SOE cryptosystems – The German view

I have already given an overview of the main cryptologic systems used by the British SOE organization in WWII here. The basic systems were double transposition using a poem as a ‘key’ generator, double transposition with prepared ‘keys’ and letter one-time pad.

A more detailed analysis of the SOE ciphers and their indicator system is available from report CSDIC (UK) S.I.R. 1106 ‘German wireless intercept and counterespionage activities’ (Nov ’44) by Miersemann, a member of Referat Vauck.

Double transposition using a poem as a key

Standard double transposition system using a poem as a ‘key’ generator. The words of the poem are assigned a letter of the alphabet and the indicator is made up of the letters assigned to the words used in the numerical key.

Then the indicator is coded by using the agents secret number. For example if the letters are MLOQY and the secret number is 23116, then the indicator becomes M+2, L+3, O+1, Q+1, Y+6= OORPE.

The indicator is placed as the first group of the message. The last group is again the indicator but coded one more time with a different procedure. Using the following table OORPE becomes EEHFO.

a	1	k	u
b	2	l	v
c	3	m	w
d	4	n	x
e	5	o	y
f	6	p	z
g	7	q
h	8	r
i	9	s
j	0	t

 

Double transposition with prepared keys

Standard double transposition system using prepared numerical keys (WOK-Worked Out keys). Each set of keys has its own indicator. The indicator is placed as the first and last group of the message. The second group of the message is the agents’ secret identity code. This is coded using the following table:

a	1	k	u
b	2	l	v
c	3	m	w
d	4	n	x
e	5	o	y
f	6	p	z
g	7	q
h	8	r
i	9	s
j	0	t

For example if the agents unique code is JTA then the 5-letter group is created as follows. First letter is random let’s say C, second letter equals a number based on the table above. For example H=8. This means that the letters of the agents code are moved forward 8 steps, so:

J+8=R

T+8=B

A+8=I

So the group will be CHRBI

 

Letter one-time pad and emergency LOP

Random letter ‘key’ coupled with the plaintext is substituted using a conversion table. According to Miersemann the indicator of each key is enciphered by adding a standard number (probably different for each agent).

For example in the indicator OORPE 2 is added to O and 8 to P, which means that the new indicator will be OQRXE. This is the first and last group of the message.

The second message group contains the agents’ secret number by using a number to letter table. This is created by using the cipher indicator OQRXE. The numbers 1-9 plus 0 at the end are written out horizontally and the indicator beneath them. Then the rest of the alphabet letters are written out, omitting those used in the indicator. In our example:

1	2	3	4	5	6	7	8	9	0
O	Q	R	X	E	A	B	C	D	F
G	H	I	G	K	L	M	N	P	S
T	U	V	W	Y	Z

If the secret number is 210 then this can be written as UGF. Two dummy letters are inserted at the beginning to form a 5-letter group.

In the emergency LOP three poems are used to create new ‘keys’ to be used with the conversion table.

Pigeon message:

While writing this piece I thought of the pigeon message mentioned some time ago in the media. The first group AOAKN is repeated at the end so it must have been the indicator.

If the message was WOK then the second group HVPKD could be interpreted as follows: H dummy letter, V=2 using the WOK conversion table, PKD=agents id code, minus two step=NIB. So maybe the Brits can find out who agent NIB was.

If the message was LOP then the group HVPKD could be interpreted as follows: using the indicator AOAKN the conversion table is:

1	2	3	4	5	6	7	8	9	0
a	o	a	k	n	b	c	d	e	f
g	h	i	j	l	m	p	q	r	s
t	u	v	w	x	y	z

Assuming that HV are dummies, then this means that PKD=748.