The document provides an overview of cryptography, distinguishing between symmetric key (secret-key) and asymmetric key (public-key) algorithms. Symmetric key algorithms, like DES, use the same key for both encryption and decryption but require secure key exchange, while asymmetric key algorithms, like RSA, enhance security by using separate keys for encryption and decryption and eliminate the need for key exchange. The document highlights the advantages and disadvantages of each cryptographic method, including speed and security considerations.

1. • A word with Greek origins, means "secret writing." However, we use
the term to refer to the science and art of transforming messages to
make them secure and immune to attacks
• It used for security purpose

2. Two Categories
We can divide all the cryptography algorithms (ciphers) into two
groups: symmetric key (also called secret-key) cryptography
algorithms and asymmetric (also called public-key) cryptography
algorithms.

3. •The same key is used by sender(for encryption ) and
the receiver(for decryption )
•The key is shared.

4. •Symmetric Key cryptography is sometimes called as
secret-key cryptography
•Examples of symmetric Key algorithms are DES, 3DES
and AES
•The most popular symmetric Key system is Data
Encryption Standard(DES)

5. •DES is a block cipher. It encrypts data in blocks of size
64 bits each. That is, 64 bits of plain text goes as the
input to DES, which produces 64 bits of cipher text.
The same algorithm and key are used for encryption
and decryption, with minor differences. The key
length is 56 bits

6. Advantages
*Simpler
*Faster
Disadvantages
*Two parties must somehow exchange the key in a
secure way.
*Public key is distributed in a non-secure way between
Client/Server.
*Easy for hackers to get the key as it is shared in
unsecure way.

8. • The most common public key algorithm is RSA, named for its
inventors Rivest,Shamir,and Adleman (RSA)
• Selecting keys
• Encryption:
1.Anyone who needs to send a message to Bob can use n and e
2.For example, if Alice needs to send a message to Bob, she can
change the message, usually a short one, to an integer. This is
the plaintext. She then calculates the ciphertext, using e and n.
3.C=P^e (modn)
4.Alice sends C, the ciphertext, to Bob.

9. • Decryption
1.Bob keeps p and d private
2.When he receives the ciphertext, he uses his private key d to
decrypt the message
3.P= C^d(modn)
4.Restriction For RSA to work, the value of P must be less than
the value of n. If P is a large number, the plaintext needs to be
divided into blocks to make P less than n.

10. ADVANTAGES
• In asymmetric or public key, cryptography there is no need for
exchanging keys, thus eliminating the key distribution problem
• The primary advantage of public-key cryptography is increased
security: the private keys do not ever need to be transmitted or
revealed to anyone
• Can provide digital signatures that can be repudiated
DISADVANTAGES
• A disadvantage of using public-key cryptography for encryption is
speed: there are popular secret-key encryption methods which are
significantly faster than any currently available public-key encryption
method.