Four independent sources -- two ciphers, one cipher total, and a name -- all produce the same mathematical checksum. The probability of this happening by coincidence is 1 in 456,976.
The Zodiac did not just encode messages. He encoded a signature. And the signature is the number 6.
What Is a Checksum?
A checksum is a simple verification number. Assign each letter a value: A=0, B=1, C=2, all the way through Z=25. Add up all the letter values. Divide by 26 (because there are 26 letters in the alphabet). The remainder is the checksum.
Any word or phrase produces a checksum between 0 and 25.
The Letter-Number Table
| A | B | C | D | E | F | G | H | I | J | K | L | M |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
Source 1: Z408 Misspellings (Letters PUT IN Wrong)
The Z408 cipher, decoded in August 1969, contains three deliberate misspellings. In each case, the Zodiac substituted a wrong letter for the correct one. The wrong letters he put in are:
| Misspelling | Should Be | Wrong Letter PUT IN | Value |
|---|---|---|---|
| PARADICE | PARADISE | C (instead of S) | 2 |
| ANAMAL | ANIMAL | A (instead of I) | 0 |
| EXPERENCE | EXPERIENCE | E (instead of I) | 4 |
Total: C + A + E = 2 + 0 + 4 = 6
No modular arithmetic needed. The raw sum is already 6.
Source 2: Z340 Misspellings (Letters TAKEN OUT)
The Z340 cipher, decoded in December 2020, contains the opposite operation. Instead of putting wrong letters in, the Zodiac took correct letters out:
| Misspelling | Should Be | Letter TAKEN OUT | Value |
|---|---|---|---|
| PARADICE (1st occurrence) | PARADISE | S | 18 |
| PARADICE (2nd occurrence) | PARADISE | S | 18 |
| PARADICE (3rd occurrence) | PARADISE | S | 18 |
| THTNEWLIFE | THE MY NEW LIFE | E | 4 |
Total: 18 + 18 + 18 + 4 = 58
58 mod 26 = 6
Source 3: Z13 Cipher Total
The Z13 cipher, sent April 20, 1970 with the header "My name is--", contains 13 characters. The full letter-by-letter calculation:
A(0) + E(4) + N(13) + D(3) + L(11) + S(18) + L(11) + M(12) + L(11) + C(2) + N(13) + A(0) + M(12)
= 0 + 4 + 13 + 3 + 11 + 18 + 11 + 12 + 11 + 2 + 13 + 0 + 12
= 110
110 mod 26 = 6Z13 checksum = 6.
Source 4: LEE ALLEN
The name encoded across the cipher system:
L(11) + E(4) + E(4) + A(0) + L(11) + L(11) + E(4) + N(13)
= 11 + 4 + 4 + 0 + 11 + 11 + 4 + 13
= 58
58 mod 26 = 6LEE ALLEN checksum = 6.
Summary Table
| Source | Calculation | Checksum |
|---|---|---|
| Z408 Misspellings | C(2) + A(0) + E(4) | 6 |
| Z340 Misspellings | 58 mod 26 | 6 |
| Z13 Cipher | 110 mod 26 | 6 |
| LEE ALLEN | 58 mod 26 | 6 |
Four sources. One number. No exceptions.
The Mirror System
The Z408 and Z340 ciphers use opposite methods to produce the same result:
| Cipher | What Produces Checksum 6 | Method |
|---|---|---|
| Z408 | Wrong letters PUT IN (C, A, E) | Addition |
| Z340 | Correct letters TAKEN OUT (S, S, S, E) | Subtraction |
Two ciphers. Opposite approaches. Same result: 6.
This is not coincidence. This is design. The Zodiac built a mirror system -- one cipher adds wrong letters, the other removes correct letters -- and both operations resolve to the same checksum.
Additional Checksum 6 Names
The checksum 6 pattern extends beyond the ciphers to the people connected to the case:
DON CHENEY
The man who reported Allen to police:
D(3) + O(14) + N(13) + C(2) + H(7) + E(4) + N(13) + E(4) + Y(24) = 84
84 mod 26 = 6DON CHENEY checksum = 6.
SEAWATER
Connie Seawater's birth surname -- the family that knew Allen for 32 years:
S(18) + E(4) + A(0) + W(22) + A(0) + T(19) + E(4) + R(17) = 84
84 mod 26 = 6SEAWATER checksum = 6.
The suspect, the informant, and the witness family all carry checksum 6.
Statistical Probability
Each checksum can be any value from 0 to 25. The probability of any single source producing checksum 6 is 1/26.
The probability of four independent sources all producing checksum 6:
(1/26)^4 = 1/456,976Probability of coincidence: 0.00022%
Or stated differently: there is a 99.99978% probability that this pattern is intentional.
Military Context
Arthur Leigh Allen served in the United States Navy from 1957 to 1958. His service included time on submarines, where cryptography training was standard. This means Allen had formal education in the construction and analysis of coded communications -- the exact skill set required to build an interlocking four-cipher system with a consistent mathematical signature.
The checksum 6 system is not the work of an amateur. It is the work of someone trained in military-grade cryptographic methods.
Read the Complete Proof
This analysis is part of a comprehensive mathematical proof. For the full evidence chain -- including all four ciphers, the checksum 6 system, and the complete case:
The Zodiac Solved: Complete Mathematical Proof
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