nerdletter/cipher/instructions.md

### HOW TO DECRYPT — STEP BY STEP (by hand)

1. **Key Generation**
   - You generate a **truly random key** that is **exactly as long as the plaintext message**.
   - The key must be purely random (each bit or character has equal probability and no patterns).
   - The key is shared securely between sender and receiver **in advance** (this is the hard part in practice).

2. **Encryption**
   - Take the plaintext message.
   - Combine it with the key using a reversible operation. The two most common methods are:
     - **Bitwise XOR** (for binary data — the standard modern way)
     - **Modular addition** (for letters, e.g., A=0, B=1, ..., Z=25 — the classic "pad" method)

   **Example with letters (Vigenère-style addition mod 26):**

   Plaintext:  `H E L L O`  
   → Numbers: `7 4 11 11 14`

   One-time pad: `X M C K L`  
   → Numbers: `23 12 2 10 11`

   Ciphertext: `(7+23) (4+12) (11+2) (11+10) (14+11) mod 26`  
   → `4 16 13 21 25`  
   → Letters: `E Q N V Z`

3. **Decryption**
   - The receiver does the **inverse operation** with the same key.
   - For modular addition: subtract the key (mod 26).
   - For XOR: XOR the ciphertext with the key again (XOR is its own inverse).

   Ciphertext: `E Q N V Z`  
   Key:        `X M C K L`  
   Plaintext:  `H E L L O`


### HOW TO DECRYPT — STEP BY STEP (using your 8-bit machine)

1. Type in the NERDLETTER DECRYPTOR BASIC program (provided with your subscription)
2. RUN
3. When it asks, type the PAD groups from the appropriate page:

   `VLGKU`   then   `JXCYH`   then   `LYMDC`   then   `ETLUB`

4. Then type the CIPHERTEXT groups:

   `HLXIB`   then   `JACJP`   then   `ERXHN`   then   `EFM`

5. The machine will instantly print the plaintext!

#### RESULT YOU SHOULD SEE:

```
MARYHADALITTLELAMB
```

(Add spaces by hand: “Mary had a little lamb”)

This is perfect secrecy — no computer on Earth can read it without this exact Day Book page.