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SIGSALY →
A-3 divided the speech frequency spectrum between 300 and 3000 Hz into five bands
of which the order could be swapped. This is why it is also known as a band-splitter.
In addition, each of the five frequency bands could be mirrored. As a result,
speech can be scrambled in 3840 orders.
As the security of the device is very limited — it is not a real encryptor —
Bell Labs preferred to call it a Speech Privacy Unit,
rather than a Speech Secrecy Unit. Furthermore, it was assumed that an adversary,
with the right tools, resources and motivation, would eventually be able to crack it.
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During WWII, transatlantic A-3 traffic was intercepted and successfully broken
by the Germans, who had setup a research facility near Noordwijk in occupied Netherlands,
known as Forschungsstelle Langeveld. This was done with the help of
several engineers of the research lab of the Dutch Post Office
(PTT). The intercept facility was later moved to Valkenswaard, near Eindhoven
(Netherlands) [2]. The A-3 traffic was solved separately in Germany,
by Wa Pruef 7 1 [3].
As far as we know, there are no surviving photographs of the A-3 scrambler.
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Wa Pruef 7, also written as Wa Prüf 7, was a test facility of
the Heereswaffenamt — the central office for technical development and
production of weapons, ammunition and equipment from the German Army.
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NOTE —
This page currently acts as a placeholder for future information about this
device.
On most telephone devices, the audio spectrum ranges from 300 to 3000 Hz.
This is sufficient for good quality speech transmission. In the A-3, the
audio spectrum is divided into five sub-bands, each of which is approx.
450 Hz wide. This band-splitting is done by means of filters.
The normal unscrambled speech is shown in the leftmost graph above.
The order of the five sub-bands is now shuffled into one of 5! = 120
possible scrambling orders. The result is shown in the rightmost graph
above. In addition, each of the sub-bands can be inverted, as shown below:
The leftmost graph above shows the same scrambling order, with with bands
2, 3 and 4 inverted. At fixed intervals, the scrambling order as well as
the inversion pattern is changed. This is shown in the rightmost graph
above. Note however, that not all scrambling orders are equally effective.
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With five sub-bands and optional inversion per sub-band, the number of
possible settings is:
5! × 25 = 3840
Not all of these settings are useful however. One of the 3840 settings
produces no scrambling at all, and there are other settings that produce
limited scrambling and are therefore unusable. In practice, only 11 of
the 3840 settings were considered secure enough for privacy [6]:
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In the 5-band band-splitter (called the A-3) installed by the
American Telephone and Telegraph Company (A.T.&T.) for radio telephone
service in 1937, only 11 of the 3840 possible codes were considered
suitable for privacy [1]. Thus availability of a large number of codes
in a scrambler should not be interpreted to mean that all of these codes
are equally useful.
- Hans Knap, Forschungstelle Langeveld
ISBN 90-6707-467-8. De Bataafse Leeuw, 1998.
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- David Kahn, The Codebreakers
Maximillian, New York, 1967. pp. 551-560, 776.
- Hans Knap, Forschungstelle Langeveld
ISBN 90-6707-467-8. De Bataafse Leeuw, 1998.
- Christos Triantafyllopoulos, Intercepted conversations -
Bell Labs A-3 Speech scrambler and German codebreakers
Christos military and intelligence corner, 2 February 2012.
- Arthur Bauer, Forschungstelle Langeveld
CDV&T, 2 October 2020.
- Carel Mackenbach, Forschungstelle Langeveld (Dutch)
Militaire Spectator, 2024. Volume 193, Issue 3, pp. 186-197.
- Voice Privacy Equipment for Law Enforcement Communication Systems
LESP-RPT-0204.00. US DoJ, May 1974. Updated 1 November 2026.
- Extracts from report on interrogation of Dr. Hans Wilhelm Thost
TICOM, 13 February 1916.
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© Crypto Museum. Created: Saturday 07 November 2026. Last changed: Wednesday, 11 November 2026 - 16:47 CET.
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