On 1 June 1931, Herbert O. Yardley's The American Black Chamber stunned the world by exposing American codebreaking activities. Among the diplomatic messages uncovered by his Cipher Bureau, "the most important and far-reaching telegram" as Yardley put it was an instruction to the Japanese plenipotentiary sent to the Washington Naval Conference in 1921 (see another article).
While there have been statements that the book caused Japan and other nations to revise their codes at once, Kahn (2004) undermined such a myth by showing the number of solutions in the British archives of secret messages of Japan and Germany did not decrease after the publication of the book (Kahn p.131-136) (A skeptical view on such a traditional "offhand statements" is also presented by the editor of ASA, Wayne G. Barker (ed.), The History of Codes and Ciphers in the United States during the Period between the World Wars, Part I. 1919-1929 p.137). Materials in Japanese archives appear to support Kahn's statements that Japan had been updating its diplomatic cryptosystems every couple of years and did not immediately change to new ones.
Table of Contents
Sensation in Japan
Code Security Measures Before and After the Publication of The American Black Chamber
Proposal of Introduction of Cipher Machines
Preparation for Facing the Parliament
Did Yardley Sell Secrets of Codebreaking to Japanese Embassy?
Yardley Continues Disclosure
Japanese Reaction to Code Security Incidents
The first notice of the publication of The American Black Chamber came in on 6 June in a telegram from the Japanese ambassador in Washington, Debuchi Katsuji, to the Foreign Minister, Shidehara Kijuro, who had been ambassador to the US during the Washington Naval Conference back in 1921-1922. It reported that the American Black Chamber had been reading foreign diplomatic secret telegrams and that the book stressed that any code could be broken with scientific methods. With regard to Japan, the book not only revealed that telegrams to and from the Japanese plenipotentiary during the Washington Naval Conference had been read but also published many telegrams translated into English (Yardley file (1) p.38).
The ambassador sent two copies of the book (Yardley file (1) p.51) and related newspaper articles (Yardley file (1) p.54).
Sakuma, Chief of the Telegraph Section of the Ministry of Foreign Affairs, wrote an extensive memorandum on the matter as early as 10 June (Yardley file (2) p.40-48). It shows that the codebreaking per se was no surprise and that the Foreign Ministry had already paying efforts to improve its cryptosystems (Sakuma understood that any code might be broken some time but still believed in cipher machines). Rather, Sakuma discusses more at length the disclosure of telegrams and inquires into ways to charge Yardley somehow (Section (2)). It also shows a bureaucratic reality that hampered the efforts to improve cryptographic security (Section (3)).
The reaction of the Foreign Ministry was slow. After the above memorandum was written, little appears to have been done until on 20 July 1931, the Tokyo Nichi Nichi newspaper carried a long article about The American Black Chamber, headed "What an Idiot." It retranslated into Japanese some of the secret telegrams disclosed by Yardley and supported them by referring to corresponding contemporary articles on the paper.
Various reactions to the revelation were carried on 22 July (also Kahn (1967) p.363). An informed source of the House of Peers accused Ambassador Shidehara, now Foreign Minister, by pointing out only Japanese telegrams, not those to the embassy in Britain but only those addressed to the embassy in the US, were read during the Washington conference and the Ambassador was unaware of it. He also questioned the attitude of the Foreign Ministry as if it were simply a matter of disgrace to the US. Baron Ikeda of the House of Peers deplored that plenipotentiaries could not request instructions but only send reports and asked for measures for the Geneva Conference scheduled next year. An anonymous naval officer directed his criticism to the breech of privacy of communication in the US. An authority of the Minstry of War said he knew of the incident by the newpaper coverage; pointed out naïveté of Foreign Ministry codes; expressed the Army would take every necessary measure for the League of Nations General Disarmament Conference (Wikipedia) scheduled next year, which would discuss reduction and limitation of the army; and sarcastically concluded he might give advice to the Foreign Ministry from good will.
Comments from an authority of the Foreign Ministry were as follows.
The newspaper on the same day carried an advertisement of imported books from a bookseller Maruzen, which highlighted The American Black Chamber (7 yen), which was to arrive in early August.
