# Modular Math and the Shift Cipher

The Caesar Cipher is a simple cipher, called the Shift Cipher that uses the modulo operator to encrypt and decrypt messages.

The Shift Cipher has a **key K**, which is an **integer from 0 to 25**. We will only share this key with people that we want to see our message.

# How to Encrypt:

For every letter in the message **M** :

**1.** Convert the letter into the number that matches its order in the alphabet starting from 0, and call this number **X**.

( A=0, B=1, C=2, ...,Y=24, Z=25)

**2.** Calculate: **Y** = **(X + K)** mod **26**

**3.** Convert the number **Y** into a letter that matches its order in the alphabet starting from 0.

(A=0, B=1, C=2, ...,Y=24, Z=25)

**For Example: **We agree with our friend to use the Shift Cipher with **key K=19** for our message.

We encrypt the message **"KHAN", **as follows:

So, after applying the Shift Cipher with key K=19 our message text "**KHAN**" gave us **cipher text **"**DATG**".

We give the message "DATG" to our friend.

# How to decrypt:

For every letter in the cipher text **C** :

1. Convert the letter into the number that matches its order in the alphabet starting from 0, and call this number Y.

(A=0, B=1, C=2, ..., Y=24, Z=25)

2. Calculate: **X= (Y - K)** mod** 26**

3. Convert the number **X** into a letter that matches its order in the alphabet starting from 0.

(A=0, B=1, C=2, ..., Y=24, Z=25)

Our friend now decodes the message using our agreed upon **key K=19. **As follows:

So, after decrypting the Shift Cipher with key K=19 our friend deciphers the cipher text "**DATG**" into the message text "**KHAN**".

# Why is the Shift Cipher insecure?

A cipher should prevent an attacker, who has a copy of the cipher text but does not know the key, from discovering the contents of the message. Since **we only have 26 choices for the key**, someone can easily__ try all of the 26 keys__, one by one, until they recover the message. This type of attack is called a **brute force attack**.

Next let's practice this cipher and earn some energy points! You deserve it.

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