Auckland cables (1918) — Key recovered from ciphertext-only

At a glanceCiphertext-only break · complete

The cables: On 18 February 1918 Alfred A. Winslow, American Consul-General at Auckland, cabled the Naval Commander at San Francisco. Washington answered on 28 February; the reply reached Auckland on 1 March.
Where: US National Archives, RG 59, Central Decimal File 763.72/9377, microfilm M367 roll 83, images 0113–0114. Catalogue item 757.
The cipher: A Vigenère cipher on the ordinary alphabet. The key is the word FEBRUARY, the name of the month, and it is the same for both cables.
How it was deciphered: The period of 8 shows in the repeats of the reply, and the column frequencies give the key. It was then frozen and applied unchanged to the outgoing cable, which had not been used: all 94 letters came out as English.
What it says: Winslow asks whether automatic wireless stations work between San Francisco and Honolulu on two-thousand-metre wavelengths. Washington says they do not, that all US wavelengths are 4,000 m or above, and that a suspicious automatic station has been heard in the US since 15 February.
Still open: The final “OK” and the number 17527 are not interpreted. The images were not seen here; the work rests on the typed transcription.
Credit: Brice Dudout found the key, read the cables and listed the garbles, working with Claude Opus 5.5 agents. He says it is the first published decipherment, to his knowledge; we have not searched independently.

01 The two cables

On 12 February 1918, at 3.50 in the morning New Zealand time, a station in New Zealand heard an unknown transmitter on about 2,000 metres, working “automatic” (too fast to tell dots from dashes). Staff Paymaster W. J. Brown, for the Naval Adviser to the New Zealand Government, wrote on 14 February that it reminded the listeners of Nauru while that station was under German control. An American journal had reported automatic working being installed between California and Honolulu, so he asked for a check by cable.

The Consul-General at Auckland cabled the question on 18 February. The reply from an unnamed office in Washington, probably the Navy Department, is dated 28 February, 10 a.m. Winslow does not say he could not read it. His despatch of 4 March says only that he did not know which office it came from “further than it was sent from Washington, D.C.” The catalogue entry, which said the recipient could not read the reply, is corrected by this.

02 The cipher and the key

Both cables are a Vigenère on the ordinary alphabet: plain letter plus key letter, modulo 26. The outgoing cable is in 5-letter groups (94 letters). The reply is in 10-letter “words” (305 letters) followed by the number 17527.

The key was found on the reply alone. The repeats TKUTZMS (104 letters apart), IFOSRLI (32) and EERMS (56) share the divisor 8, and the index of coincidence at period 8 is 0.062. A frequency fit on each of the eight columns gives FEBRUARY. The key was frozen, with its SHA-256 recorded, and then applied without change to the outgoing cable, which had not been used in finding it. That cable decrypts to 94 of 94 letters of continuous English. None of 1,000 shuffles of the same ciphertext, decrypted with the same key, reaches its English letter-frequency score.

A monthly key word gave almost no security in 1918, as the period shows in the repeats. The State Department had codes of its own; these two cables show a consul and the Navy making do with an elementary method.

We re-decrypted both cables from the typed transcription with FEBRUARY on 2 November 2026, and the output matches the reading below letter for letter, including the garbles.

03 The reading

18 February (Winslow to the Naval Commander, San Francisco): “Are automatic wireless stations working between San Francisco and Honolulu on two thousand metre wavelengths?”

28 February (Washington to Auckland): “No automatic wireless stations used between Honolulu and San Francisco. Stations can receive while sending. All wavelengths used four thousand meters or above. Suspicious automatic station heard in this country since February fifteenth on following wavelengths: forty-two hundred, ten thousand, eleven thousand and fifteen thousand meters.”

So the transmitter heard on 2,000 m was not American. We have not tried to identify the 4,200, 10,000, 11,000 and 15,000 m transmitters Washington mentions.

