FOR YOUR EYES ONLY // MODULE 04

ENCRYPTION

ESTIMATED: 25 MINUTES
OPERATIONAL BRIEF

The encryption pipeline: plaintext → checkerboard encode → mod-10 add key → five-group format. Each step is independent and reversible.

4.1 Preparation Checklist

Before encrypting:

[ ] Plaintext is ready and finalized. No last-minute changes. [ ] Checkerboard is available (memorized or on reference card). [ ] Correct pad page is selected. Page identifier noted. [ ] Starting position on pad noted. [ ] Work surface is private and secure. [ ] Pencils, not pens (pen presses through; can be read from next sheet). [ ] Post-encryption destruction plan confirmed.

4.2 Step 1: Prepare the Plaintext

Write out the full plaintext. This is your last chance to review it.

Convert to uppercase. Remove any formatting. Decide on space convention — we drop spaces (the receiver infers word boundaries from context).

CONVENTION

In our reference system, spaces are dropped. Some traditions encode spaces as a special character. Dropping spaces is simpler and leaks less information about word lengths.

4.3 Step 2: Encode with Checkerboard

Working letter by letter, look up each character in the reference checkerboard and write its digit code below it.

Reminder of the reference layout:

0123456789
ROW 1 E T A O N I FIG / / FIG
7_ SRHLDCUMWF
8_ GYPBVKJXQZ

Concatenate all codes into a single digit string, then group into 5s. This intermediate string is not the ciphertext — it is the input to the OTP encryption step.

4.4 The Encryption Pipeline

-- TRAINING REFERENCE --
OTP / SC OPERATIONAL PIPELINE
MOD-10 WITH STRADDLING CHECKERBOARD — FIGURE 1
ENCRYPT
INPUT
PLAINTEXT
English text, uppercase
Spaces dropped
STEP 1 — ENCODE
CHECKERBOARD
Each char → 1 or 2 digits
High-freq = single digit
Escape digits: 7, 8
avg ~1.4 digits/char
INTERMEDIATE — SENSITIVE
DIGIT STRING
Concatenated digit codes
Grouped in 5s
Destroy after use
STEP 2 — ENCRYPT
MOD-10 ADDITION
Per digit: P + K (mod 10)
No carry. No borrow.
Column-by-column.
C = (P + K) mod 10
OUTPUT — TRANSMIT
CIPHERTEXT
5-digit groups
Page ID + start pos
Safe to transmit
DECRYPT
INPUT — RECEIVED
CIPHERTEXT
5-digit groups
Record page ID + start pos
Do not rearrange
STEP 1 — DECRYPT
MOD-10 SUBTRACT
Add tens complement of K
Never subtract directly
Column-by-column.
P = (C + K′) mod 10
INTERMEDIATE — SENSITIVE
DIGIT STRING
Recovered plaintext digits
Not yet readable
Destroy after use
STEP 2 — DECODE
CHECKERBOARD
Left to right, strictly
Escape rule: 7/8 = 2-digit
No lookahead
digit(s) → char
OUTPUT
PLAINTEXT
Verify against crib
Destroy all workings
Mark pad as used
SHARED SECRET — BOTH SIDES REQUIRE
KEY MATERIAL: ONE-TIME PAD + CHECKERBOARD KEY
Pad: true random digits • Generated from d10 dice • Pre-shared by physical courier
Used once only • Destroyed after use • Never transmitted electronically
Checkerboard: keyed layout • Pre-shared separately • Stored apart from pad
PROCESS STEP
DATA STATE
SHARED SECRET
OTP-SC COURSE — FIGURE 1 — OPERATIONAL PIPELINE — V1.0

4.5 Step 3: Mod-10 Addition

Extract key digits from your pad starting at the noted position. You need exactly as many key digits as there are plaintext digits. Write plaintext digits above, key digits below, and compute each column independently:

C = (P + K) mod 10 Work each column. No carry. No borrow. Example: (7+5)=12 → 2 (0+8)=8 (1+6)=7

Group the resulting ciphertext digits into 5-digit transmission groups. If the final group is incomplete, pad with random null digits.

Authentication Note

Plain OTP provides confidentiality but not authentication. The receiver who decrypts a valid-looking message has no cryptographic proof it came from the expected sender. For training purposes, authentication is out of scope.

