The Syko machine was a British strip cipher used during
WWII for encoding radio communications between aircraft.
There's not much information on the Web about these machines;
a scan of the Syko Manual
(PDF, 2.4MB) is available but it's rather hard to read
as it's an old document and has had extra annotations added.
In this post, I have transcribed the entire document
to make it easier to read and find information about
how the Syko machines were operated.
FOR OFFICIAL USE ONLY
R.A.A.F. MANUAL OF INSTRUCTIONS
This book is the property of the Air Board.
Attention is called to the penalties attaching to
any infraction of the Crimes Act, 1914-1937.
The contents of this book are not to be
communicated either directly or indirectly to the
press; nor to any person NOT authorised to use it
in the course of their duties.
The book is invariably to be kept locked
up when not in use, and is not to be taken into the
air or outside the establishment for which it is
issued, without the express permission of the
Promulgated for the information and guidance
of all concerned.
By command of the air board.
S e c r e t a r y.
Air Force Head-Quarters
[Previous page repeats.]
C O N T E N T SUse .. .. Paragraph 1 Page 1
Description Paragraphs 2-11 " 1 - 2
Cards .. .. " 12-17 " 2 - 3
To Cypher a Message .. " 18-25 " 3 - 4
To Decypher .. " 27-27 " 4
Examples .. .. " 4
R.A.F SYKO MACHINE.
1. The Syko machine is intended to provide a means
whereby secret communication may be attained between air-
craft in flight, when circumstances render such
communications necessary. Aircraft on extended flights
and cruises may use the Syko machine for point to point
secret communication when circumstances render this
necessary. It may also be used by units at advanced
operational bases when copies of S.D.45 - R.A.F. Cypher -
are not available.
The machine must be used for recoding the
Air Force Code whenever recoding is laid down as necessary
and supersedes the "Recoding Tables for Self-Evident Code"
for this purpose. It may also be used for cyphering
plain language messages to and from aircraft in flight and
must be used for recoding the 'Q' code (using the civil
edition cards) when communicating with aeradio stations.
It supersedes the "Recoding Tables for 'Q' Code"for this
2. The Syko machine consists of a metal case
carrying 32 metal elements (hereafter referred to as
sliding bars) each of which may be moved longitudinally
so as to uncover letters and figures on a card underneath.
3. Each sliding bar is engraved from top to bottom
with the letters of the alphabet, starting at the top with
the letter "A"; after "Z" are the numerals arranged from
9 to 0 in a descending order and finally a dash making 37
symbols in all. The provision of the dash enables the
"short break" to be cyphered. In order to avoid confusion
between the letter O and the figure O, the symbol 0 is *
used for the figure. Along the top the case is engraved
with white figures from 1 to 32, thus giving each sliding
bar a number.* [In the text, the numeral 0 (zero) is shown with a bar
through the center; the letter has no such bar. I have
used the standard ASCII characters; their rendering will
vary depending on the font you are using.]
4. The case is provided with two metal covers. The
upper one is hinged at the top and when closed covers the
upper half of the machine. The lower cover is a sliding
one, and has a channel section cross piece fixed to its
underside. When the lower cover is slid upwards this
channel section piece pushes all the bars upwards so that
the A's are set in line at the top of the machine. When
this cover is in its lower position it is retained by two
5. The machine may be opened to reveal the cypher
card by depressing a metal catch at its lower edge. The
cypher card is slid into position on the lipped platform
directly below the sliding bars.
6. A special pencil is provided housed in clips in
a cavity provided on the right hand side of the machine.
It is of the propelling lead type, carrying at the end
remote from the lead a pointed brass pin which is used to
move the bars, a notch below the appropriate letter being
engaged by the pin, i.e., actually placing the pin on the
symbol required. It is necessary to release the catch
and open the case in order to remove the pencil.
- 2 -
7. A flat transverse counter bar is provided. The bar
is marked off so as to facilitate arranging the cypher into
four element groups. The bar is capable of an endwise
movement in order to retain this counting facility in the
event of commencing to cypher at a bar other than number one.
