From: "Geoffrey Sinclair" gsinclair@froggy

From: "Geoffrey Sinclair" gsinclair@froggy.com.au
Subject: JN-25 factsheet

[Received via Rich Leonard, 26 May 2004]

Start proposed JN-25 fact sheet, this is the text I would have available if
required or requested. This is a pre publication version for comment,
please only use it for review purposes at the moment. This release is
going out to the mailing list maintained by David Aiken and also the
Midway Round Table Forum.

Firstly thank you to Phil Jacobsen for supplying the copies of the
documents referenced in this article, his willingness to answer my
questions, aid in understanding the documents and suggestions to help
accurate presentation of the material. Any remaining errors are the
author's responsibility.

JN-25 code information until 5 August 1943, Comment and any
corrections welcome, this summary is based on US Navy documents
on IJN codes and as such will likely have some errors in them when
it comes to the finer points of the codes.

References,

[1] RG38 CNSG Library, Box 115 5750/197 The activities and
accomplishments of GY-1 during 1941, 1942 and 1943.

[2] RG38 CNSG Library, Box 115 5750/198 OP-20-GY code
breaking progress reports.

[3] RG38, CNSG Library Box 116 5750/199 (2 of 3), OP-20-GYP
History WWII. (OP-20-GYP was the USN organisation charged
with breaking Japanese codes, it was originally called OP-20-GY)

[4] RG438, CNSG Library Box 116 5750/202, The History of GYP-1.

[5] RG457 HCC Box 578 NR 1391 JN-25 Cryptographic System Archives
II, College Park MD. (Note: HCC is Historical Cryptographic Collection)
This reference gives the commencement, end dates and designations of
the different versions of JN-25 from 1 June 1939 to early 1945 in a
spreadsheet. It also gives the number of key and text additives per version
from A-5 onwards.

JN-25, a code book of 5 digit numbers (also called five numeral groups at
times), divisible by three, to disguise the numbers a book of 5 digit random
numbers, additives, was used, a page of the additive book was a 10x10 table
of five numeral groups. At a random point in the additive book the code
clerk selected a sequence of additives and added them to the message code
groups using false arithmetic (no carrying), these were the numbers then
transmitted. In addition another 5 digit code group was added at a known
point in the message to tell the receiver where the random numbers were
taken from, this was called the indicator group, it was sent twice to insure
against any garbles.

Ignoring for a moment the indicator group, date/time group, call signs and
the tables for super enciphering date/time, and so forth the fundamental
JN-25 system for the war was as follows,

Message,

From Kaga Estimated time of arrival 19th 2130.

Code groups,

21936 48222 01905 38832 87039 64527 11520 99708

meaning, one word per group

from NGUC Kaga stop ETA 19th 2130 stop.

NGUC = next group upper case

additive key

02923 41338 00989 15861 28959 90024 23693 18229

code plus additive

23859 89550 01884 43693 05988 54541 34113 07927

encrypted text as transmitted,

23859 89550 01884 43693 05988 54541 34113 07927

[Example from Code Breakers by Hinsely and Stripp]

The USN called the book where it wrote down the basic information on
the JN-25 code R.I.P 74, Registered Intelligence Publication number 74.
The basic information being the complete system of encryption and hints
for decryption, and things like date of use but not the recovered code and
cipher values.

The allies called the random numbers used to disguise the code groups
text additives and the random numbers used to disguise the indicators
key additives. Since the basic JN-25 system did not change during the
war the methods developed in 1940 and 1941 could be used and refined,
there was no need for the allies to start again from square one. What
did change during the war was the way the text preambles were done,
things like where in the message the indicators were sent to tell the
receiver which additives were used and also how the methods by which
the indicator values were encrypted. When multiple versions of JN-25
were in use the text preamble had to include a version identifier. From
August 1942 onward several channels, that is different code or additive
books, within JN-25 were set up for separate groups of subscribers
which made recoveries more difficult and tedious.

The "preamble" of a message is the radio "heading" with communication
data such as call signs, precedence, message number (external) action
addressee, information addressee, originator and group or word count.

JN-25 timeline, with usage dates and code designations from reference 5,

1 June 1939 the A version or JN-25A introduced along with additive
book 1, A-1 or D IPPAN RA1. At the time called AN by the USN,
code book D by the IJN. It is understood only one code group book
was issued, with the code groups assigned in alphabetical order, the
code therefore being described as unhatted, but this single book made
it very time consuming to decrypt a message.