Next day, on 23 July, a correspondent reported words from Yardley, whom he met in Yardley's hometime in Indiana where he then lived. Yardley pointed out that every state other than the US had a black chamber and there was no option for the US but to establish one or conclude a treaty to make other states abolish it. He said he offered to the US government a new measure for undecipherable code by means of a "code machine", for which he had not received any response. He declared that there was no conventional code/cipher that could not be broken.
The Tokyo Nichi Nichi daily carried partial translation of The American Black Chamber from 24 July. (The whole book would be published by its parent company, Osaka Mainichi Shimbunsha, in August.)
Apparently, the Tokyo Nichi Nichi's scoop did not arouse a nationwide sensation. For example, the Asahi newspaper does not appear to have carried a follow-up article. Still, an American in Japan felt a "wave of anti-American feeling [Yardley's book] stirred up among all classes of the Japanese people" (ASA op.cit., p.136).
English-language newspapers referred to "mild sensation" or "serious sensation." At least, there was a persistent and considerable interest in the issue (Kahn (1967), p.363-364).
Moreover, it did add fuel to partisan criticism of the Foreign Minister Shidehara. The Nippon newspaper carried a series of articles from 15 August titled "Revealed History of Failures of Shidehara Diplomacy", which accused diplomacy of the Foreign Minister Shidehara as complaisant. The Yardley files include clips of these articles. .
The second article on 16 August mentioned Yardley's book. The article did not make much of codebreaking itself. Instead, it criticized breaking into the Japanese General Cosulate in New York to get access to a codebook and suspected other dubious means for breaking Japanese code. The third article on 17 August insinuated a possibility that Shidehara's treatment of the staff at the embassy might have produced cooperators to the Black Chamber.
The article reported comments of Shiratori (Wikipedia (in Japanese)), Head of Intelligence Department of the Foreign Ministry: "Secretive reading of messages in code is a problem during pendency at the telegraph office, not after their reception at the embassy. Moreover, there was once an offer of a deal from the author and others but we turned them down." The reporter added he heard there had been an offer as early as at the time of the Washington Conference of a deal about reading of secret telegrams but the Foreign Ministry also took no notice of it, which the reporter criticized by saying at least the veracity of the reading should have been checked.
The newspaper coverage appears to have roused those concerned from inactivity. On 28 July, Sakuma drafted replies to possible questions that may be brought up in the parliament about the issue and on 31 July further inquiries were made to the ambassador in the US. On 29 July, Ito Risaburo, a naval officer, renewed his proposal for the Foreign Ministry to use cipher machines. On 15 August, instructions about security of code were drafted and sent to overseas establishments on 17 September. These will be described in the following.
In a document titled "Regarding Protection of Security of Telegraphic Code" dated 9 September 1930, the Foreign Minister alerted overseas establishments about storage of codes as well as telegrams in code. It even asked for caution when quoting a secret telegram so as not to give any clue to codebreakers. (JACAR B12080889500 p.9)
In this fall, the Foreign Ministry assigned personnel to check outgoing telegrams, one of whose duty was to make sure that the code was fully used. There was tendency that only relatively simple parts of the code were used. Failure to use the full range of the code results in not only high telegraph costs due to increase in the number of words but also vulnerability of even a relatively complex code. (JACAR B12080889500 p.15-16)
Sakuma, Chief of the Telegraph Section, drafted a document again titled "Regarding Protection of Security of Telegraphic Code" dated 1 April 1931 (Yardley file (3), p.32-35).
Based on reports from the overseas establishments solicited in the above document of 9 September 1930, it focused on handling and storage of not only codes as well as drafts/translations of telegrams in code and required use of a steel cabinet with a combination lock etc.
It is silent about cryptologic security as discussed in the memorandum of 10 June 1931 after the publication of Yardley's The American Black Chamber. But then, it was only natural because preparation and distribution of codes are the responsibilities of the Foreign Ministry, not the embassies or consulates.
