This is How I Started My Learning of Cryptography.

Cryptography is often called “The Art of Secret Writing.”

Cryptography is often called “The Art of Secret Writing.” It is more than that. It encompasses everything from invisible inks to transmitting messages by quantum entanglement of photons.

In particular, cryptography includes the making and breaking of codes and ciphers.

What is Plaintext?

Plaintext or cleartext is the message or document that you wish to keep secret. In traditional cryptography, the message was text written in some language known to both sender and receiver. In a computer setting, this could be any type of file such as a PDF (text), JPG (image), MP3 (audio), or AVI (multimedia).

What is cipher?

A cipher is a method, or algorithm, for garbling a message to make it unreadable: for example, by changing the order of the characters or by replacing some characters with different characters. In general, ciphers operate on individual characters or groups of characters in the text without regard for their meaning.

Caesar Cipher (Shift Cipher)

This cipher shifts each letter in the alphabet by a fixed number.

Let’s use a shift of 3.

Alphabet shift rule

Plain : ABCDEFGHIJKLMNOPQRSTUVWXYZCipher: DEFGHIJKLMNOPQRSTUVWXYZAB

So:

• A → D

• B → E

• C → F

• X → A

• Y → B

• Z → C

What is a Key?

A key is a secret piece of information known only to the sender and the legitimate receiver(s) that selects which transformation is used for each message.

For example, if the cipher (method) is to change the order of the letters in a message, the key might specify which order to use for that day’s messages. A key can be a letter, a word or phrase, a number, or a sequence of letters, words and numbers. The strength of a cipher is highly dependent on the total size of the keys it uses.

Caesar Cipher Key

Cipher: Caesar cipherKey = 3

This means: shift every letter forward by 3.

So the key is literally the number:

Key = 3

Using this key:

• A → D

• B → E

• C → F

Without the key, someone wouldn’t know how many letters to shift.

What is Encryption?

Encryption or encipherment is the process of changing the plaintext into an unreadable garble by the legitimate sender who knows the key.

What is Ciphertext?

Ciphertext is the resulting garbled unreadable message or document, which will be transmitted or stored.

What is Decryption?

Decryption or decipherment is the process that the legitimate receiver, who knows the method and the key, uses to turn the garbled ciphertext back into the original plaintext message.

What is Code?

A code is also a method for garbling a message to make it unreadable. By contrast to a cipher, a code normally operates on words or phrases in a message. A typical code replaces words or phrases with groups of digits or letters. (Confusingly, the word code is also used to mean a standardized representation for letters, such as Morse code. Hopefully the meaning will be clear from the context.)

A code hides meaning by swapping whole words or phrases using a secret codebook.