(This will not be done without special orders). In the
centre of the guide in which the bar travels are four datum
marks, and the bar is moved endwise so that the centre
marking on the bar coincides with the appropriate datum mark
which will give the required grouping depending on the bar
chosen for commencement. One of the datum marks is coloured
red. This is the one used when commencement is made at bar
number one, the usual procedure. The bar is pushed to the
left or right by inserting the brass pin on the pencil into
8. Beneath the lower platform is provided a holder
for stowing, when necessary, reserve cypher cards.
9. For convenience two holes are drilled in the side
of the case. This enables the machine to be anchored in the
aircraft by means of a cord which may be fitted as convenient
for different positions in aircraft. The cord should be
fastened so as not to interfere with the operation of the
10. It will be seen that two spring clips are provided,
one on the inside of the top cover and one on the face of the
lower slide cover for carrying the messages which are being
cyphered or decyphered. For convenience of handling in
aircraft, the top cover can be turned back under the case
being then retained by a spring fastener.
11. A carrying case to house the machine is provided
in special cases such as when aircraft are away from their
base for an extended period. It will not be necessary for
the case to be carried in aircraft.
12. As indicated in paragraph 5, the use of the Syko
machine necessitates the provision of cypher cards which must
be inserted within the lipped edges of the platform inside the
machine. Two editions of Syko cards are issued - a Service
edition for intercommunicating between service aircraft and
units and a civil edition for use when service aircraft or
units wish to communicate with aeradio stations. In addition
certain civil aircraft use the civil edition for communicating
with aeradio stations and certain aeradio stations use it for
intercommunicating among themselves.
13. The card consists of 32 vertical columns of symbols,
corresponding to the 32 sliding bars. Each column contains
the twenty-six letters of the alphabet, the letters 0 to 9 and
the additional letter P arranged in random order. The letter
P is included to correspond with the dash mentioned in paragraph 3.
14. It is important that the card be inserted accurately in
order to ensure that the symbols register correctly when the
sliding bars are moved down in cyphering and decyphering.
15. Syko cards are issued in packs of 31 dated to
correspond with the days of a 31 day month. The card corres-
ponding with the date is used and is changed at midnight G.M.T.
each day. Service edition cards are used for one day only.
Civil edition cards remain in force
until ordered otherwise by the Air Board, the
same cards being used month after month.
- 3 -
16. Packs of service edition Syko cards at present
on issue are marked alphabetically from A to Z and once
one set has been brought into force each set will be brought
into force consecutively with the first day of each month,
i.e. if set "M" is ordered to be brought into force on 1st
September, set "N" will be brought into force on 1st October.
Obsolete packs of service edition cards are to be destroyed
by fire on the 3rd day of each month, destruction certificates
being forwarded to the distribution authority through the
Instructions for dealing with obsolete
packs of civil edition cards will be promulgated,
as necessary, to all holders by the Air Board.
In addition advise by Signal if you consider it
17. [The beginning of paragraph 17 has been struck out and
replaced by the previous insertion at the end of paragraph 16.]
Aircraft proceeding on extended flights or on
long cruises may be authorised to carry the appropriate
cards covering each flight and cruise.
To Cypher a Message:
18. See that the correct Syko card for the day is
accurately inserted in the platform.
At end of paragraph 18 add - "This may be checked from
the front of the machine by observation through the
vertical slit to the right of Sliding Bar No.32.".
19. Set all the sliding bars at the ready position by
sliding the lower cover to the top of the machine. Then
return this cover to the bottom of its travel and see that
it clicks home.
20. Commence at the extreme left-hand sliding bar,
insert the metal pin of the pencil ON the first symbol to be
cyphered and draw down the sliding bar until the pencil comes
to rest against the transverse counter bar. In a similar
manner deal with the remaining symbols to be cyphered from
left to right taking each sliding bar in turn. If the
cyphering has been carried out accurately it will be seen
that the symbols on the sliding bars are in one line
immediately above the transverse counter bar; this should
always be observed as a check. The cyphered version consists
of the symbols shown ON THIS SYKO CARD appearing immediately
above the tops of the sliding bars which ave been drawn down,
i.e., immediately above the A's. The symbols should be read
off from left to right and written down in groups of four
symbols on the message pad. An incomplete group at the end
of the cyphering is NOT to be completed to a group of four.
