Cryptography involves converting plain text into cipher text to secure communication. There are two main types: private/symmetric key cryptography uses a single shared key for encryption and decryption, while public/asymmetric key cryptography uses two keys - a public key to encrypt and a private key to decrypt. Private key cryptography is faster but requires secure key exchange, while public key allows communication without pre-shared keys and enables authentication and digital signatures.

1. Cryptography
Source: Content by William Stallings,
Lawrie Brown ,Triad Ltd.

2. Introduction to Cryptography
 Cryptography is the practice and study of techniques for
securing communication and data by converting
messages into format not understandable by
unauthenticated users.
Terms:
Plain text-Understandable message
Cipher – Algorithm to convert plain text to cipher text
Cipher text- Scramble form of message only understandable
by authenticated users

3. Example of Cryptography
 Suppose you want to send Message –Hello
If shift cipher is used for cryptography ,then it shift each
character by key value
 Assume key =3
 For Encryption
Plain text enryptedcipher text
Hello->Khoor(added key of 3 to each char)
 For Decryption
Cipher text decrypted Plaintext
KhoorHello(sub key-3 )

4. Encryption
Encryption is a way of converting data from plaintext to
cipher text so that unauthorized parties cannot understand
the information.
Source:https://www.nodejsera.com/assets/img/encrypt.jpg

5. Decryption
 Decryption is the process of converting ciphertext
toplaintext.
Source:https://www.nodejsera.com/assets/img/Decrypt.png

6. Types of Cryptography
 Private or Symmetric key cryptography
 Public or Asymmetric key cryptography

7. Private Key cryptography also called as symmetric
key cryptography.In symmetric-key cryptography,
the same key is used by the sender(for encryption)
and the receiver (for decryption).
The key is shared.
Algorithm: DES,3DES
Private Key cryptography

8. Private-Key Cryptography
 traditional private/secret/single key cryptography uses
one key
 Same Key is shared by both sender and receiver
 Sender uses this key and encryption algorithm to
encrypt data
 Receiver uses this key and decryption algorithm to
decrypt data
 if this key is disclosed communications are compromised
 also is symmetric, parties are equal
 hence does not protect sender from receiver forging a
message & claiming is sent by sender

9. Private-key cryptography

10. Private-key cryptography
 Advantages:
 Simple
 Faster
 Disadvantages:
 Key must exchanges in secure way
 Easy for hacker to get a key as it is passed in unsecure
way.

11. An asymmetric-key (or public-key) cipher uses two
keys: one private (To encrypt data) and one public(To
decrypt data).
Asymmetric-key(Public key) cryptography

12. Public-Key Cryptography?
 developed to address two key issues:
 key distribution – how to have secure communications
 digital signatures – how to verify a message comes intact from the claimed
sender
 public invention due to Whitfield Diffie & Martin Hellman at Stanford Uni
in 1976
 known earlier in classified community

13.  Asymmetric encryption use two keys, one to
encrypt the data, and another key to decrypt the
data.
 These keys are generated together
 One is named as Public key and is distributed
freely. The other is named as Private Key and it
is kept hidden.
 Both Sender & Recipient has to share their
Public Keys for Encryption and has to use their
Private Keys for Decryption.

14. Public-key cryptography

15.  Asymmetric encryption use two keys:
Public Key - to encrypt the data
Private Key - to decrypt the data
 These keys are generated together.
 The Public key(s) is distributed freely between the sender and
receiver.
 The other is named as Private Key and it is kept hidden.
 The Private Key is only used for Decryption and will not be shared
between the sender and receiver.
Key Points in Asymmetric Encryption

16. Merits:
 Two parties don't need to have their private keys already shared in order to
communicate using encryption.
 Authentication and Non-Repudiation are possible. (Authentication means that you can
encrypt the message with my public key and only I can decrypt it with my private key.
Non-repudiation means that you can "sign" the message with your private key and I can
verify that it came from you with your public key.)
De-Merits:
Asymmetric Encryption algorithms are comparatively complex.
Time consuming process for Encryption and Decryption.
MERITS & DE-MERITS

17. Public-Key Cryptography
Applications
 can classify uses into 3 categories:
 encryption/decryption (provide secrecy)
 digital signatures (provide authentication)
 key exchange (of session keys)
 some algorithms are suitable for all uses, others are specific to one

19. Comparison between two categories of cryptography