Sakuma relied on expertise of Captain Inoue of the Reserve Navy, who worked for the Telegraph Section since 1929 as a temporary member and would join the regular staff in 1934 (Shiraishi p.103, n.(9)). Inoue headed a Code Research Group in the Telegraph Section. The group had been established in 1921 in the Telegraph Section of the Foreign Ministry by the ministries of War, Navy, Foreign Affairs, and Communication and had broken English and American codes (Hori Eizo, Daihonei Sanbo no Joho Senki; Kahn (1967) p.495). The group took over compilation of codes from the telegraph staff in the Telegraph Section. Sakuma describes as follows in a memorandum dated 9 October 1933 (JACAR Ref. B12080889500).
After the publication of Yardley's The American Black Chamber in June 1931 and its newspaper coverage in late July, needs for additional measures for code security were felt. On 15 August 1931, instructions about proper use of telegraph codes were drafted (Yardley file (3) p.75).
In particular, it warns against using a code to encode a report known to another country and stipulates use of a proper code among various codes provided, depending on the nature of the particular message to be encoded.
The draft was revised and sent out to overseas establishments on 17 September 1931 (its text is at JACAR B12080889500 p.10).
The above is followed by an explanation of proper use of various codes depending on the nature of the message to be encrypted (see another article). In addition, it was stressed that the full range of a code should be used so as to avoid spelling a word letter by letter (i.e., kana by kana) and that alternative groups should be used when the same word/phrase is frequently used in a telegram.
The Japanese translation of The American Black Chamber was published in a book form in August 1931 (priced at one yen per copy). The publisher, Osaka Mainichi Shimbunsha, was the parent company of the Tokyo Nichi Nichi newspaper that covered the book in July.
On 22 August, Sakuma, Chief of the Telegraph Section, drafted a proposal on behalf of the Foreign Ministry asking for Cabinet approval to purchase 138 copies for distribution to overseas establishments, explaining that the book "describes in detail how the US codebreaking unit tried every vicious means to break codes of Japan and other countries and it would be very beneficial for protection of security of our code to let the staff of the overseas establishments, in particular telegraph staff, be informed of its contents." (Yardley file (1) p.72) (The "vicious means" appears to refer to breaking into consulates to get access to a code (cf. JACAR Ref. B12080889500 p.14) or an attempt to cause specific information to be telegraphed in order to get a crib (ibid. p.13), the kind of means also used by the Japanese.) The books were despatched as of 18 September (Yardley file (1) p.70).
The Army also drew some lesson from The American Black Chamber. In August 1931, the "7th Section" of the Army drafted a memorandum titled "Measures for Communication Interception from the Viewpoint of Military Communication." Its introduction observes that although Yardley's book mainly deals with peacetime diplomacy, experience from the First World War necessitates measures against communication interception in peacetime as well as in wartime. The overall structure is as follows.
1. Communication Medium for Intelligence and Communication Policy (C14010457000) ... As evident from The American Black Chamber, as long as a foreign communication network is used, the foreign government may obtain a copy of a telegram. Short-wave radio may be used for intelligence agents but it involves a high risk of being discovered.
2. Choice of Communication Medium (C14010457100) ... Wireless media is prone to interception and wired media should mainly be used.
3. Communication Instruments (C14010457200) ... In amphibious operations and air-ground communication, wireless communication is prerequisite. Development of various technologies should be conducted.
4. Countermeasures against Enemy Communication Intelligence on the Battlefield (C14010457300) ... Countermeasures for wireless and wired communications are discussed. Among the quotations of the measures adoped by the British Navy is "7. Different codebooks should be used for different theatres of operations and should be frequently changed."
5. Communication Tactics (C14010457400) ... Active measures to deceive the enemy, rather than passive countermeasures, are discussed.
6. Cryptologic Service (C14010457500) ... Importance of codes/ciphers and prevention of codebreaking are discussed in view of The American Black Chamber. The following observation is made regarding preparation of new codes/ciphers.
7. Format of Communication Message (C14010457600) ... many and long telegrams, repetition of common phrases, regular construction of telegrams, disclosure of a material to be compared with the communication content, etc. would help codebreaking. In particular, the descriptions of Chapter 15 of The American Black Chamber (p.281) are quoted to warn against routine telegram construction.
8. Detection of Enemy Communication Intelligence (C14010457700)
9. Defense agianst Espionage (C14010457800)
Comments of the Ministers of the Navy and the Army appeared in the Osaka Mainichi newspaper on 31 August 1931.
