KEY MATERIAL

3.1 Requirements for the Key

The security of the entire system depends entirely on the quality and handling of the key material. No cipher engineering compensates for bad key hygiene.

The key stream (also called the pad) must satisfy all of the following:

1. Random: Each digit is statistically independent of all others. No pattern. No pseudorandomness. True physical randomness.
2. Secret: No unauthorized person has seen any part of the pad.
3. Never reused: Each page, each digit, used exactly once.
4. Destroyed after use: Immediately and completely.
5. Longer than the message: The key must be at least as long as the encoded plaintext digit string.

3.2 Generating Key Material with Dice

The gold standard for hand-generated key material is physical dice. We use ten-sided dice (d10) with faces 0–9.

Method

1. Roll one d10 for each digit needed.
2. Record immediately, sequentially.
3. Do not “correct” any result. Do not re-roll because it looks non-random. Accept every result.
4. Work in a private, secure location.
5. Never photograph or allow phone cameras ambient line-of-sight.

How many digits per message?

Rule of thumb: English text encodes to approximately 1.4 digits per character using our checkerboard. For a 50-character message, prepare 70–80 key digits, and round up to a clean group boundary (multiple of 5).

Always prepare more than you think you need. Truncating mid-message because you ran out of key is an operational failure.

3.3 The Pad Format

A one-time pad is typically organized as:

• A series of numbered pages
• Each page has a unique page identifier (printed or handwritten)
• Digits are arranged in 5-digit groups, typically 10 groups per line (50 digits per line)
• Pages are typically 10–20 lines (500–1000 digits)
• The pad has two identical copies: one per party

The page identifier is transmitted with the message (in the clear or via agreed protocol) so the receiver knows which pad page to use.

3.4 Key Discipline

Before encryption

• Confirm you have the correct page.
• Note your starting position (group and digit within group).
• Never start a new message in the middle of a page without marking exactly where you are.

During encryption

• Track position with a finger or pencil dot.
• Announce each key digit aloud (or subvocalize) before writing.
• Do not look ahead in the key stream.

After encryption

• Mark the page as USED up to the position consumed.
• Never use those digits again.
• If the page is fully consumed, destroy it.

Destruction methods

• Physical: burn. Verify ash. No ash = not burned.
• Alternative: shred to confetti, soak in water, dispose.
• Nitrocellulose paper ignites instantly and leaves no residue. This is why intelligence services used it.
• Rice paper and mulberry paper burn cleanly.

Key Distribution

This course does not solve the key distribution problem. It assumes you have securely pre-distributed identical pads to both parties by physical means (courier, personal handoff, dead drop). Key distribution is the hardest part of the OTP system in practice.

Never:

• Transmit key material over any electronic channel.
• Generate key material algorithmically (makes it pseudorandom, not random).
• Store key material digitally.
• Use a “key” derived from a passphrase (deterministic, low entropy).

3.5 Alternative Random Sources

Besides d10 dice, other physical random sources include:

• Coin flips: four flips yield one binary digit; convert sets of four flips into a decimal digit. Less efficient than d10 but universally available.
• Radioactive decay: hardware random number generators based on nuclear decay counts are considered gold-standard entropy sources, but require equipment.
• Shuffled cards: a well-shuffled deck of cards provides approximately 225 bits of entropy. Map sequences to digits.

What does not qualify:

• Computer random() functions (pseudorandom, seeded, deterministic).
• Keyboard mashing or “random typing.”
• Atmospheric noise websites (you cannot verify the source).
• Any source where a third party could reproduce the output.

EXERCISES — MODULE 3