There were 30,000 text additives, a book of 300 pages each with a 10 by
10 grid of 5 digit random numbers, clerks were to always start using the
additives from column 1. There were 999 key additives, to be chosen
by date. Code clerks were meant to start encoding somewhere in the text
of the message in order to avoid the weakness of stereotyped message
beginnings, a begin message here group was inserted into the transmission
to show where the message really began. The code groups themselves
were 5 digit numbers divisible by 3, so a possible 33,333 valid groups,
of which around 30,000 were being used, the rest reserved for expansion.
[ref 3, pages 4-6] [ref 4 pages 2-4]

Indicators could be used to replace repeated words, names like Manila
could be rendered as a single word or as 3 Kana particles or as an
area designator or as Kanji characters or as six separate letters, since
the code had an English alphabet. [ref 4 page 2]

Each unit was assigned a start point in the text additive book and was
meant to start working through the book from that point, not repeating
the use of the same additives until they had returned to the starting point.
[ref 3, pages 4-6]

The indicator group consisted of the page number of the text additive
book multiplied by three, then one was subtracted, the result then
multiplied by 100, the line number was multiplied by 10 and added to
the page number calculation and a random number was used as the 5th
number, in the units column. So page 213, line 5, random number 7
would come out as 63857. The five digit key additive, selected from
the list according to the current date, was used to disguise the indicator
number, this group was transmitted at the start and end of the message.
[ref 4 page 3]. Finally as well as the text preamble, call signs, number
of message groups, serial number and so forth was added and enciphered
using a substitution code. A plain text date/time group was added to the
end of the message, after the second indicator group. [ref 3, pages 4-6]

From the start it is clear many clerks ignored the "start in the middle of
the message" and "work through the additive book" orders. When
added to the stereotyped openings and reports this became a way into
the code. As a further aid once the system was understood by the allies
the conscientious clerks, the ones who are following the text additive
instructions, were tracked as they gave insight into the whole additive
book and keying system, already compromised by the heavy use starting
on page 1 line 1, or else page x (where x is a low number) line 1. It
appears 10,000 of the text additives were enough to read 60% of the
messages even when the additive book had 50,000 entries. [ref 3 chapter
V pages 8-9]

Intercepted traffic from the Pacific intercept stations, which in the
period just before Pearl Harbor were Corregidor, Guam and Heeia,
Oahu, Hawaii was forwarded to the US via the weekly sailing of the
Dollar line "President" liners. A small amount of priority traffic
could be sent airmail via the Pan Am clippers, which had a small
strong box in their hulls. [ref 4 page 5]

1 September 1939 additive book 2, A-2 or D IPPAN RA2

1 December 1939 additive book 3, A-3 or D IPPAN RA3

1 June 1940 additive book 4, A-4 or D IPPAN RA4

"Early (Northern) Fall" 1940 the US code breakers realised the system
for numbers in JN-25A was the same as an old code the US has the
code book for, the code group was the number itself multiplied
by a constant. In a single day of checking the high frequency code
groups the code groups for the numbers 0 to 999 fell out, 1/30 of
the total code. Since all code these group values were divisible by
three it confirmed the tentative code group values previously
assigned were correct and told the code breakers valid code group
values were divisible by three. [ref 3, page 9 and ref 4 pages 11-13]

After this discovery the task of further recoveries and exploiting
traffic was largely shifted to Corregidor, which for a time in 1940/41
was the largest USN code breaking unit, having completely absorbed
the Shanghai unit in December 1940. [ref 3, page 10] (SRH-179
notes that the personnel from Shanghai were transferred to Corregidor
in phases from August 1940 to December 1940. The last group of ten
men from Shanghai reported to Corregidor on 16 December 1940. Just
before that time the intercept and decryption efforts at Shanghai were
shut down.)

