Editing Openai/692c8201-7a4c-800d-9ca0-5220fef3e400 (section)

=== Example 3 — 1944 San Francisco message, with additional mod‑26 letter-based OTP demonstration ===

Source: Venona — San Francisco KGB Station: Pacific Cables, 1944 (Wilson Center / Library of Congress), message #789, pages pp. 210–212 — excerpt from translation:

: “… contact established at midnight”

Part A: Digit super‑encipherment (reconstruction)
* Plaintext excerpt: “contact established midnight” (3 words).
* Assigned 5‑digit codebook groups (reconstruction): - contact     →  55555 - established →  66666 - midnight    →  77777
* Random 5‑digit pad fragment (reconstruction): - pad groups →  12345  98765  43210

Encryption (plaintext + pad → ciphertext)

| Plain | Pad   | Cipher |
| ----- | ----- | ------ |
| 55555 | 12345 | 67890  |
| 66666 | 98765 | 65431  |
| 77777 | 43210 | 10987  |

Decryption (ciphertext − pad → recovered)

| Cipher | Pad   | Plain |
| ------ | ----- | ----- |
| 67890  | 12345 | 55555 |
| 65431  | 98765 | 66666 |
| 10987  | 43210 | 77777 |

Recovered groups map to “contact established midnight.”

Part B: Letter‑based OTP demonstration (mod 26)
To show how a letter‑based OTP differs from Venona’s digit super‑encipherment, take the plaintext phrase:

: “MEET AT MOON”

(Spaces ignored; only letters; letters A–Z → 0–25)
* Plaintext letters → numeric (A=0…Z=25): M(12) E(4) E(4) T(19)  A(0)  T(19)  M(12)  O(14)  O(14)  N(13)
* Random key stream (2-digit per letter) — reconstruction: key:  03  15  08  22  11  19  04  25  07  09
* Encryption (letter + key mod 26): - 12+3 = 15 → P - 4+15 = 19 → T - 4+8 = 12 → M - 19+22 = 41 mod26 = 15 → P - 0+11 = 11 → L - 19+19 = 38 mod26 = 12 → M - 12+4 = 16 → Q - 14+25 = 39 mod26 = 13 → N - 14+7 = 21 → V - 13+9 = 22 → W

Ciphertext letters: “PTMPLMQNVW”
* Decryption (ciphertext − key mod 26) recovers original plaintext letters exactly.

Note on difference from Venona digit‑system:
The letter‑based OTP uses mod 26 on letter‑value units; Venona used digit groups (decimal digits) and mod‑10 addition on each digit individually. The structure is different: a letter‑OTP works on letters; Venona works on numeric code‑groups, making it suitable for superenciphering numeric code‑groups, not text directly.