On October 23rd, 2014, we updated our
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• Cryptography (or cryptology;
from Greek κρυπτός, "hidden, secret";
and γράφειν, graphein, "writing", or -λογία, logia, "study", respectively) 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.
• Computer Security - generic name for the collection
of tools designed to protect data from the hackers
• Network Security - measures to protect data during
• Internet Security - measures to protect data during
their transmission over a collection of interconnected
• 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.
• 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
Cryptography- is a “Greek word” which means “Secret
• 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
• 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.
• 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
• The sender uses this key and an encryption algorithm to
• 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.
• 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
• The Public and Private key – used in Asymmetric
• 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.