Encryption is everywhere in our lives, from WhatsApp chats to online banking. But what if you wanted to learn the logic of encryption without a computer? The old style with paper and pen. You can do that with a fun puzzle system that mixes numbers and letters into strange-looking codes?

In this article, we’ll explore a homemade cipher idea based on numbers, calculation, and the alphabet. It’s not secure for real-world security, but it teaches and is a fun puzzle to show to your friends.

The Core Idea

Instead of mixing letters in a direct way, we use a random string of digits and arithmetic rules to turn each plaintext letter into a token that looks like numbers with a letter in the middle (e.g., 78O00, 18Q00). That I will explain in a bit.

Each token depends on:

• The seed digits we consume,
• Simple math operations (subtraction, multiplication, division),
• A sum that is the number from the alphabet (e.g., a = 1, b = 2).
• The results are a hybrid number-letter code that feels like random scrambling on paper but is simple enough to calculate on paper.

Step-by-Step Rules

Let’s say you want to encrypt a message. Both sender and receiver must agree on a numeric seed. For example:

Seed: 3183602946

For each letter you will need 4 digits. For each group of 4 digits, d1 d2 d3 d4, do the following:

Subtract the first two digits:

a = d1 - d2 If a < 0, add 10. If a = -2 ⇒ a = 10-2 = 8

Turn this into a “block” value:

block = (a + 1) × 2600

Multiply the last two digits:

b = d3 × d4

Divide b by 5200 (integer division):

c = b // 5200

Divide by d1 (integer division; if d1=0, then set result to 0):

d = c // d1

Subtract from the block:

e = block - d

Sum all four digits and pick a letter:

s = d1 + d2 + d3 + d4 letter = ((s-1) mod 26) + 1 → map to alphabet

Format the token:

Take the first two digits of e, then the chosen letter, then the last two digits of e.

That token is the ciphertext for one letter.

Worked Example

Let’s encrypt the word “HI” with the seed 3183602946.

Encrypting “H”

Digits: 3 1 8 3

a = 3 - 1 = 2

block = (2+1) × 2600 = 7800

b = 8 × 3 = 24

c = 24 // 5200 = 0

d = 0 // 3 = 0

e = 7800 - 0 = 7800

s = 3+1+8+3 = 15 → O (15th letter of the alphabet)

Token = 78O00

Encrypting “I”

Digits: 6 0 2 9

a = 6 - 0 = 6

block = (6+1) × 2600 = 18200

b = 2 × 9 = 18

c = 18 // 5200 = 0

d = 0 // 6 = 0

e = 18200

s = 6+0+2+9 = 17 → Q (17th letter)

Token = 18Q00

Final ciphertext for “HI”:

78O00 18Q00

Why It’s Fun (But Not Secure)

This system is not cryptographically secure. A determined attacker could reverse it quickly and with little ease. Because the math is simple. But that’s not the point.

Puzzle system: create challenges for friends. or game night.

Teaching tool: show how encryption mixes rules, keys, and the ability to think logically.

Brain exercise: practice and learn arithmetic and alphabet mapping in a playful way.

Think of it as Sudoku meets cryptography.

Variations to Try

You can make puzzles harder by:

Changing the number of digits per group (3, 5, or 6).

Reversing the order of digits before applying rules. to make it even harder.

Each twist makes decoding more of a game. And at the end it's a game; you can add the twists you find fun.

Final Thoughts

Real encryption is built on strong mathematics like AES or RSA and is far more secure than just number games. Scrambling numbers and letters. But puzzle ciphers like this have their own way of interesting us. They let us step inside the mindset of cryptographers: normalizing input, applying repeatable rules, and producing ciphertext that looks strange but is very much reproducible.

Who knows? It might spark the same curiosity that led mathematicians to invent real encryption in the first place. You just need to look far.