Cryptography involves secret writing and is used to secure sensitive information. It has two main types: symmetric-key cryptography where encryption and decryption keys are the same, and asymmetric-key cryptography where the keys are different. Common symmetric algorithms include transposition ciphers while asymmetric algorithms include RSA which uses public and private key pairs. Steganography hides messages within other innocent-looking files like images and is used for digital copyright and covert communication.

2.  What is meant by cryptography ?
• Its Greek word
• Crypto means “ Secret”
• Graphie means “ Writing “
 Why Cryptography ?
• To convert the plain text to unreadable text
 What is need of Cryptography ?
 To secure the Sensitive Information
• Username – Password
• Credit Card Number etc..

3.  CRYPTOGRAPHY COMPONENT

4.  Cryptanalysis
basic component of cryptography is a cryptosystem
 Cryptanalyst
• Person working for unauthorized interceptor
 Cryptographer
• Person working for legitimate sender or receiver
 Cryptology
• encryption and decryption

5.  Cryptosystem
 A cryptosystem is a 5-tuple (Ε , D , M , K , C)
• M is the collection of plaintexts
• K is the set of keys
• C is the set of cipher texts
 How it works ?
o Ε = Μ × K → C is the set of encipher (encryption)
o D = C × K → M is the set of deciphering (decryption)
 Note
• Plaintext M : collection of original messages
• Cipher text C : set of messages in encrypted form

6.  Cryptosystem
 Encryption
• Process of encoding (enciphering / encryption ) a message
 Decryption
• Process of decoding (deciphering or decryption) an
encrypted message to its original form.
Alisa Kishan
data, control
channel
messages
data secure secure data
sender receiver
Trudy

7.  CRYPTOGRAPHY
CRYPTOGRAP
HY
SYMMETRIC-KEY ASYMMETRIC-
CRYPTOGRAPHY KEY
CRYPTOGRAPHY

8.  Types of Cryptosystems
 Symmetric Key Cryptosystems
• Also called single-key cryptosystems
• M = D(K, E(K, M))
• The encryption key and decryption key are the same

9.  Symmetric Cryptosystems
 Basic techniques for classical ciphers (Encryption)
 Substitution
• One letter is exchanged for another
 Transposition
• The order of the letters is rearranged Classical ciphers
 Mono-alphabetic
• Letters of the plaintext alphabet are mapped into
other unique letters
 Poly-alphabetic
• Letters of the plaintext alphabet are mapped into
letters of the cipher text space depending on their
positions in the text
 Stream
• Key stream is generated and used to encrypt the plaintext
(Symmetric Key Algorithms)

10.  Transposition
 Example :-
Cipher-Text :- TRHCEEIETGSSMAIAEASS
T 5 T R
H 3 H C E E
E 1 I E T G
K 4 S S M A
E 2 I A E S
Y 6 S S
Refer (by key): IETGIAESHCEESSMATRSS T H E K E Y
5 3 1 4 2 6
 Columnar T H I S I S
A S E C R E
• IEEIRSHSMESCSTATGSEA T M E S S A
G E

11.  Types of Cryptosystems
 Asymmetric cryptosystem
• M = D (Kd , E (Ke , M ) )
• The encryption key and decryption key are not the same
• Kd ≠ Ke

12.  Asymmetric Key Cryptosystem
(Public Key Cryptosystem)
 Uses public and private keys
• Public key for encryption
• Private key for decryption
 Examples
• RSA
• Trapdoor one-way function
• Elliptical curve cryptography

13.  RSA Principle
 Developed by Ron Rivets , Adi Shamir, and Leonard Adleman.
(MIT, 1977)
 C = me mod (pq)
• P , q : any two large prime numbers (more than 512 bits)
• e: public key
• d: private key
• Knowing C and e, it is “infeasible” to calculate m without
knowing d , p and q
• Knowing d, p and q, it is easy to find the original
plaintext message m = Cd mod (pq)

14.  RSA Principle
{m, e, p, q; {C, e, p, q;
C= me mod(pq)} m=Cd mod(pq)}
Sender Receiver
Intruder
{C, e}

16.  What is Steganography?
 Its Greek Words:
• STEGANOS – “Covered”
• GRAPHIE – “Writing”
 Steganography is about Hiding messages
 Digital Applications of Steganography
• Can be hidden in digital data
- MS Word (doc)
- Web pages (html)
- Sound files (mp3, wav, cda)
- Video files (mpeg, avi)
- Digital images (bmp, gif, jpg)
 Today, steganography is used primarily to protect
digital rights
• “watermarking” copyright notices
• “fingerprinting” a serial ID

17.  History of Steganography
 Chinese silk balls
 Invisible ink-jet technology
 Microdots
 Easter eggs
 Hiding a message within a text
 Shift some words by one point/pixel
- Shifted words (or their first letters) make the sentence
 Use different fonts
- Letter by letter or word by word

18.  A Real Example
 During WW2 the following cipher message
was actually sent by a German spy
“Apparently, neutral’s protest is thoroughly
discounted and ignored. Isman hard hit.
Blockade issue affect pretext for embargo on
by products, ejecting suets and vegetable
oils.”
 Hidden Message
• “ Pershing sails from NY June 1”

19.  Modern Steganography
 Hiding one message within another
(“container”)
 Most containers are rich media
• Images , audio, video ,mp3 etc.
 Copyright notices embedded in digital art
• Prove ownership
 Digital cameras EXIF tags
• Not secretive, but hidden from the eyez
• Embed info such as camera type, date, shutter speed ,
focal length,..
 Similarly , possible to embed messages in invisible parts of html pages

20.  Steganography System

21.  Comparison
Cryptography
C = Ek (P) Plain text
P = Dk (C) f Ciphertext
Key
Steganography
secret message
f Stego message
cover image

22.  Hiding a Message in an Image
Original Picture Embedded picture