferring


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Attack: Interruption

Cut wire

lines,Jam wirelesssignals,Droppackets,


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Attack: Interception

Wiring,eavesdrop


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Attack: Modification

intercept

Replaced info


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Attack: Fabrication

Also called impersonation


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Need for Security

Authentication Do you trust the sender of a message?

Integrity Is the message changed during transit?

Confidentiality

Is the message seen by someone else?

Non-repudiation Can the sender refute the message?


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Authentication

A B

C

I am user A

Who am I?


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Integrity

Has the Message Changed?

A B

C

Transfer $100to C Transfer $1000

to C

Ideal route of the message


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Confidentiality

Has someone seen it?

A B

C

Secret


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Non-repudiation

A BI never sent

that

message!

A sends a message and refutes it later


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How to achieve Security?

Use Cryptography

Art of transforming messages

Messages become unreadable


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Cryptography

Cryptography is the study of

Secret (crypto-) writing (-graphy)

Concerned with developing algorithms:

Conceal the context of some message fromall except the sender and recipient (privacy

or secrecy), and/orVerify the correctness of a message to the

recipient (authentication)


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Cryptography Basics

Plain text

All understandable messages

Example: My name is Simson

Cipher text

All non-understandable messages

Example: G%er@17*0-1>-


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Cryptography Mechanisms

Substitution Techniques

Replace one or more characters with other

characters Example: Replace each a with d, b with e, etc

Transposition Techniques

Rearrange the text

Example: Replace 1st character with 4th, 2nd with5th, etc

Combinational Technique: Mostly used


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Cryptography is simple!

Hi Simson,

Hope you are doing fine.

How about meeting at thetrain station this Friday at

5 pm? Please let me know

if it is ok with you.

Regards.Pricilla

Kl Dplw,

Krsh brx duh grlqj ilqh. Krz

derxw phhwlqj dw wkhwudlq vwdwlrq wklv Iulgdb

dw 5 sp? Sohdvh ohw ph

nqrz li lw lv rn zlwk brx.

Uhjdugv.

Dwxo


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Encryption and Decryption

Encryption Conversion ofplain textinto cipher text

Decryption Conversion ofcipher textinto plain text

Two Aspects Algorithm

Key


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Encryption

Plain text

Encrypt

Cipher text

My name is Simy

Encrypt

G%er@17*


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Decryption

Cipher text

Decrypt

Plain text

G%er@17*

Decrypt

My name is Simy


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Algorithm and Key

Encryption/Decryption Algorithm Specifies the operations to be performed

Examples: DES, IDEA, AES Known to everybody

Key Must be kept secret

Usually at least 56 bits


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Key Range

A 2-bit binary number has four possible states:00011011

If we have one more bit to make it a 3-bit binary number, the number of possible statesalso doubles to eight, as follows:000001010011100101110111

In general, if ann bit binary number hask possible states, ann+1 bit binary number willhave2k possible states.


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Brute Force Attack

Cipher text

R$9O0-=-P;. Decrypt

Try with key = 0

Plain text

V^510->o2

Cipher text

R$9O0-=-P;. Decrypt

Try with key = 1

Plain text

7/5vc_=;a1

Cipher text

R$9O0-=-P;. Decrypt

Try with key =

90,171,451,191Plain text

To: Payroll

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Revised Network Security