In effect Washington declared JN-25A "completely solved" and
"completely broken", that is the system was understood and the
remaining work was in recovering and assigning meanings to code
groups, then exploiting the results. The result was most work was
directed at the earlier, now superceded versions, to try and recover
the underlying code groups, since there were many more messages
in those versions, the keys of the latest text additive book were also
targeted as intercepts built up. No attempt was made to read current
traffic. [ref 4 page 14]

1 October 1940 additive book 5, A-5 or D IPPAN RA5

The text additive book is changed to 500 pages of 100 numbers, so
50,000 numbers and clerks can start selecting numbers in any column,
rather than being mandated to start in column 1. The indicator
group (key) system is changed, it is now the third and fourth groups
transmitted, disguised by a 10 digit random number. The Japanese
chose the subset of 10 digit numbers using all 10 digits, rather than
allow digits to be repeated, so 1,234,567,890 is valid 1,234,567,899
is not. This number is no longer selected by date. Now the additive
page number is multiplied by 2, then 1 is subtracted and the result
multiplied by 100, the line number is multiplied by 10 and it, along
with the column number are added to the result of the page number
calculation, so page 123, line 6, column 9 gives a value of 24569.
The first and second groups of each message are the Secret Message
Serial number and operate as a key additive selector. [ref 4 pages 16-17]

Some 1,300 A code groups meanings had been recovered by the end of
November. [ref 2, December 1 1940 report] In November 1940 it was hoped
that the code breakers could move to reading current JN-25A traffic by the
end of the year. "This hope, which was held throughout November, 1940,
vanished, ironically enough, on December first .." [ref 4 page 17], the
IJN had changed the version of JN-25 in use.

It should be noted when counting the number of code recoveries that the
literature has many examples of the problem of erroneous recoveries of
both code book and cipher additive values. That is values that were
subsequently found to be wrong, meaning new ones had to be substituted
in their place. When the British and Americans combined their effort in
1941 for example, the British were critical of many of the U.S.N. recoveries
as being erroneous. Corrections to previous bad values was an on-going
effort. So it is not simply a case of assuming a steady increase in the
number of recoveries and that each recovery was completely correct. See
for example the number of changes made when in 1945 the USN did more
work on the B7 and B8 versions of JN-25 (see below).

On 1 December 1940 the B version (or JN-25B), a new code book but using
same indicators and additives, B-5 or D IPPAN RA5 was introduced, replacing
the A version.

This version was called AN-1 by the USN at the time, it was known as code
book D by the IJN. It introduced upper case or Auxiliary table (two
meanings per code word), hatted (code groups no longer in alphabetical
order so 2 books, one in numeric, the other alphabetical order), and 3
extra tables for 1) position, 2) geographic designations and 3) date time
groups. The third "table" for date time groups is not like the geographical
designations but a separate 12 x 31 substitution cipher table rather than a
one for one code group substitution.

The first 2/3 of the code groups in the book were given 2 meanings, so
some 55,000 meanings were in use. Common words had multiple code
groups assigned.

According to an article in the US Naval Institute Proceedings in
June 1979 by/during 1941 the additive book had 50,000 numbers.
The article is a translation by Edwin Layton of a section of chapter
13, volume 45 of the Japanese Defence Agency War History series.
The article notes the geographic designators and date time tables
remained the same for the rest of the war.

By working on the A-5 and B-5 messages, the former for text additives
to apply to B-5 messages, the latter for code group values, code group
values could be recovered. By the end of February 1941 some 1,600
A code group meanings had been recovered. [ref 2 February 1941 report]

1 February 1941 additive book 6, B-6 or D IPPAN RA6. In this additive
book the Japanese simply rearranged the key additives, rather than
changing them, making the key recovery simple. [ref 4 page 19]

Initially Corregidor is put on B-6, Washington continues work on A-5
and B-5 messages. [ref 4 page 19] The GY-1 code breaking section in
Washington is 10 people, all military personnel, in the first quarter of
1941. [ref 1 page 4] It should be noted this unit was also tasked with
the ever increasing work in decrypting Japanese diplomatic messages
beginning in the northern Spring of 1941.

In early 1941 the US and the UK code breakers (mainly at Singapore,
the Far East Combined Bureau [FECB]) joined forces, both countries
have around the same amount of code and additive recoveries, with
many overlaps but the bulk of the recoveries are new to the other
country. [ref 4 pages 21-22]. An experimental teletype line is laid to
Winter Harbor, Maine and brought into use in March 1941. The first
exchange of raw traffic by cable to London was in May 1941,
though these did not include JN-25 messages. [ref 4 page 22]

In 1941 Washington was receiving on average 7,000 JN-25 original
messages plus 11,000 duplicates per month. This average level continued
until mid 1942, [ref 1, page 7] These made up approximately 60 to 75% of
the IJN messages (depending on the month) received in Washington [ref 1,
page 1]

