• Cryptography is the science of using mathematics to encrypt and decrypt data.

• Cryptography enables you to store sensitive information or transmit it across insecure networks (like the Internet) so that it cannot be read by anyone except the intended recipient. A cryptographic algorithm works in combination with a key-a word, number, or phrase-to encrypt the plaintext.

• The same plaintext encrypts to different ciphertext with different keys. The security of encrypted data is entirely dependent on two things: the strength of the cryptographic algorithm and the secrecy of the key.

Terms related to Cryptography

a. Plaintext: It is the human language that is commonly understood.

b. Ciphertext: Encoded messages which can only be understood by the sender and the intended receiver.

c.Encryption: Encoding plaintext to ciphertext

d. Decryption: Decoding cipertext to plaintext

e. Key: It used to transform palintext to ciphertext and vice versa

f. Process:

1. Sender writes message in plaintext

2. Plain text message is converted to ciphertext using key K.

3. Ciphertext is sent over the channel to the receiver.

4. The receiver uses key K to convert ciphertext back to plaintext.

g. Substitution Ciphers: In this each letter or a group of letters are replaced by another letter or group of letters

Network Technology for Digital Devices simplification:

1. Monoalphabetic Substituion: Each alphabet is replaced by an alphabet that is X alphabets after the actual alphabet in the set of english alphabets.

$\hspace{1.5cm}$Suppose X=2

$\hspace{1.5cm}$Then

$\hspace{1.5cm}$A B C D becomes

$\hspace{1.5cm}$C D E F

1. Polyalphabetic Substitution: Here substitution depends on

$\hspace{1.5cm}$i. The character itself.

$\hspace{1.5cm}$II. Position of character in the text

h. Transposition Ciphers: In this case the characters do not change, only the position of the characters change.

Process and example:

1. select a key. Let the key be COMP.

2. Number the alphabets in the key according to the way they occur in the set of English alphabets.

$\hspace{1.5cm}$C O M P gives 1 3 2 4

1. Let the message to be sent be "I GOT THE MONEY"

2. Write the message to be sent in the table as shown in figure and form the ciphertext.