It will be noted that the resultant groups consist of mixed
letters and figures.
21. Should the message consist of more than 32 symbols
the sliding bars must be reset and the cyphering continued
from left to right. Before resetting the sliding bars,
however, it is essential that the portion of the message
already cyphered be written down, since the resetting of the
sliding bars removes all trace of the cyphered version.
22. The dash on the sliding bars must always be used
to cypher a "short break" which forms an integral part of a
signal, e.g., in the self-evident code of the Air Force Code.
23. Since no provision is made for provisioning the link
sign ┌─ or the pendant sign PT these symbols are omitted
altogether. No ambiguity is likely to arise but this
limitation should be borne in mind.
"23.(a) When using the machine to recode "Q" Code
signals a dummy word, consisting of any
proper name, is to be inserted as the first
word of the recode. The same name is not
to be used in the same series of trans-
missions and should be avoided as far as
possible in subsequent transmissions. The
purpose of this instruction is to avoid
the recode always beginning with the same
To Decypher a Message:[This heading has been written in by hand, replacing the
heading above paragraph 26, which has been struck out.]
24. The method of decyphering is similar to that of
25. Commencing with the left hand sliding bar place
the brass pin of the pencil ON the first symbol to be
decyphered and draw down the sliding bar until the pencil
- 4 -
comes to rest against the transverse counter bar. Continue
in a similar manner with the remaining symbols to be de-
cyphered using each sliding bar in turn working from left to
right.[Here the original "To Decypher a Message" heading
has been struck out.]
26. The decyphered version consists of the symbols
exposed ON THE SYKO CARD immediately above the top of the
sliding bars which have been drawn down, i.e.e, above the A's.
It should be noted that the cypher version will appear
immediately above the counter bar and should be checked.
If the message to be decyphered consists of more than 32
symbols the sliding bars must be reset and decyphering
continued from left to right, care being taken, however, to
write down the first decyphered portion of the message before
the machine is reset.
27. It must be remembered that, as stated in paragraph 13,
the dash on the sliding bar corresponds to accented P and there-
fore when the letter occurs in the text of a message to be de-
cyphered it is the dash that must be drawn down. Consequently
in checking the cypher version as mentioned in paragraph 26, a
dash should be read as P.
28. Transmission of Syko Machine through P.M.G Channels.
When preparing Syko messages for transmission
through P.M.G. channels, care must be taken in the use
of P and 0 (figure nought) symbols. As it is
impracticable for the Postmaster-General's Department to
transmit these symbols on their automatic circuits, they
are to be written thus on the telegraph form :-
P as (P)
0 as (O)
(a) Plain language message to aircraft:-
Message = Line squall centred on King Island = 0930.
by machine = M37AAB0XCJP1L7Z2ODM5Q6PSH08P9 =
for despatch = M37A AB0X CJP1 L7Z2 ODM5
Q6PS H08P 9 = 0930.
(b) Enemy report using self-evident code:-
Message as coded
by S.E.Code = 2CR153 - 9 - 085 - DNPL4359 X10 = 1000
As cyphered by
machine = EVCG0ODNPJKO26H7XQIVKP8A
for despatch = EVCG 0ODN PJKO 26H7 XQIV KP8A = 1000
"(c) Request by aircraft for homing bearing and
altimiter correction -
Message VZA de QY8S QDM? QFE?
for coding Charlie QDM IMI QFE IMI
Message as coded
by machine P5HCLPQ0OLJT24A9M0L
for despatch VZA de QY8A P5HC LPQ0 OLJT
Decoded reply BOBQDM090QTE1002
Reply - actual
meaning QDN 090 QTD 1002"
W/T Operators when transmitting messages are trained to
separate cypher groups by inserting the "short break" between
consecutive groups. Where the cypher consists of similar
groups, as in Syko, it is not usual for the receiving
operator to write in the "short break" but where the groups
are dissimilar as in the Air Force Code when not recoded, the
"short break" is invariably written in.
It is possible that operators will pass in messages in Syko
where they have written in the "short break". These "short
breaks" are not to be confused with "short breaks" which may
have been encyphered in the text of the message as in
example (b) above. THEY MUST BE IGNORED WHEN DECYPHERING