Summary

The Shift (or Caesar) Cipher is a type of substitution cipher that offers slightly more security than the Atbash Cipher but remains relatively easy to break by modern standards. This cipher was historically used by Julius Caesar to send secure messages, as noted by Suetonius. Caesar encrypted messages by shifting letters of the alphabet; for example, with a shift of three, "A" would be replaced by "D", "B" by "E", and so on.
This technique effectively obscured messages from adversaries. The cipher’s strength is slightly enhanced by the use of a key, which is the number of positions each letter is shifted. While still not highly secure, this method was more robust than the Atbash Cipher due to the variability of the shift, exemplified by Caesar’s own use of a shift of three..

Key selection Choose any number from 1 to 25. Remember, the security of the Caesar Cipher doesn't depend on the complexity of the key but rather on keeping it secret from unintended recipients!.
Consistency: Use the same key for encrypting and decrypting your message. If the key used to encrypt the message is not the same as the one used to decrypt it, the original text will not be recovered correctly.

How it works: Encryption

Encryption using the Shift Cipher involves creating a ciphertext alphabet by shifting the alphabet a specific number of places to the left, as determined by the key. For instance, a shift of 1 moves "A" to the end of the alphabet, and a shift of 15 would rearrange "A" to the position of "P". This shift creates a new order of letters, which is then used to encode the plaintext. For example, using a key of 3, the plaintext "julius caesar" would be encrypted to "MXOLXV FDHVDU", by matching each letter to its shifted counterpart in the ciphertext alphabet.

How it works: Decryption

Decryption is equally straightforward and involves reversing the encryption process using the same shifted alphabet. If the ciphertext received is "PDUFXV EUXWXV" and it’s known to have been encrypted with a key of 3, the decryption process would simply require aligning the ciphertext with the shifted alphabet to convert it back to plaintext, revealing "marcus brutus". This decryption uses the same table used for encryption, making the process efficient but vulnerable to brute force attacks, as only 26 possible shifts exist, each easily tested until coherent text emerges.

[1] Adriano, "Caesar Cipher: What It Is and How It Works" - Online source.
[1] Cryptop Corner, "Caesar Cipher" - Online source.