TSM Crptography

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CRYPTOGRAPHY



• Cryptology – Science of encryption

• Cryptography

Kryptos-hidden

Graphein- to write

• Cryptanalysis

Obtaining original message from encrypted message

without knowing algorithms or keys.



• Encryption:

Process of converting original message in to

form that is unreadable to unauthorized

person.

Decryption:

Process of encrypted text message back in tooriginal form.



Security provided by Encryption

Confidentiality:

Hide information from unauthorizedindividuals either in transit or in storage.

Interception causes loss of messageconfidenttality

Integrity:

It identifies changes of information either intransit or in storage. Modification causes lossof message integrity.



Accountability :

Enables authentication of the origin of message

and prevents information from repudiating.

Nonrepudiation:

It does not allow sender of message to refute the

claim of not sending that message.

Access Control

Availability



Physical Cryptography

(Weak keys)

• Substitution Ciphers:

Changing one character/ symbol in to another

Transposition Cipher:

Transposing or scrambling the letters in acertain manner.

Stegnography

Process of hiding one message in another.

Hybrid Systems



• Substitution Poly alphabatic

A B C D E F G……. Plain Text

J K L M N O P…… Substitution 1

Q R S T U V W….. Substitution 2

Transposition Cipher( Permutation) From right

1-4, 2-8, 3-1, 4-5, 5-7, 6-2, 7-6, 8-3

00100101 01101011

00001011 10111010



Transposition Ciphers

• Rearrange order of characters(permutation)

• SIMPLE COLUMNAR CIPHER

Using a grid, write plaintexthorizontally, read ciphertext.

vertically.

P launchmissilesnow

launch

missil

esnow

C LMEAISUSNNSOCIWHL

• ATTACK ON COLUMNAR CIPHER Ciphertext has same letterfrequencies as plaintext -> Easy

• MULTIPLE TRANSPOSITION

CIPHERS Pass a plaintext through two ormore transposition ciphers ->Much harder to attack.



One-time Pad

• Use a random key as long as themessage. Must not reuse the keysequence ever again.

• Both parties must have key sequence

• Hotline between USA and USSR wasrumoured to use a one-time pad.

• Destroy key sequence after use

• Advantages?

• Disadvantages?

• EXAMPLE

Key is number of places to shift

letter

K 321424

P launch

C OCVREL • Suggest a good 1-time pad

function for binary data?



Transposition Ciphers

• Rearrange order of characters(permutation)

• SIMPLE COLUMNAR CIPHER

Using a grid, write plaintexthorizontally, read ciphertext.

vertically.

P launchmissilesnow

launch

missil

esnow

C LMEAISUSNNSOCIWHL

• ATTACK ON COLUMNAR CIPHER Ciphertext has same letterfrequencies as plaintext -> Easy

• MULTIPLE TRANSPOSITION

CIPHERS Pass a plaintext through two ormore transposition ciphers ->Much harder to attack.



Steganography

Dear George, 3rdMarch

Greetings to all at Oxford. Many thanks for your

letter and for the Summer examination package.

All Entry Forms and Fees Forms should be

readyfor final dispatch to the Syndicate by Friday

20th or at the very least, I’m told, by the 21st.

Admin has improved here, though there’s room

for improvement still; just give us all two orthree

more years and we’ll really show you! Please don’t let these wretched 16+ proposals destroy

your basic O and A pattern. Certainly this

sort of change, if implemented immediately,

would bring chaos.

Conceal existence ofmessage, e.g. 1stletter of each word,least sig. bit ofgraphic image

Useless once methoddiscovered

Peter Wayner,Disappearing Cryptography , 2nd ed,Morgan Kaufmann,2002



Steganography **





ready for final dispatch to the Syndicate by Friday







would bring chaos.



Moon beans are nice.

Moon Beams Are Nice

on Mo amsBe reA Ce.Ni



Mathematical Cryptography

• HASH Function

Mathematical algorithms.

Message : this

ASCII Values: 116 104 105 115

Calculated value: 232 202 210 230

Hash value Calculation:

(232+208+210+230)/10

Hash Value: 88



DES



• Symmetric Algorithms:

Both ends of an encrypted message have thesame key and processing algorithms.

Secret key is generated which is some timescalled as Private key.

Asymmetric Algorithms:

Two keys to encrypt & decrypt data,

referred as Public Key & Private key.Public key used by sender to encrypt.

Private key is used by receiver to decrypt.



Steganography **





ready for final dispatch to the Syndicate by Friday







would bring chaos.



Steganography





readyfor final dispatch to the Syndicate by Friday







would bring chaos.

Conceal existence ofmessage, e.g. 1stletter of each word,least sig. bit ofgraphic image

Useless once methoddiscovered

Peter Wayner,Disappearing Cryptography , 2nd ed,Morgan Kaufmann,2002

TSM Crptography

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