CAPSTONE EXERCISES
Full operational exercises combining every skill. Generate pads, construct keyed checkerboards, encrypt, decrypt, track pad consumption, and recover from errors.
These exercises integrate all skills. Work them in full, on paper, using only the reference material in Module 11.
Track your total time across all exercises. Target: complete all four capstone exercises within 45 minutes.
12.1 Full Cycle: Encrypt and Decrypt
Step 1: Generate Materials
- Generate a practice pad of 200 digits (roll d10 200 times).
- Construct a keyed checkerboard using keyword PHANTOM.
Step 2: Encrypt Three Messages
Using your generated pad and checkerboard, encrypt each of the following messages. For each: show checkerboard encoding, key extraction, mod-10 addition, grouped ciphertext. Track pad consumption across all three messages.
ALL UNITS STAND DOWN AWAIT ORDERS
CONFIRM RECEIPT OF LAST TRANSMISSION
SITE IS CLEAR PROCEED
Log your pad usage for each message. Track page, start position, end position, and digits consumed.
Step 3: Decrypt
Give your ciphertext (without key or plaintext) to a partner. They should decrypt from the ciphertext groups, pad page, and page/position metadata alone. Verify results match.
If working alone: wait 24 hours before decrypting your own ciphertext. The memory of the plaintext should have faded enough to give an honest test.
12.2 Error Recovery Scenario
Receive the following key and ciphertext:
Part A: Normal Decryption
Decrypt the message normally using the tens complement method.
Part B: Error Correction
The 8th digit of the ciphertext was transmitted with an error — the correct digit is 6, not 7. Re-decrypt with the correction.
What changed? With mod-10, a single-digit error in the ciphertext produces a single-digit error in the recovered plaintext digit string, which may or may not affect one or two characters depending on whether the wrong digit falls in a single-code or double-code position.
Identify corrupted digits in a transmitted ciphertext.
12.3 Checkerboard Construction and Use
Construct a complete keyed checkerboard from the keyword:
NIGHTFALL
Using the construction method in Module 6. Choose blank positions at columns 4 and 8. Verify that the 8 highest-frequency English letters are in Row 1.
Build your NIGHTFALL checkerboard interactively. Verify your paper work against this tool.
Encrypt with Your Checkerboard
Using your constructed NIGHTFALL checkerboard, encrypt this message (generate your own key from dice):
THE OPERATION COMMENCES AT NIGHTFALL
Document the full procedure: checkerboard encoding, key extraction, mod-10 addition, grouped ciphertext, pad position tracking.
12.4 Traffic Analysis Resistance
You need to maintain a daily communication schedule with a counterpart over an observed channel. Messages vary from 3 to 30 characters. An observer will see the length of your transmissions.
Design Challenge
Design a protocol using null padding that makes all messages appear to be the same length (say, 10 groups = 50 ciphertext digits).
- All transmissions must be exactly 10 five-digit groups (50 digits).
- The receiver must be able to distinguish real message from padding.
- An observer seeing only ciphertext must not be able to determine where the real message ends.
Write out the full encoding and null-padding convention. Consider:
- How do you signal to the receiver where the real message ends and padding begins?
- What do you use for the padding digits? (They must be indistinguishable from ciphertext.)
- How does this affect pad consumption? (Every message now uses 50 key digits regardless of length.)
Traffic analysis was one of the most productive intelligence techniques in the VENONA era. Message length alone revealed whether a communication was a routine check-in or an urgent operational message. Fixed-length padding eliminates this channel.
Course Complete
If you have completed all capstone exercises on paper, you have demonstrated proficiency in the complete OTP/Straddling Checkerboard system: pad generation, checkerboard construction, encryption, decryption, error recovery, position tracking, and traffic analysis resistance.
The cipher is provably secure. Your operational discipline is what makes it work.