4.6 Worked Example

Let us encrypt the plaintext MEETATDAWN using our reference checkerboard.

-- TRAINING REFERENCE --
WORKED ENCRYPTION EXAMPLE
FULL PIPELINE: PLAINTEXT → CHECKERBOARD → MOD-10 → CIPHERTEXT — FIGURE 4
INPUT
PLAINTEXT
MEETATDAWN
Uppercase. Spaces dropped. 10 characters total.
STEP 1
CHECKERBOARD ENCODE
M
↓
77
E
↓
0
E
↓
0
T
↓
1
A
↓
2
T
↓
1
D
↓
74
A
↓
2
W
↓
78
N
↓
4
DIGIT STRING 7 7 0 0 1 2 1 7 4 2 7 8 4 13 DIGITS
STEP 2
MOD-10 ADDITION — (P + K) mod 10
PLAINTEXT (P) 7700121742784
+++++++++++++
KEY (K) 5382604719362
=============
CIPHER (C) 2082725451046
EXAMPLES: (7+5)=12→2 • (0+8)=8 • (1+6)=7 • (7+7)=14→4 • (2+9)=11→1 • NO CARRYING
OUTPUT
CIPHERTEXT — 5-DIGIT GROUPS
2
0
8
2
7
GRP 01
2
5
4
5
1
GRP 02
0
4
6
_
_
GRP 03 (NULL PAD)
TRANSMIT PAGE: 003   START: G01/D01   GROUPS: 3
MESSAGE 20827   25451   046__
✓ Plaintext “MEETATDAWN” correctly encoded to 13 digits and encrypted into 3 transmission groups. Receiver requires: ciphertext, pad page 003, and checkerboard key.
OTP-SC COURSE — FIGURE 4 — WORKED ENCRYPTION EXAMPLE — V1.0
WORKED EX.

4.7 Step-by-Step Walkthrough

Walk through the encryption of MEETATDAWN one step at a time. Click NEXT STEP to reveal each stage of the pipeline.

INTERACTIVE: ENCRYPTION STEPPER

EXERCISES — MODULE 4

EXERCISE 4.1 — FULL ENCRYPTION

Using the reference checkerboard, encode and encrypt the following plaintext. Generate your own key digits from dice (or use random digits).

PACKAGEDELIVERED

Show all steps: plaintext, checkerboard encoding, key extraction, mod-10 addition, grouped ciphertext.

ANSWER KEY (encoding only)

P=82, A=2, C=75, K=85, A=2, G=80, E=0, D=74, E=0, L=73, I=5, V=84, E=0, R=71, E=0, D=74

Digit string: 82 2 75 85 2 80 0 74 0 73 5 84 0 71 0 74

Concatenated: 8 2 2 7 5 8 5 2 8 0 0 7 4 0 7 3 5 8 4 0 7 1 0 7 4 (25 digits)

Grouped: 82275 85280 07407 35840 71074

Your ciphertext will depend on your key digits. Add mod-10 column by column.

EXERCISE 4.2 — SECOND ENCRYPTION

Repeat for:

EXTRACTIONCOMPROMISEDABORT
EXERCISE 4.3 — DECRYPT AND DECODE

Given this ciphertext and key, decrypt and decode:

Ciphertext groups: 31482 90173 65542 Key (page 001, position 001): 23971 44826 51037
ANSWER KEY

Key complement: 8 7 1 3 9   6 6 2 8 4   5 9 1 7 3

Cipher:         3 1 4 8 2   9 0 1 7 3   6 5 5 4 2

Plaintext:     1 8 5 1 1   5 6 3 5 7   1 4 6 1 5

Decode: 1=T, 8_ ??? — work it yourself with the checkerboard!

EXERCISE 4.4 — KEY ESTIMATION

You have a 30-character message to send. How many key digits do you need to prepare?

ANSWER KEY

Worst case: all 2-digit codes: 30 × 2 = 60 digits.

Best case: all 1-digit codes: 30 × 1 = 30 digits.

Reasonable estimate: 30 × 1.4 = 42 digits → round up to 45 (9 groups of 5).

Prepare at least 45 key digits. Prefer 50 for safety margin.

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