Cesar encryption

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One of the simplest forms of cryptography is Cesar encryption. This form of cryptographygets its name from Julius Cesar as it is said that the Roman emperor was the one who invented it.

The encryption involves using numbers to represent the 26 letters of the alphabet with 0-A, 1-B,...up to 25-Z.

A key is then chosen which is a number between 1 and 25. This is then added to the numeric version of the message if we want to encrypt the message or subtracted if we have a message we want to decrypt.

If the resulting number is less than 0, or more than 25, then 26 is added, or subtracted, respectively until the number is between 0 and 25. This is written as mod 26 but this just really means the remainder when divided by 26.

These numbers are then translated back into letters to form the encrypted message.

For example if I wanted to encrypt the word maths, using the key 5, this would be done by:

Text message: M A T H S

Numeric message: 12 0 19 7 18

Plus 5 mod 26

Encrypted numeric message: 17 5 24 12 23

Encrypted text message: R F Y M X

For an example of how to decrypt a message; to decrypt the title of this blog post using the key 10 would be done by:

Encrypted text message: M B I Z D Y Q B K Z R I

Encrypted numeric message: 12 1 8 25 3 24 16 1 10 25 17 8

Minus 10 mod 26

Decrypted numeric message: 2 17 24 15 19 14 6 17 0 15 7 24

Decrypted text message: C R Y P T O G R A P H Y

Can you decrypt the message R C S Q C A using this method with the key equal to 8?