Cryptography is the practice of hiding information by converting intelligible data into unintelligible ciphertext, and back again. It provides confidentiality, integrity, and authentication. There are three eras of cryptography - manual using pen and paper, mechanical using cipher machines, and modern using computers. Encryption is converting plaintext to ciphertext while decryption is converting ciphertext back to plaintext. Symmetric cryptography uses a single shared key for encryption and decryption, while asymmetric cryptography uses public and private key pairs.

1. What is Cryptography?
Cryptography derived its name from a
Greek word called “Kryptos” which means
“Hidden Secrets”.
Cryptography is the practice and study of
hiding information. It is the Art or Science
of converting a plain intelligible data into
an unintelligible data and again
retransforming that message into its
original form.
It provides Confidentiality, Integrity,
Authentication.

2.  There are three eras in the history of Cryptography:
 The Manual era
 The Mechanical era
 The Modern era
 Manual era refers to Pen and Paper Cryptography and dates back to 2000 B.C.eg :
Scytale, Atbash , Caesar, Vigenere.
 Mechanical era refers to the invention of cipher machines. E.g.: Japanese Red and
Purple Machines , German Enigma.
 The modern era of cryptography refers to computers.
 There are infinite permutations of cryptography available using computers. E.g.: Lucifer,
Rijndael , RSA , ElGamal.
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3. What is Encryption / Decryption
 Encryption –
 The process of converting plain text into an unintelligible format (cipher text)
is called Encryption.
 Decryption –
 The process of converting cipher text into a plain text is called Decryption.

4. Cryptography
Symmetric key Asymmetric key
cryptography cryptography
(Public key
cryptography)
Classical Modern
cryptography cryptography
Transposition Substitution Stream Block
cipher cipher cipher cipher
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5. Symmetric
o Even if an attacker captures the data, the attacker
will not be able to manipulate it in any meaningful
way.
o Symmetric algorithms use a single key shared by
two communicating parties.
o The same key is used for both encryption and
decryption.

6. Classic or Symmetric Cryptography
•have two basic components of
classical ciphers: substitution and
transposition
•in substitution ciphers letters
are replaced by other letters
•in transposition ciphers the
letters are arranged in a different
order.

7. Example:
 “Hello world” becomes “ehlol owrdl” is a trivally is a
simple rearrangement schemes.

8. Asymmetric
oMathematically related key
pairs for encryption and
decryption.
oPublic and private keys.

9. PUBLIC KEY or Assymmatric CRYPTOGRAPHY
 A form of cryptography in which the key used to encrypt a
message differs from the key used to decrypt it.
 In public key cryptography, a user has a pair of cryptographic
keys—a public key and a private key. The private key is kept
secret, while the public key may be widely distributed.
 The two main branches of public key cryptography are:
1. Public key encryption
2. Digital signatures