04 What remains uncertain

Eight letters of the reply do not decrypt directly: six are transmission errors, mostly typical Morse confusions (i/t, g/w, z/p, z/x), and two are overstruck on the Auckland copy (groups 10 and 28). Each is forced by its word, and all eight are listed in the decryption file. Restored in the reading: wireles[s], peri[o]d, w[h]ile, [p]eriod, febr[u]ary, [f]ollowing, fort[y], [a]nd, [t]housand.
“PERIOD” stands for the full stop, and in one place the encipherer left out an O in the clear while the key stayed in step.
The final “OK” and the number 17527 are not interpreted.
The transcription was made by AI from the NARA images and has not been checked by a person against them. The images were not fetched for this page, so it carries no snip of the cipher.

Full texts

The transcription and the decryption of every document on this page, so that the reading can be used and checked. Short files are shown here; all are in the repository.

Winslow to Naval Commander San Francisco, 18 Feb 1918

Are automatic wireless stations working between San Francisco and Honolulu on 2,000-metre wavelengths?

Transcription

transcription.txt

# NARA RG 59, Central Decimal File 1910-29, 763.72/9377, microfilm M367 roll 83.
# Transcribed from the NARA images (letters as typed on the Auckland copies). Images not included: see links in NOTES.md.

## Cable 1: image 0113, sent 18 Feb 1918, American Consul-General Auckland -> Naval Commander, San Francisco
FVFRO TFKFX JTQII CQITJ MTRRN SOJQO IINRH SYTNC JRTRH FIYSG JJWOR LILPE ILLJZ SOKQO KFTYT RHDDC YVFNU VVJJR HKBS

## Cable 2: image 0114, "Office of Origin: Washington D C", "Via Pacific", 37 words, dated 28 [Feb] 10 am,
## remarks "usgovt pbam", received 1 Mar 1918, to American Consul General Auckland; closing number and signature as typed
# groups 10 (fexieehisz, letter 7) and 28 (ermsvjunuj, letter 10) are overstruck on the copy
ssblnodyym dncrvjjwij nakgtrtlme uzjxxvynym ssmlfurliw bezrrlhmtt ipvpnhtkut zmswdrhrva jmwvqgzjjw fexieehisz idrjqabmyl vllxijosvb ksvinhfsxe ougekcwwpi ubftjtfico uqzwqzwifs xevkimrrng tkutzmslfr ldzlyljjwo llyvzjcntc kicidaiwkm gkyeermsog ilcmbmoxqa mcqioxnhjd tvuznwffzr eiydkcsxif osrliimvpe ermsvjunuj shgzztvcsz ifosrliqfk yrjmp 17527
Dnc

Decryption

decryption.txt

# Vigenere decryption, key FEBRUARY (P = C - K mod 26, ordinary alphabet), key restarted at the first letter of each cable.

## Cable 1 (18 Feb 1918)
FVFRO TFKFX JTQII CQITJ MTRRN SOJQO IINRH SYTNC JRTRH FIYSG JJWOR LILPE ILLJZ SOKQO KFTYT RHDDC YVFNU VVJJR HKBS
FEBRU ARYFE BRUAR YFEBR UARYF EBRUA RYFEB RUARY FEBRU ARYFE BRUAR YFEBR UARYF EBRUA RYFEB RUARY FEBRU ARYFE BRUA
AREAU TOMAT ICWIR ELESS STATI ONSWO RKING BETWE ENSAN FRANC ISCOA NDHON OLULU ONTWO THOUS ANDME TREWA VELEN GTHS
= ARE AUTOMATIC WIRELESS STATIONS WORKING BETWEEN SAN FRANCISCO AND HONOLULU ON TWO THOUSAND METRE WAVELENGTHS