Even before the publication of The American Black Chamber, vulnerability of code was recognized and during the London Naval Conference of 1930, cipher machines made by the Navy were used by the Foreign Ministry. There was a proposal from the Navy after the conference that a cipher machine should be consistently used for all the important telegrams but it was only after the publication of The American Black Chamber that the proposal was renewed and adopted.
On 9 July 1930, Ito Risaburo, Chief of the Telegraph Section of the Naval Ministry, sent a document titled "Observations on Code/Cipher for Naval Conference" to several persons in the Foreign Ministry on behalf of the Navy (JACAR Ref. B04122581000). It explained techniques of codebreaking and superiority of cipher machines, which had been used to the great advantage during the London Naval Conference in January to April 1930. Ito recommended use of cipher machines for all the telegrams during the next major international conference because concurrent use of a weaker code might compromise the security of the machine cipher.
The proposal was forgotten until after the publication of The American Black Chamber in June 1931 and its newspaper coverage in late July. It may be because the new gadget had been unsatisfactory for the Foreign Ministry. Not that cryptologic security was questioned but there were several problems in practice. The Navy improved the design and Ito renewed the proposal by telephone to Sakuma on 29 July 1931 in view of the situation now that "the Yardley incident is much clamored".
Now Sakuma took up the proposal seriously and visited Ito. Satisfied with his explanation of improvements, he drafted a proposal on behalf of the Foreign Ministry, dated 6 August 1931, asking for Cabinet approval of introducing the new cipher machine of the Navy for use during the Geneva Naval Conference (Wikipedia) scheduled in 1932 (Yardley file (3), p.36-39). He pointed out the machine cipher would be more secure than the conventional code system and the defects of the previous model used during the London Naval Conference in 1930 were to be largely corrected.
This cipher machine, introduced in 1931 (or the Japanese Imperial Year 2591), would be known as RED as designated by the US codebreakers (Wikipedia).
(See another article for development of the RED machine.)
On 28 July 1931, Chief Sakuma drafted replies to possible questions about the Yardley issue that may be brought up in the next session of the parliament (Yardley file (1) p.1-13, (2) p.51-66, (3) p.40-53). The first question concerns the responsibility of the Foreign Ministry and is adapted from Section (4) of the above memorandum of 10 June 1931 drafted shortly after the publication of The American Black Chamber.
The other questions are as follows. None of these items concerns cryptologic security of diplomatic codes. (Their answers, occasionally bureaucratic, are of some interest but are off the main topic here.)
Regarding (5), Sakuma noted reports were expected from the Ambassador to the US and further information would be available by the parliament session.
Indeed, on 31 July 1931, the Ambassador was instructed to inquire into whether Yardley's disclosure was a breach of federal law or state law by infringing on privacy of communication. The Ambassador reported on 11 August after an interview with an official of the Department of State that, while one should wait and see the proceedings of the investigation by the Department of Justice, the only legal problem if any was how he could obtain the telegrams.
The Ambassador was further instructed to collect information about US federal and state laws as well as rules of telegraph companies about censorship and privacy of communication. Exchanges went through September, November (Yardley file (1) p.75-78, (2) p.7-11), and December (Yardley file (2) p.18-36).
Sakuma compiled information from the Ambassador in a memorandum dated 21 March 1932 about privacy of communication in the US (Yardley file (1) p.27, 39, 40, (3) p.1). Sakuma used this as a basis for a supplement of the possible questions for the parliament dated 15 May 1932 (Yardley file (1) p.15, (2) p.66, (3) p.54). Further inquiries were made on 20 May 1932 (Yardley file (2) p.49) and, with reports in response, Sakuma prepared a supplement dated 10 August 1932 for the memorandum of 21 March 1932 (Yardley file (1) p.34, (3) p.8).
These efforts did not find any politically practical means to charge Yardley or telegraph companies. Nor did Sakuma intended that. As a bureaucrat, he was doing his best to discharge accountability. His very first memorandum of 10 June 1931, drafted shortly after the publication of The American Black Chamber, states "Since any of the above measures would not be appropriate, after all, it would be sufficient to consider domestic measures, that is, how to respond when the issue is taken up by the parliament or domestic newspapers."
