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@@ -45,6 +45,8 @@ Information about the parts of the WROLPi interface.
     * [Summary](modules/inventory/summary.md)
     * [Ration](modules/inventory/ration.md)
     * [Export](modules/inventory/export.md)
+* Calculators
+    * [One-Time Pad](modules/calculators/one-time-pad.md)
 
 ## Other File Types
 
diff --git a/docs/modules/calculators/one-time-pad.md b/docs/modules/calculators/one-time-pad.md
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+# One-Time Pad
+
+A One-Time Pad lets you encrypt short messages so they can be passed over an insecure channel — read aloud over
+radio, handed over on paper, or sent by courier. WROLPi can generate a pad for you, and can encrypt and decrypt
+messages with it. You can also do the same work with nothing but the printed pad and a pencil.
+
+> To open it, click **More** → **Calculators** → **One Time Pad**.
+
+![The One-Time Pad page](otp-advanced.png)
+
+Everything on this page happens **inside your browser**. Your pad, your message, and the encrypted result are
+never sent to the WROLPi server, and they are never stored. Reload the page and they are gone.
+
+## Generate a Pad
+
+> Click **Generate New Pad**.
+
+A new page opens containing 8 numbered messages. Each message is 320 random characters, printed as groups of five
+characters, 16 groups per line.
+
+![A generated One-Time Pad](otp-generated-pad.png)
+
+The characters are chosen with your browser's cryptographically secure random number generator, so the pad is
+unpredictable. Every pad is unique — the page is generated fresh each time you click the button, and WROLPi keeps
+no copy of it.
+
+Print the page and give a copy to everyone you trust to exchange messages with. Everyone in the group must hold
+the **same** pad, because the sender and receiver both use the same characters.
+
+**Warning!** Use each message **only once**. A pad character that is reused can be broken. Cut off and burn each
+message as it is used.
+
+## Encrypt
+
+Both the sender and the receiver need the same two things: the **key** (an unused message from the pad) and the
+character set the pad was built from.
+
+![Encrypting a message](otp-encrypt.png)
+
+* **Key** — type or paste the random characters from your One-Time Pad.
+* **Plaintext** — the message you want to send.
+* **Ciphertext** — the encrypted result, updated as you type.
+
+Spaces and line breaks are ignored in both boxes, so you can paste the pad exactly as it is printed. When the
+character set contains only uppercase characters (as both built-in sets do) your typing is uppercased for you.
+The ciphertext is printed in groups of five characters, which is the traditional format for reading a message
+aloud.
+
+The key must be at least as long as the plaintext. If it is shorter you will see *Plaintext is longer than OTP*.
+If either box contains a character that is not in the pad's character set, you will see an *invalid characters*
+error — punctuation is not encryptable, so spell it out (for example `STOP` instead of `.`).
+
+## Decrypt
+
+![Decrypting a message](otp-decrypt.png)
+
+* **Key** — the same pad message the sender used.
+* **Ciphertext** — the message you received.
+* **Plaintext** — the decrypted message.
+
+The plaintext is also shown in groups of five, so word breaks are not preserved: `MEET AT DAWN` comes back as
+`MEETA TDAWN`.
+
+## Advanced
+
+### Pad Characters
+
+The character set is a property of the pad — generating, encrypting, and decrypting all use whichever set is
+selected here, so the whole group must agree on it before any pads are printed.
+
+| Setting | Characters |
+| --- | --- |
+| **A–Z** | The 26 uppercase letters. The easiest set to work by hand. |
+| **A–Z and 0–9** | The 26 letters plus the 10 digits (the default). Lets you send numbers without spelling them out. |
+| **Custom** | Any set you define. |
+
+A custom set must have between 2 and 256 characters, they must all be unique, and it cannot contain spaces. A
+custom set that includes lowercase characters is treated as case-sensitive — nothing is uppercased for you, so
+`a` and `A` are two different characters (and both must appear in the set).
+
+**Warning!** The [cheat sheet](#by-hand) table is built for the **A–Z and 0–9** set only. If you change the
+character set, the printed table no longer applies and the pad can only be used by hand with a table you make
+yourself.
+
+### Error Detection
+
+> Tick **Append error-detection checksum**.
+
+With this enabled, encryption adds one extra character to the end of the ciphertext. That character is calculated
+from the **ciphertext**, not from your plaintext, so it gives away nothing about your message — it is worked out
+from the encrypted characters that get sent anyway. If a character is misheard, mistyped, or two characters are
+swapped in transit, the receiver is told about it.
+
+When decrypting with the checkbox ticked, WROLPi shows **Checksum is valid** or **Checksum is invalid** above the
+plaintext. An invalid checksum means the message arrived corrupted — ask the sender to repeat it rather than
+trusting what you decrypted.
+
+Both the sender and receiver must have the setting turned on. If the receiver has it on and the message does not
+actually carry a checksum, WROLPi says the checksum is invalid and decrypts the whole message anyway, so no
+character is silently dropped.
+
+## By Hand
+
+A One-Time Pad needs no computer. WROLPi ships a printable reference table so you can encrypt and decrypt with a
+pencil.
+
+> Click **Cheat Sheet PDF** to open it, then print it along with your pads.
+
+The sheet is a grid of the **A–Z and 0–9** characters:
+
+* **Sending / encrypting** — find the intersection of the row for your pad character and the column for the
+  character you want to send. The character in that cell is what you send.
+* **Receiving / decrypting** — follow the row of your pad character until you find the character you received;
+  the column header is the decrypted character.
+
+Work one character at a time, striking through each pad character as you use it.
+
+## Rules to Follow
+
+1. Every member of the group has their own copy of the same pad.
+2. Each pad message is used once, then destroyed.
+3. The pad is only as secret as its worst copy — a pad that has been photographed, emailed, or left lying around
+   is no longer a secret.
+4. Only the characters in the pad's character set can be sent. Spell out punctuation and word breaks.
+5. A pad message limits how long your message can be. Keep messages short.
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