Allied code group meaning recovery status, for JN-25B,
April 1, 1941: "approximately 300 values recovered" (also there were
1,800 A code groups recovered showing the continued work on that
system, up from 1,400 at the end of February) Selected personnel
were receiving 1 hour per day Japanese language training.
May 1, 1941: "approximately 400 values recovered"
June 1, 1941: "approximately 1100 values recovered"
July 1, 1941: "approximately 1100 values recovered"
August 1, 1941: "approximately 2000 values recovered"
[ref 2, April, May, June, July and August 1941 reports]

1 August 1941 additive book 7, B-7 or D IPPAN RA7 introduced.

In September 1941 it was noted the code groups used to indicate following
upper case or auxiliary table values invariably began with a thrice repeated
digit, since these indicators appeared frequently in messages it gave a
quicker way to remove the text additives. The method was called the
difference table, discovered independently by Washington and Corregidor
[ref 4 pages 24-25]

Allied code group meaning recoveries,
Sept 1, 1941: "approximately 2000 values recovered"
Oct 1, 1941: "approximately 2400 values recovered"
"Completed during October 1941" Nov 1, 1941: "600 values recovered" [i.e.,
a total of 3,000 values as of this date]
"Completed during November 1941" Dec 1, 1941: "800 values recovered"
(i.e., a total of 3,800 values as of this date)
[ref 2, September, October, November and December 1941 reports]

4 December 1941 additive book 8, B-8 or D IPPAN RA8. The indicator
groups are altered so the page number is being multiplied by 100, so
the 5 digit group is now the actual starting point, disguised by the
key additive. [ref 4 pages 27-28]

7/8 December 1941, Pacific war begins.

On 9 December Hawaii started on JN-25 but it looks like the station did
not have the materials to really start working on JN-25B until about 11
December 1941 or a few days later. Another date given is 10 December.
The materials had been sent out by Washington around September 1941,
they were misrouted and were finally found in a Registered Publications
depot. The disruption of the Pacific mail system meant Washington had
to mark time for a month before B-7 traffic was available to it in quantity
though it did do some key recovery work. [ref 4 page 27]. In December
1941 or early 1942 use of the secure "Copek" radio channel was allowed
to link the code breaking groups. [ref 4 page 29]. The channel had been
set up some time previously for top secret information, now the code
breakers were allowed to use it for exchanging information.

Allied code group meaning recoveries, "Unfinished or continuous"
Jan 1, 1942: "Approximately 6,180 values recovered". [ref 2, January
1942 report] In the final quarter of 1941 GY-1 is 22 personnel, 9
officers, 10 enlisted men and 3 civilian men, this is an increase of 1
man per month on average since the start of 1941. [ref 1, page 4]

Total messages read by the USN per month in Washington during 1941,
none. [ref 1, page 8]. The code breakers in the Philippines are said to
have managed to break out some simple messages, "invariably ship
movement reports: arrivals and departures, together with some
fragmentary schedules. The cryptanalysis of the ciphers had outstripped
the facilities for code recovery." [ref 4 pages 25-26].

Contradicting this claim is the fact none of these claimed decrypted JN-25B
messages was of such intelligence value as to be included in Corregidor's
daily Comint Summary. Also only one so-called "movement report" was
included in the Corregidor Comint Summary, on 30 November 1941 and
this report could have been from one of two other methods of sending this
information by radio besides JN-25B. The personnel from Corregidor, such
as Commander Rudolph Fabian and Captain Duane Whitlock , indicate
nothing of any intelligence value was ever obtained from JN-25B decrypts
before the war broke out. It should be noted reference 4 is a wartime
document written in Washington, with problems in obtaining input from the
people who were on Corregidor in 1941 and therefore appears in error on
this point.