## Cable 2 (28 Feb 1918), letters as transcribed (garbles left in)
SSBLNODYYM DNCRVJJWIJ NAKGTRTLME UZJXXVYNYM SSMLFURLIW BEZRRLHMTT IPVPNHTKUT ZMSWDRHRVA JMWVQGZJJW FEXIEEHISZ IDRJQABMYL VLLXIJOSVB KSVINHFSXE OUGEKCWWPI UBFTJTFICO UQZWQZWIFS XEVKIMRRNG TKUTZMSLFR LDZLYLJJWO LLYVZJCNTC KICIDAIWKM GKYEERMSOG ILCMBMOXQA MCQIOXNHJD TVUZNWFFZR EIYDKCSXIF OSRLIIMVPE ERMSVJUNUJ SHGZZTVCSZ IFOSRLIQFK YRJMP
FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYFEBRUA RYFEBRUARY FEBRUARYFE BRUARYFEBR UARYF
NOAUTOMATI CWIRELESHS TATIONSUSE DBETWEENHO NOLULUANDS ANFRANCISC OPERIDSTAT IONSCANREC EIVEWGILES ENDINGCERI ODALLWAVEL ENGTHSUSED FOURTHOUSA NDMETERSOR ABOVEPERIO DSUSPICIOU SAUTOMATIC STATIONHEA RDINTHISCO UNTRYSINCE FEBRJARYFI FTEENTHONP OLLOWINGWA VELENGTHSF ORTITWOHUN DREDTENTHO USANDELEVE NTHOUSANDL NDFIFTEENV HOUSANDMET ERSOK
= NO AUTOMATIC WIRELES[S] STATIONS USED BETWEEN HONOLULU AND SAN FRANCISCO PERI[O]D STATIONS CAN RECEIVE W[H]ILE SENDING [P]ERIOD ALL WAVELENGTHS USED FOUR THOUSAND METERS OR ABOVE PERIOD SUSPICIOUS AUTOMATIC STATION HEARD IN THIS COUNTRY SINCE FEBR[U]ARY FIFTEENTH ON [F]OLLOWING WAVELENGTHS FORT[Y] TWO HUNDRED TEN THOUSAND ELEVEN THOUSAND [A]ND FIFTEEN [T]HOUSAND METERS <OK>

## Garbles in cable 2 (group.letter: cipher read -> cipher needed; Morse read / needed; word)
2.9	i -> t	.. / -	WIRELES[S]
9.6	g -> h	--. / ....	W[H]ILE
10.7	h -> u	.... / ..-	[P]ERIOD *
21.5	d -> o	-.. / ---	FEBR[U]ARY
22.10	g -> w	--. / .--	[F]OLLOWING
25.4	z -> p	--.. / .--.	FORT[Y]
28.10	j -> y	.--- / -.--	[A]ND *
29.10	z -> x	--.. / -..-	[T]HOUSAND
* overstruck letter on the Auckland copy.
PERI[O]D: an O left out of the plaintext by the encipherer (the key stays in step).
The final OK and the number 17527 are outside the text and not interpreted.

Reading

reading.md

# Established text (English, telegraphic; punctuation added, PERIOD written as a comma + period)

Restored letters (the cipher has another letter, see decryption.txt): wireles[s], peri[o]d, w[h]ile, [p]eriod, febr[u]ary, [f]ollowing, fort[y], [a]nd, [t]housand. The two final letters OK are not interpreted.

**Cable 1, Auckland to San Francisco, 18 Feb 1918.** Are automatic wireless stations working between San Francisco and Honolulu on two thousand metre wavelengths?

**Cable 2, Washington to Auckland, 28 Feb 1918.** No automatic wireless stations used between Honolulu and San Francisco, period. Stations can receive while sending, period. All wavelengths used four thousand meters or above, period. Suspicious automatic station heard in this country since February fifteenth on following wavelengths: forty two hundred, ten thousand, eleven thousand and fifteen thousand meters.

Translation

translation.md

# Plain sense

18 February (Winslow): “Are automatic wireless stations working between San Francisco and Honolulu on two thousand metre wavelengths?”

28 February (Washington): “No automatic wireless stations used between Honolulu and San Francisco. Stations can receive while sending. All wavelengths used four thousand meters or above. Suspicious automatic station heard in this country since February fifteenth on following wavelengths: forty-two hundred, ten thousand, eleven thousand and fifteen thousand meters.”

Washington to Consul-General Auckland, 28 Feb 1918

No automatic stations between Honolulu and San Francisco; wavelengths 4,000 m or above; a suspicious automatic station heard in the US since 15 February.