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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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Cryptography and NetworkSecurity 6
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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Cryptography and NetworkSecurity 7
Information Transferring
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Cryptography and NetworkSecurity 8
Attack: Interruption
Cut wire
lines,Jam wirelesssignals,Droppackets,
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Cryptography and NetworkSecurity 9
Attack: Interception
Wiring,eavesdrop
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Cryptography and NetworkSecurity 10
Attack: Modification
intercept
Replaced info
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Cryptography and NetworkSecurity 11
Information Transferring
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Cryptography and NetworkSecurity 12
Attack: Interruption
Cut wire
lines,Jam wirelesssignals,Droppackets,
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Cryptography and NetworkSecurity 13
Attack: Interception
Wiring,eavesdrop
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Cryptography and NetworkSecurity 14
Attack: Modification
intercept
Replaced info
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Cryptography and NetworkSecurity 15
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 and NetworkSecurity 22
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
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