Cryptography

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.

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.

Security Trends

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 information-based systems

• often threat & attack used to mean same thing• have a wide range of attacks• can focus of generic types of attacks– passive– active

Passive Attacks

Active Attacks

contd.,Cryptography- is a “Greek word” which means “Secret

writing”.

• We use this term to make secure and immune to attacks.

• 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

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.

Two categories• The ciphers( cryptography algorithm) into two groups:– Symmetric key(also called secret key)

– Asymmetric( Also called Public key)

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.

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.

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.

Types of keys• The secret key – is the shared key used in symmetric-key

cryptography.

• The Public and Private key – used in Asymmetric cryptography.

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.

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