FOR YOUR EYES ONLY // MODULE 11
REFERENCE MATERIAL
ESTIMATED: 10 MINUTES
OPERATIONAL BRIEF
Quick reference for the complete encrypt and decrypt procedures, plus glossary and practice templates.
For Mod-26 Vernam reference, see the main reference site.
11.1 Quick Reference: Complete Procedure
ENCRYPT
- Write plaintext in capitals, decide space convention.
- Encode each character via checkerboard → digit string.
- Group digit string in 5s.
- Select correct pad page, note page ID and starting position.
- Extract key digits (one per plaintext digit).
- Add plaintext digit + key digit mod 10 → ciphertext digit.
- Group ciphertext in 5s.
- Transmit: page ID, start position, ciphertext groups.
- Mark pad page as used to current position.
- Destroy all intermediate materials.
DECRYPT
- Record ciphertext, page ID, start position.
- Load correct pad page at specified position.
- Extract key digits (one per ciphertext digit).
- For each ciphertext digit: add tens complement of key digit mod 10 → plaintext digit.
- Decode digit string via checkerboard (left to right, escape rule).
- Mark pad page as used.
- Destroy intermediate materials.
11.2 Mod-10 Addition Table
| + | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 |
| 2 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | 1 |
| 3 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | 1 | 2 |
| 4 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | 1 | 2 | 3 |
| 5 | 5 | 6 | 7 | 8 | 9 | 0 | 1 | 2 | 3 | 4 |
| 6 | 6 | 7 | 8 | 9 | 0 | 1 | 2 | 3 | 4 | 5 |
| 7 | 7 | 8 | 9 | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
| 8 | 8 | 9 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 9 | 9 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
11.3 Tens Complements
| DIGIT | COMPLEMENT | DIGIT | COMPLEMENT | |
|---|---|---|---|---|
| 0 | 0 | 5 | 5 | |
| 1 | 9 | 6 | 4 | |
| 2 | 8 | 7 | 3 | |
| 3 | 7 | 8 | 2 | |
| 4 | 6 | 9 | 1 |
11.4 Reference Checkerboard
| COL | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|---|
| ROW 1 | E | T | A | — | O | N | I | — | S | H |
| ROW 2 (esc=3) | B | C | D | F | G | J | K | L | M | P |
| ROW 3 (esc=7) | Q | R | U | V | W | X | Y | Z | . | / |
Code Reference
Single-digit codes (Row 1): E=0, T=1, A=2, O=4, N=5, I=6, S=8, H=9
Two-digit codes, Row 2 (escape=3): B=30, C=31, D=32, F=33, G=34, J=35, K=36, L=37, M=38, P=39
Two-digit codes, Row 3 (escape=7): Q=70, R=71, U=72, V=73, W=74, X=75, Y=76, Z=77, .=78, /=79
11.5 Encoding Frequency Notes
With this checkerboard, average digits per character (English text):
- 8 letters with 1-digit code: E T A O N I S H — combined frequency ~57.6% of English text
- 18 letters with 2-digit code: all others — combined frequency ~42.4%
Average = (0.576 × 1) + (0.424 × 2) = 0.576 + 0.848 = 1.424 digits/character
Compare to naive 2.0: saves approximately 29% in pad consumption.
11.6 Common Mistakes Checklist
- ☐ Using regular subtraction instead of tens complement + addition
- ☐ Treating a ciphertext digit as an escape digit during decode
- ☐ Starting decryption at the wrong pad position
- ☐ Failing to destroy intermediate plaintext after encryption
- ☐ Re-using a pad page (ANY portion of it)
- ☐ Computing (P − K) instead of (P + K) for encryption
- ☐ Grouping digit strings differently than in 5-group convention before and after encryption
- ☐ Failing to null-pad the final group (leaves message length information)
- ☐ Leaving scratch paper with intermediate calculations
- ☐ Not marking consumed pad positions immediately after use
Glossary
- Checkerboard
- A tabular encoding scheme converting text characters to digit sequences of variable length.
- Ciphertext
- The output of encryption. Safe to transmit.
- Crib
- A known or expected piece of plaintext used to align key streams or recover from errors.
- Depth
- Two ciphertexts encrypted with the same key segment. Exploitable.
- Escape digit
- A digit in a checkerboard that signals a two-digit code follows (not a direct single-digit encoding).
- Figure shift / Letter shift
- Convention markers used to switch between digit encoding mode and letter encoding mode in extended checkerboards.
- Five-group format
- Organizing ciphertext in groups of five digits for transmission and error detection. Standard for hand ciphers.
- Intermediate digit string
- The output of checkerboard encoding, before OTP addition. Equivalent in sensitivity to plaintext.
- Key stream
- The sequence of random digits from the pad used for encryption.
- Message indicator
- A short agreed-upon standard phrase at the start of a message that confirms correct key alignment.
- Mod-10 addition
- Addition modulo 10, keeping only the ones digit. No carrying.
- Null padding
- Random digits appended to a message to reach a group boundary or standardize message length.
- One-time pad (OTP)
- A cryptographic system using a key as long as the plaintext, used once and only once, providing perfect secrecy.
- Pad page
- A numbered sheet of pre-generated random digits used as a key stream for one or more messages.
- Perfect secrecy
- Shannon's definition: a cipher in which the ciphertext reveals no information about the plaintext to an adversary without the key.
- Plaintext
- The unencrypted original message.
- Straddling checkerboard
- A checkerboard that "straddles" by using variable-length codes (1 or 2 digits) to achieve encoding efficiency.
- Tens complement
- The value (10 − a) mod 10, used to perform mod-10 subtraction as addition.
- Traffic analysis
- Inference from message metadata (timing, length, frequency, direction) rather than content.
- VENONA
- US/UK intelligence project (1943–1980) that exploited Soviet OTP misuse to decrypt significant quantities of traffic.
Practice Form Templates
Print these templates for paper-based exercises. Use pencil for easy destruction.
Pad Page Template
PAD: _________ PAGE: ___ COPY: ___ DATE: ___________ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ CONSUMED THROUGH: GROUP ___ DIGIT ___ DESTROYED: ___________
Encryption Worksheet
MESSAGE ID: ____________ DATE: ____________ PAGE: ___ POS: ___ PLAINTEXT: ________________________________________________________________________ CHECKERBOARD ENCODING: CHAR: | | | | | | | | | | | | | | | | CODE: | | | | | | | | | | | | | | | | DIGIT STRING: _________________________________________________________ KEY DIGITS: _________________________________________________________ CIPHERTEXT: _________________________________________________________ GROUPED: _____ _____ _____ _____ _____ _____ _____ _____ _____ _____
Decryption Worksheet
MESSAGE ID: ____________ DATE: ____________ PAGE: ___ POS: ___ RECEIVED CIPHERTEXT: _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ DIGIT STRING: _________________________________________________________ KEY DIGITS: _________________________________________________________ K' (COMPL): _________________________________________________________ PLAIN DIGITS: _________________________________________________________ DECODED PLAINTEXT: ________________________________________________________________________
Checkerboard Template
Col: 0 1 2 3 4 5 6 7 8 9
-------|---|---|---|---|---|---|---|---|---|---|
Row 1:| | | | | | | | | | |
Row 2:| | | | | | | | | | | (escape = _)
Row 3:| | | | | | | | | | | (escape = _)
-------|---|---|---|---|---|---|---|---|---|---|
KEYWORD: ____________________
BLANK POSITIONS: ___, ___
DATE / EDITION: ____________________