Swedish Naval World War I cipher

Description

The letter	is substituted by		To be used on the following days	Cipher	Keyfigures		The figure	is substituted by
A	18		1 - 3	F	2 1 3 4 0 5		1	E
B	15						2	P
C	4		4 - 6	G	3 0 4		3	U
D	17						4	C
E	1		7 - 9	H	1 8 9 4		5	F
F	5						6	W
G	10		10 - 12	I	3 2 5		7	J
H	16						8	Å
I	20		13 - 15	J	4 2 3		9	Ö
J	7						10	G
K	21		16 - 19	K	2 3 1		11	V
L	29						12	Q
M	22		20 - 23	L	5 2 0 1		13	X
N	23						14	Ä
O	24		24 - 27	M	6 9 3		15	B
P	2						16	H
Q	12		28 - 31	N	2 3 1 0		17	D
R	25						18	A
S	26			O			19	Y
T	27						20	I
U	3						21	K
V	11						22	M
W	6						23	N
X	13						24	O
Y	19						25	R
Z	28						26	S
Å	8						27	T
Ä	14						28	Z
Ö	9						29	L

The cipher is used in the following way:

1. The letters of the cleartext is substituted by figures according to the two leftmost columns in the table above.
2. Under each of these figures one keyfigure is written according to the order given for that day.
3. The figures representing the letters are added to the keyfigures directly beneeth, and if the sum exceeds 29 it is subtracted by 29 (i.e. addition modulo 29) so that the result always lies in the range 1- 29.
4. The new sums are looked up in the next-to-the-right column, and substituted back to the letters given in the rightmost column.

Cleartext:	F	O	U	R	D	E	S	T	R	O	Y	E	R	S
Figures:	5	24	3	25	17	1	26	27	25	24	19	1	25	26
Key G:	3	0	4	3	0	4	3	0	4	3	0	4	3	0
Sum mod 29:	8	24	7	28	17	5	29	27	29	27	19	5	28	26
Cryptogram:	Å	O	J	Z	D	F	L	T	L	T	Y	F	Z	S

To decrypt a received message, one uses the same procedure, but the keyfigures are subtracted from the numbers representing the cryptotext instead of added. If the result is a negative number or zero, one must add 29 to reach the right answer.

The regulations stated that the cryptograms should be sent in 12-letter groups. Because the lengths of the keys used are all factors of 12, the first figure of the key used must fall under the first letter of every such 12-letter group, and the last figure under the last letter. The cryptoclerk could thus easily spot if a letter was missing from a cryptogroup (Apparently, one hadn't begun using standard five-letter groups at that time).

The reader might think I've made a typo, and forgot to provide the key O. That's not the case. This was a special key only to be used between the King and the Supreme Commander of the Coastal Navy, and was issued only to the Supreme Commander of the Coastal Navy and the King.

Break a Navy Ciphermessage

= King Gustaf, Royal Castle Stockholm = 18/3 1916 = GR 12 = TXT =

PDGAIFPEGLAT = WVAEIJFÅCSÅT = CEÖUURLZÄZSL =
RYFUZZLRUKRR = BYÖAICFZCRSU = LRBÖMFIZRCWU =
ZJUFTTTFGTRM = ZYFIZFZUGLLT = SYJCYRLSLÄIJ =
EZCJNNZEPTRF = IPJÖUEPIÄNTC = P =
Supreme Commander of the Coastal Navy =

There's no reward to the successful breaking of this message, since I believe breaking a crypto is a kind of reward in it self! Good Luck, though!