In the mean time, the session of the 60th Imperial Parliament this winter had been from 26 December 1931 to 21 January 1932. As far as can be known from the proceedings of the Parliament (NDL), the plenary session under a new cabinet inaugurated in December 1931 was dominated by eagerness to approve the marching of the army in Manchuria since the Mukden Incident in September 1931.
Sakuma's memorandum dated 10 June 1931, translated above, mentions more than once a deal whereby the Japanese embassy paid 7000 dollars to Yardley for copies of decoded telegrams etc. This memorandum is mentioned on p.273 of Kahn (2004) (David Kahn Official Website) as a source of two American authors reporting this story of Yardley's betrayal.
Kahn doubts the story and believes that it was fabricated to denigrate Yardley and save Japanese face. This sounds right as for the alleged offer of a deal during the Washington Conference, which was mentioned in the Foreign Ministry's comments on the 22 July issue of the Tokyo Nichi Nichi and which the reporter of the Nippon said he had heard.
However, Sakuma's memorandum refers to June 1930 and the references sound like more than a mere fabrication. Rather than trying to denigrate Yardley to save face of Japan or the Foreign Ministry, the author seems to be really concerned that Yardley might disclose his selling the documents.
While Kahn mentions "great motivation" for the allegation, Yardley also had his own motivation. In October 1929, when Yardley's Cipher Bureau was abolished, the Great Depression plunged the world into a devastating economic crisis. Yardley gave up his fancy apartment in New York and removed to his hometown in Indiana. (Kahn (2004), p.104) Report has it from his friends that Yardley offered his documents to the US government when he left the service but the government refused them (Yardley file (3) p.14, citing New York Times, 21 February 1933). In April 1930, his application to the navy for a position of an instructor in cryptanalysis was turned down. So was his proposal in the spring to a publisher to write about his activities, after which he "did nothing for a few months", until he found a contact which led to the publication of The American Black Chamber (Kahn (2004), p.104-105). The date "June 1930" of the Telegram No. 105, albeit not discovered, coincides with the blank in Yardley's career during the nadir of his fortune.
What happened in Japanese code security measures after June 1930?
Chief Sakuma of the Telegraph Section replaced his predecessor Sawada in July 1930. He wrote in a later memorandum dated 9 October 1933 (see above) that at the time he took office "there was the Yardley issue" (JACAR Ref. B12080889500 p.20) but the expression "at the time" may broadly refer to "ever since he took office." (Considering he was concerned that Yardley might disclose the deal, it is unlikely he casually mentioned it even in a memorandum stamped "top secret".)
On 9 July 1930, Ito submitted his first proposal of introduction of cipher machines. The Foreign Ministry did have used cipher machines of the Navy during the London Naval Conference in January to April 1930 but Ito thought it fit to make a proposal at this timing. However, Ito's first proposal is silent about Yardley, while it does mention older code security incidents (see another article). Thus, it appears Ito did not know the deal.
At the end of 1930, a naval code was updated from Red to Blue (dubbed by the US Navy codebreakers) (NSA; Budiansky, Battle of Wits, p.83; "Red" should not be confused with the RED cipher machine mentioned above). But of course, the Navy no more needed "the Yardley issue" than the Foreign Ministry to be motivated to revise its code.
Thus, no more convincing evidence for the deal was found than the really concerned tone of Sakuma's memorandum of 10 June 1931 and the coincidence of the timing mentioned. Although I am inclined to think the offer in June 1930 was actually made, I would then have to explain why Shiratori, Head of Intelligence Department, mentioned it to the media, when Sakuma, Chief of Telegraph Section, feared its disclosure.
At least, the question should be left open until someone finds further evidence, either positive or negative. Needless to say, even if there was the deal, it does not have to detract from Yardley's achievement in codebreaking.
