Beaufort cipher

The Beaufort cipher is a classical polygram substitution method and a variant of the polyalphabetic cipher family. It was developed by British Admiral Sir Francis Beaufort in the early 19th century, though it gained broader recognition due to its similarity to the well-known Vigenère cipher.

What makes the Beaufort cipher unique is its simplicity and symmetry: the same algorithm is used for both encryption and decryption. While elegant and easy to implement, the Beaufort cipher today is mostly used for educational purposes, cryptographic puzzles, and demonstrations of basic encryption principles. It is considered insecure for practical applications compared to modern standards like AES or RSA, but it remains valuable for historical and instructional use.

How the Beaufort Cipher Works

The Beaufort encryption algorithm is simple enough to be performed by hand and easy to implement in code. It involves a few straightforward steps.

Step 1: Preparing the data

• Select the plaintext message you want to encrypt.
• Choose a keyword. If the keyword is shorter than the message, repeat it cyclically to match the length.

Step 2: Encrypting the message

Each letter of the message is encrypted using the following formula:

\[ C_i = (K_i - P_i) \mod N \]

• \(C_i\) — the encrypted character (ciphertext);
• \(K_i\) — the corresponding character from the keyword;
• \(P_i\) — the original plaintext character;
• \(N\) — the size of the alphabet (for English, \(N = 26\)).

Example

Let’s encrypt the word SECRET using the keyword CODE.

Plaintext	S	E	C	R	E	T
Index	18	4	2	17	4	19

Keyword	C	O	D	E	C	O
Index	2	14	3	4	2	14

Step	Calculation	Resulting Index	Letter
1	(2 – 18) mod 26	10	K
2	(14 – 4) mod 26	10	K
3	(3 – 2) mod 26	1	B
4	(4 – 17) mod 26	13	N
5	(2 – 4) mod 26	24	Y
6	(14 – 19) mod 26	21	V

The resulting ciphertext is: KKBNYV.

Decryption with the Beaufort Cipher

Thanks to the cipher’s symmetric nature, the decryption process is identical to encryption and follows the same formula:

\[ P_i = (K_i - C_i) \mod N \]

Decryption Example

Let’s decrypt the ciphertext KKBNYV using the keyword CODE:

Ciphertext	K	K	B	N	Y	V
Index	10	10	1	13	24	21

Keyword	C	O	D	E	C	O
Index	2	14	3	4	2	14

Step	Calculation	Resulting Index	Letter
1	(2 – 10) mod 26	18	S
2	(14 – 10) mod 26	4	E
3	(3 – 1) mod 26	2	C
4	(4 – 13) mod 26	17	R
5	(2 – 24) mod 26	4	E
6	(14 – 21) mod 26	19	T

The original plaintext is: SECRET.

Modern Applications of the Beaufort Cipher

Although the Beaufort cipher belongs to the category of historical cryptographic methods, it continues to have value in modern educational and recreational contexts. Today, it's commonly used in the following ways:

• Cryptography education: The cipher is perfect for demonstrating the core ideas of polyalphabetic encryption in schools, universities, and online courses.
• Puzzles and escape rooms: Popular in logic games and escape quests, where decoding skills and analytical thinking are required.
• Online tools and simulators: Web-based cipher tools make it easy to experiment with Beaufort encryption and decryption without needing to do manual calculations.

It’s important to note that the Beaufort cipher is not secure by modern standards and should not be used to protect sensitive or confidential information.

Security Tips and Best Practices

If you're using the Beaufort cipher for educational purposes or puzzle creation, it's still worth following a few basic security guidelines:

• Choose a strong key: The key should be sufficiently long and not easy to guess.
• Use random characters: Avoid common or predictable words as your key.
• Rotate keys regularly: Change the key periodically to enhance variability.

Key Type	Examples	Recommendation
Recommended ✅	X9KPZ, FHTY63, GN7PD	Random mix of letters and numbers
Not Recommended ❌	PASSWORD, 123456, QWERTY	Too simple or widely used

By following these simple tips, you can use the Beaufort cipher more effectively in learning environments, games, and interactive scenarios.

Conclusion

The Beaufort cipher is a great example of a historical polyalphabetic encryption method. It offers a clear and hands-on way to explore how cryptographic algorithms work.

Its simplicity and ease of understanding make it especially popular in educational settings and recreational challenges. However, it should not be used to secure sensitive or private information. Instead, its true value lies in teaching, puzzles, and online demonstrations.