Also "JN-25 played no part in the Radio Intelligence story of Pearl Harbor."
[ref 3 page 11] and "JN-25, therefore, has no part in the story of Pearl
Harbor. What "might have been" there is a story confined entirely to the
Japanese diplomatic, not to the Naval communication channels." [ref 4 page
26]

In the first quarter of 1942 GY-1 is 41 personnel, 9 officers, 30 enlisted
men, 1 civilian man and 1 civilian woman. [ref 1, page 4]

On 18 March 1942 the Washington code breaking group was ordered to
Begin decrypting current JN-25 traffic. On 18 April the last of the code
breakers was evacuated from the Philippines. [ref 4 page 29]

28 May 1942 C version plus new additive book 9, C-9 or D IPPAN RA9,
(The Japanese called it code book D-1, from Proceedings article)

In the second quarter of 1942 GY-1 is 98 personnel, 12 officers, 70 enlisted
men, 1 civilian man and 15 civilian woman. [ref 1, page 4]

In the period December 1941 to June 1942 Hawaii recovered some 25,000
text additives, in the March to May period Washington supplied over 16,000
more. [ref 4 page 51]

Early August Marines on Guadalcanal capture a JN25C code book.

15 August 1942 D version plus additive book 10 D-10 Jo or RO 1 RA 1
This runs to 19 October 1942. It replaces the C code book, from now on
there will be more than one combination of code and additive book in
use at the same time. The number of text additives is changed from
50,000 to 100,000, key additives from 999 to 1,000. This version was
meant as a general purpose system for all naval forces on land and sea
excluding picket boats, gunboats, hospital ships, rescue ships and labour
battalions. (For the information on which code book was for what
purpose see [ref 3 chapter V page 3])

15 August 1942 D-16-A Jo version, RO 2 RA 1. This is the first case of
multiple versions of the code running at the same time, it appears to use
additive book 16. This runs to 15 October 1942.

15 August 1942 D-16-B Jo version, RO 3 RA 2, running to 15 October 1942.
The D-16-A and B versions were general purpose systems for the high
command. The traffic in the high command systems was so limited the
D-16 versions were never broken and this state of affairs continued for the
rest of the war for the high command "channel". (For the information on
which code book was for what purpose see [ref 3 chapter V page 3])

A series of 3 digit numbers from a 9 x 9 garble table assigned in 10 day
blocks was used to distinguish which version a message was being sent in.
[ref 3 chapter V page 4]

Total JN-25 messages read by Washington per month during the first 9 months
of 1942 estimated to be 2,500. [ref 1, page 8]

In the third quarter of 1942 GY-1 is 207 personnel, 17 officers, 125 enlisted
men, 15 civilian man and 50 civilian woman. [ref 1, page 4]

1 October 1942 additive book 11, E-11, HA 1 RA 1, for E version code, this
runs until 30 November. First appearance of the E code book. So we now
have 2 versions of the code book in use plus 4 additive books. Note unlike
the D codes in use only 50,000 text additives are used. This version was
meant as a general purpose system for larger shore bases and all fleet units
except submarines. (For the information on which code book was for what
purpose see [ref 3 chapter V page 3])

During October 1942 the IJN begins a program of deception and drill traffic,
continuing irregularly through 1943 [ref 1, page 2]

16 October 1942, D-16-A Go, RO 2 RA 1, and D-16 B Go, RO 3 RA 2,
revisions of the two 15 August versions. Changing from the Jo to Go additive
book, until 13 December.

20 October 1942 additive book "D-10 Go", for D version, RO 1 RA 1
Until 17 December.

24 October 1942, EFU, until 14 April 1943. Probably a minor cipher book.

1 November F-10 Go, KO 3 RO 1 RA 1 Go, this is first appearance of the
F code book. Until 31 May 1943 after which it is retired. So we have the
D, E and F code books in use. The F code was meant for auxiliary vessels,
minor shore stations and resident naval officers. (For the information on
which code book was for what purpose see [ref 3 chapter V page 3])

1 December 1942 additive book 12 for E code, HA 1 RA 1, until 2 January
1943. Note unlike the D codes in use only 50,000 text additives are used.

14 December 1942 D-17-A, RO 2 RA 2 and D-17-B, RO 3 RA 2, additive
book 17 for the D code books replacing 16 where it was in use. Until
14 February 1943.

17 December 1942 Additive book 13 for the D version, RO 1 RA 2, until 14
February 1943

Total JN-25 messages read by Washington per month during the last 3 months
of 1942 estimated to be 6,000. [ref 1, page 8]

Total JN-25 messages sent to Washington in the last 6 months of 1942, on
average per month 17,000 plus 20,000 duplicates. [ref 1, page 7]

In the fourth quarter of 1942 GY-1 is 299 personnel, 32 officers, 15 Waves
officers, 168 enlisted men, 29 civilian man and 55 civilian woman. [ref 1, page 4]

3 January 1943 additive book 14 for the E version, RA 1 RA 2, until 14
February 1943. Number of text additives raised to 100,000, from now until
the J-36 version in December 1943 this will be the standard text additive size.

