Cryptography is the practice and study of techniques for secure communication in the presence of third parties. It involves encrypting plaintext messages into ciphertext that can only be decrypted by authorized parties. There are two main categories of cryptography: symmetric key cryptography which uses the same key for encryption and decryption, and asymmetric key cryptography which uses separate public and private keys. Network security relies heavily on cryptography to protect data transmission over the internet.

1. Cryptography

2. Introduction
• Cryptography (or cryptology;
from Greek κρυπτός, "hidden, secret";
and γράφειν, graphein, "writing", or -λογία, logia, "study", respectively)[1] is the practice
and study of techniques for secure
communication in the presence of third parties.
• Network security is mostly achieved through
the use of cryptography.
• A science based on abstract algebra.

3. Definitions
• Computer Security - generic name for the collection
of tools designed to protect data from the hackers
• Network Security - measures to protect data during
their transmission.
• Internet Security - measures to protect data during
their transmission over a collection of interconnected
networks.
• our focus is on Internet Security
• which consists of measures to determine, prevent,
detect, and correct security violations that involve the
transmission & storage of information.

4. Security Trends

5. Security Attack
• any action that compromises the security of
information owned by an organization
• information security is about how to prevent attacks,
or failing that, to detect attacks on informationbased systems
• often threat & attack used to mean same thing
• have a wide range of attacks
• can focus of generic types of attacks
– passive
– active

6. Passive Attacks

7. Active Attacks

8. contd.,
Cryptography- is a “Greek word” which means “Secret
writing”.
• We use this term to make secure and immune to attacks.

9. • Plaintext– the original (or) user message, before being
transformed is called plain text.
• Ciphertext – After wards the plaintext transformed, it is
called ciphertext. An encryption algorithm transforms the
plaintext into ciphertext; a decryption algorithm transforms
the ciphertext back into plaintext.
• The sender uses an encryption algorithm and the receiver
uses a decryption algorithm.
• Cipher – we refer both encryption and decryption
algorithms as cipher. it is also refers to different categories
of algorithms in cryptography.
• Key – a key is number(or set of numbers) used to make
cipher

10. Contd.,
• An encryption algorithm, need an encryption key and the
plaintext.[as same for decryption].
• Alice, Bob and Eve –in cryptography, normally used three
characters in an information exchange.
• Alice is a person who needs to send secure data.
• Bob- is the recipient of the data.
• Eve- is the person who somehow disturb the communication
between Alice and bob.

11. Two categories
• The ciphers( cryptography algorithm) into two groups:
– Symmetric key(also called secret key)
– Asymmetric( Also called Public key)

12. Symmetric key cryptography
• Here, the same key is used by both parties(the key is
shared).
• The sender uses this key and an encryption algorithm to
encrypt data;
• The receiver uses the same key and decryption algorithm to
decrypt the data.

13. Asymmetric key cryptography
• There are two keys; a private key and a public key.
• The private key is kept by the receiver.
• The public key is announced to the public.

14. Contd.,
• Imagine Alice wants to send a message to Bob. Alice uses
the public key to encrypt the message. The Bob use the
private key to decrypt the message.

15. Types of keys
• The secret key – is the shared key used in symmetric-key
cryptography.
• The Public and Private key – used in Asymmetric
cryptography.

16. comparison
• Encryption can be thought of as electronic locking;
decryption as electronic unlocking.
• The sender puts the message in a box and locks the box by
using a key; the receiver unlocks the box with a key and
takes the message.
• In symmetric key cryptography- same key used for both
locks and unlocks the box.
• In asymmetric key cryptography- one key used for lock the
box and another key needed to unlock it.

18. Symmetric key cryptography
• It started thousands of years ago and still used.