(Notes added in November 2014) A recollection by a witness is recorded in Kosaka Masataka and Nagata Junko, "Nichi-bei Ango Senso 2 -- Kimitsu Ango Purple" in Rekishi-heno Shotai 27, NHK, Japan (1983), p.151-152, based on a TV program in 1982 (NHK On-Demand). Kase Toshikazu (1903-2004) (National Diet Library (in Japanese), Wikipedia (in Japanese)), assistant secretary in his late twenties to the Japanese embassy in Washington at the time, says he was present when secretary Togo Shigenori (Wikipedia) received Yardley, who brought many deciphered papers (but Kase does not mention any monetary request). Deciphering was one of assistant secretary's jobs and Kase confirmed to Togo that Yardley's deciphering was correct.
Kase was also a prolific writer and he provides more details in his memoirs (Kase Toshikazu Kaisoroku (1), 1986, p.20-21). The memoirs date Yardley's production of his deciphering on the day after his first visit. He further identifies the code he recognized as a decipherment table "P go" ("the Issue P"). (Of course, he could not have recognized a code used back in 1921-1922, since he was in the US as assistant only from June 1928.)
Further according to Kase's memoirs, Yardley not only asked the embassy to buy the deciphering with 10,000 dollars but also offered to teach the art of deciphering. The home ministry decided to accept the offer and Kase was chosen as the pupil because he had already met Yardley. But Yardley disappeared without giving his lessons.
That Togo's memoirs and biographies do not tell the episode may be natural because he must have been more concerned about what was told in telegrams rather than the ciphering of the telegrams. The problem of Kase's testimony, however, is that he identifies the date as "the spring of 1929". He appears to have relied on the time of the inauguration of President Hoover and Secretary of State Stimson (March 1929) but it is highly improbable that Yardley ever thought of selling the secret of the Cipher Bureau so early, considering that Yardley seems to have had no concern as of the end of May (Kahn (2004), p.267, the sixth note; p.97). (Moreover, the Cipher Bureau was paid until the end of October 1929 (Kahn (2004), p.97-99).) One is tempted to consider it was an error for "the spring of 1930", which would match Sakuma's memorandum and the nadir of Yardley's career. However, Togo left Washington in May 1929 (Kase helped his packing) (Sofu Togo Shigenori no Shogai [Life of My Father, Togo Shigenori], p.111). Is it possible that Kase mistook who was his superior secretary at the time?
Even after the publication of The American Black Chamber, Yardley did not stop taking advantage of the materials he retained after the Cipher Bureau was abolished. They were reportedly kept in safe deposit boxes at various places. Though Kahn considers it unlikely, at least when Yardley needed them in late 1931, Yardley asked D. Laurans Chambers, the editor of Bobbs-Merrill, the publisher of The American Black Chamber, for "the Japanese telegrams that are in your safe" (Kahn (2004) p.278, p.158, Yardley file (3) p.14, citing New York Times, 21 February 1933).
On 11 June 1931, Yardley revealed in Chicago Daily News what is alleged to be a Japanese diplomatic message sent in 1921 containing instructions for propaganda for the coming Washington Naval Conference (Yardley file (1) p.50).
In the issue of 10 October 1931 of Liberty, Yardley further disclosed more recent Japanese telegrams: one dated 14 May 1927 from Tokyo to London and another dated 17 May 1927 from London to Tokyo (Yardley file (1) p.79-86). The Yardley files also include the original Japanese messages (Yardley file (2) p.1-7).
Yardley's activities could not escape attention of the Japanese embassy. On 2 June 1932, the Ambassador in the US sent two copies of Yardleygrams (Yardley file (2) p.51), which presented codebreaking puzzles in story form for the general public.
More alarming was a report from New York in the spring of 1933 that Yardley's manuscripts titled Japanese Diplomatic Secrets were seized by the authority (Yardley file (3) p.12). Written by a ghostwriter, it was a bore, interspersing the intercepted dispatches (hundreds of dispatches during the Washington Naval Conference) with scraps of dry, impersonal text (Kahn (2004) p.159).
The renewed attempt to publish intercepted telegrams might have deteriorated foreign relations for the US. Thus it led to an introduction in the US Congress a bill "For the Protection of Government Records" on 27 March 1933, though Yardley was never mentioned when it was first proposed (Kahn (2004) p.162, 164).