12 January 1943 ?? code (name appears crossed out), in use until 14 April 1943

20 January 1943 dummy "SUB CODE" until the end of the month. This is
the first appearance of a dummy code.

15 February 1943 D-17-A, RO 2 RA 2 and also D-17-B, RO 3 RA 2, unknown
change, both until 25 February, after which they appear to be retired.

15 February 1943 additive book 14 for the D code book, HA 1 RA 2, until 31
March

15 February 1943 additive book 15 for the E code book HA 1 RA 3, until
14 April. In this system a message begins with the relevant to/from
information, a six digit plain text date/time group, a code group indicating
what version of JN-25 is being used and the indicator group repeated twice.
The final three groups of the message are an ending indicator, the last 5
digits of the date time group and finally a repeat of the version indicator group.

15 February 1943 DQ code, RO 1 RA 3, until 31 March, when it appears to
be retired.

February/March 1943, US recovers significant code materials from the I-1
sunk off Guadalcanal. Including a superceded JN-25 code book, but no
additive book.

1 March 1943 E-20-A (44) RO 2 RA 3 and E-20-B (44) RO 3 RA 3, additive
book 20 for these two codes. In use until 30 April.

Total JN-25 messages read by Washington per month during the first 3 months
of 1943 estimated to be 8,000. Of which 4,800 were current. Total messages
sent to Washington per month 19,000 plus 50,000 duplicates. [ref 1, page 7-8]

In the first quarter of 1943 GY-1 is 387 personnel, 46 officers, 23 Waves
officers, 196 enlisted men, 60 enlisted waves, 26 civilian man and 36
civilian woman. [ref 1, page 4]

13 March 1943 Dummy code, used until sometime in April.

1 April 1943 D-18a, additive book 18a for D code book, RO 1 RA 4, until 14 April.

With D-18 the number of key additives is raised from 1,000 to 2,000, this becomes

standard until the J-36 version introduced in December 1943.

5 April 1943, or around this date another dummy code which stays in service
until 13 April.

15 April 1943 D-18b, additive book 18b for D code RO 1 RA 4 , D-18
ends 31 May and this appears to be the final use of the D code book, at
least before 4 August. Also E-19 additive book 19 for E code HA 1 RA 4,
E-19 ends on 14 June.

15 April 1943 change made to ??, in use until 16 July and then appears
to end. Also a code called EFU, in use until 1 May. EFU appears to end after 1 May.

18 April 1943 the aircraft carrying Admiral Yamamoto is intercepted and
shot down, killing the Admiral.

1 May 1943 E-23-A RO 2 RA 4 and E-23-B RO 3 RA 4, additive book 23
for these two codes. In use until 30 June.

1 June 1943 H-21 additive book 21 for H version, RO 1 RA 5, until 7 July.
Also K-25 RU 1 RA 1 to 4 August 1943. So the first appearance of the H
and K code books.

15 June 1943 E-22 additive book 22 for E version HA 1 RA 5, until 4 August
1943.

Total JN-25 messages read by Washington per month during the second 3
months of 1943 11,000. Of which 5,100 were current. Total messages sent
to Washington per month 22,000 plus 70,000 duplicates. [ref 1, page 7-8]

In the second quarter of 1943 GY-1 is 472 personnel, 47 officers, 29 Waves
officers, 160 enlisted men, 185 enlisted waves, 20 civilian man and 31
civilian woman. [ref 1, page 4]

1 July 1943 E-29-A RO 2 RA 5 and E-29-B RO 3 RA 5, additive book 29
for these two codes. Until 4 August.

7 July 1943 H-24 additive book 24 for H version RO 1 RA 6 to 4 August

Total JN-25 messages read by Washington during July 1943 16,800. Of which
15,700 were current. [ref 1, page 8]

Big JN-25 reorganisation on 5 August 1943, no less than 5 changes, J code
books introduced, E code books retired.

Things continue to become more complex, including old code and
additive books being brought back into service. The above basic JN-25
version information comes from 4 pages of spreadsheets, in reference 5
covering over 4 years from June 1939 to August 1943. The JN-25
information for August 1943 to the end of 1944 is another 5 spreadsheet
pages. It includes further changes to the key additives system and even
the return of previously retired versions for supplemental or other uses.
The final entry is for JN-25 version N-77 (RO 2 RA 18) on 1 January 1945.