The Foreign Ministry of Japan followed these issues intently (Yardley file (3) p.15-29). They detected that the bill was motivated by Yardley's seized manuscript (Yardley file (3) p.23, 18, 20). They were alarmed in particular because there was information that the seized manuscripts contained Japanese telegrams related to the Mukden Incident in 1931 (Yardley file (3) p.18). The Embassy in the US contacted the publisher, Macmillan, through an agent and, relievingly, had an impression that Yardley's manuscript was a recompilation of old materials, not a product of new materials after the Mukden Incident (Yardley file (3) p.21).
In the US, the bill drew protests as infringing upon freedom of speech. The closed hearings in the Senate committee limited the bill to "code", thus effectively directly aiming at Yardley (Kahn (2004) p.165, Yardley file (3) p.25-27). When the bill was discussed in the Senate on 10 May, the advocates of the bill denied it was designed to punish a particular person but Yardley's name came up in the debate (Kahn (2004) p.165-166). One senator was convinced that "there are other motives behind this proposed legislation, and entirely unrelated to Mr. Yardley" by calling the bill a "gag law." (Kahn (2004) p.168-169) But the bill became law in June (Kahn (2004) p.169-171; US Code 952 of Title 18).
Upon the publication of The American Black Chamber, the efforts of the Foreign Ministry of Japan were directed to legal issues concerning privacy of communication rather than replacement of codes. Apparently, it was because the very code used during the Washington Naval Conference was no longer used and the code currently in use as of 1931 was a more complex one, albeit the same hybrid two-letter/four-letter system was employed (see another article).
The reason that the same system as the one broken by Yardley was still in use is explained in Sakuma's memorandum dated 9 October 1933 mentioned above (JACAR Ref. B12080889500).
It was recognized that any code could be broken and the codebreaking per se was no surprise. As long as the current code was not compromised, it was no serious concern.
Such an attitude may be contrasted with the reaction to other code security incidents. When there was an incident of telegram theft at the embassy in Turkey in June 1932, the RO Code was replaced by the HA Code, which had already been in use in the embassy in Russia. When it was found that the HA Code was known to Russia in 1933, it was discontinued for establishments in Russia and superencryption with a memorable key was introduced (see another article). At the latter occasion, detailed inquiries were made as to whether the leak was due to codebreaking or theft by burglary (JACAR Ref. B12080889300, esp. B12080889500).
Yardley's publication did not cause replacement of Japanese diplomatic code outright but it was mentioned when there was a renewed proposal from the Navy for use of a cipher machine, which had been tentatively used during the London Naval Conference in 1930. While it was recognized that conventional code with superencryption with a memorable key was not absolutely secure, a cipher machine was believed to provide the maximum security.
David Kahn (2004), The Reader of Gentlemen's Mail, Herbert O. Yardley and the Birth of American Codebreaking
David Kahn (1967), The Codebreakers
Japan Center for Asian Historical Records (JACAR)
Yardley-related materials are under Reference Codes B13080930400, B13080930500, and B13080930600 (referred to as Yardley file (1), (2), and (3) herein; the page numbers are those of digital images). The following lists some related documents.
Navy's proposal of introduction of cipher machine (9 July 1930) by Chief Ito Risaburo of the Telegraph Section of the Naval Ministry (B04122581000)
Memorandum shortly after the publication of The American Black Chamber (10 June 1931) by Chief Sakuma of the Telegraph Section of the Foreign Ministry (Yardley file (2) p.40)
Anticipated Questions and Proposed Answers of 28 July 1931 by Sakuma (Yardley file (1) p.1-13; Yardley file (2) p.51-66, Yardley file (3) p.40-53)
Proposal of Introduction of Cipher Machine of 6 August 1931 by Sakum (Yardley file (3) p.36-39)
Memorandum on a code security incident (9 October 1933) by Sakuma (B12080889500 p.20-36), printed in Shiraishi Masaaki (1998), "Iwayuru 'Kaibunsho Jiken' ni kansuru Sakuma Denshin Kacho Kiso Chosho ni tsuite", Gaikoshiryokanpo, June 1998, pp.97-104 （白石仁章「いわゆる"怪文書事件"に関する佐久間電信課長起草調書について」，『外交史料館報』，1998.6，pp.97-104）