As of 20 April 1945 the USN found it had 35,761 additives of JN-25B7,
the version in use 1 August to 3 December 1941, with the probability
several hundred might be bad. The recent decision to rework the traffic,
which was available in far greater amount than traffic in any other version
meant it was considered possible to recover most of the missing additives.
JN-25B8, in use 4 December 1941 to 27 May 1942 had 47,340 text
additives recovered but they included a large number of bad values, a quick
check resulting in around 1,000 changes. In addition many of the B8 work
book pages had been destroyed, requiring them to be rewritten but it was
hoped almost all additives could be recovered. [ref 5 workable traffic list
dated 20 April 1945]. As of 21 August 1945 the recoveries by version were
[ref 5 JN-25 key and additive recovery table],

JN-25A5 999 out of 999 key additives, 4,907 out of 50,000 text additives.
JN-25B6 999 out of 999 key additives, 45,321 out of 50,000 text additives.
JN-25B7 999 out of 999 key additives, 47,590 out of 50,000 text additives.
JN-25B8 999 out of 999 key additives, 48,835 out of 50,000 text additives.

The following table is a restatement of the number of JN-25 messages read
in Washington per month, table is time period, total read per month, total
read that were in the current cipher,

1941 none and none
January to September 1942 2,500 (estimated) and unknown
October to December 1942 6,000 (estimated) and unknown
January to March 1943 8,000 (estimated) and 4,800 (estimated)
April to June 1943 11,000 and 5,100
July 1943 16,800 and 15,700
August 1943 13,000 and 2,100
September 1943 14,300 and 12,900
October 1943 17,800 and 9,300
November 1943 20,900 and 17,200
December 1943 16,600 and 1,700

JN-25 Messages received per month in Washington, table is time period,
number of original messages, number of duplicates,

1941 7,000 and 11,000
January to June 1942 7,000 and 11,000
July to December 1942 17,000 and 20,000
January to March 1943 19,000 and 50,000
April to June 1943 22,000 and 70,000
July to September 1943 26,000 and 84,000
October to December 1943 30,000 and 96,000

Some 60 to 75% of the IJN messages received in Washington were
in JN-25, in the period 1941 to 1943.

Size of GY-1,USN code breaking unit in Washington

In the first quarter of 1941 GY-1 is 10 personnel, 7 officers and 3 enlisted men.
In the second quarter of 1941 GY-1 is 16 personnel, 9 officers, 5 enlisted
men and 2 civilian men.
In the third quarter of 1941 GY-1 is 20 personnel, 8 officers, 10 enlisted
men and 2 civilian men.
In the final quarter of 1941 GY-1 is 22 personnel, 9 officers, 10 enlisted
men and 3 civilian men.
In the first quarter of 1942 GY-1 is 41 personnel, 9 officers, 30 enlisted
men, 1 civilian man and 1 civilian woman.
In the second quarter of 1942 GY-1 is 98 personnel, 12 officers, 70
Enlisted men, 1 civilian man and 15 civilian woman.
In the third quarter of 1942 GY-1 is 207 personnel, 17 officers, 125
Enlisted men, 15 civilian man and 50 civilian woman.
In the fourth quarter of 1942 GY-1 is 299 personnel, 32 officers, 15 Waves
officers, 168 enlisted men, 29 civilian man and 55 civilian woman.
In the first quarter of 1943 GY-1 is 387 personnel, 46 officers, 23 Waves
officers, 196 enlisted men, 60 enlisted waves, 26 civilian man and 36
civilian woman.
In the second quarter of 1943 GY-1 is 472 personnel, 47 officers, 29 Waves
officers, 160 enlisted men, 185 enlisted waves, 20 civilian man and 31
civilian woman.
In the third quarter of 1943 GY-1 is 606 personnel, 60 officers, 47 Waves
officers, 117 enlisted men, 336 enlisted waves, 12 civilian man and 34
civilian woman.
In the fourth quarter of 1943 GY-1 is 656 personnel, 70 officers, 72 Waves
officers, 108 enlisted men, 367 enlisted waves, 5 civilian man and 34
civilian woman.

End proposed JN-25 fact sheet.