The Nissho Maru Incident (日章丸事件) (Wikipedia) was a 1953 incident in which the Japanese oil tanker Nissho Maru broke an embargo on Iranian oil posed by Britain. The mission required utmost secrecy, and coded messages played a role.
When in 1951 Iran nationalized the Anglo-Iranian Oil Company (AIOC), which had been drawing off most of the profits from oil business in Iran, Britain imposed economic sanctions on Iran and, in July 1952, the Royal Navy seized the Italian tanker Rose Mary, claiming that its oil was stolen property. However, seeing that the nationalization was being accepted internationally (『ペルシャ湾上の日章丸』p.53-54, 61, 148-149, etc.), the Japanese petroleum firm Idemitsu Kosan acted quickly to purchase the Iranian oil before other companies in the world still dared not challenge the British blockade.
The Nissho Maru set sail on 23 March 1953, purportedly for Saudi Arabia. When cruising in the Indian Ocean, a radio message from the home office revealed the real purpose of the voyage was to receive petroleum at Abadan, Iran. The sealed packet entrusted with the captain contained a message to the crew from the president of the firm, declaring his resolution to have a direct access to the oil resources without interference from the monopolozing major oil companies (ibid. p.152-155). After this, the Nissho Maru kept radio silence until 30 April, when it came back as far as the East China Sea. The Nissho Maru received a hero's welcome when she safely arrived at Kawasaki, Japan, on 9 May 1953 (ibid. p.172). No less welcome awaited her on her second, no longer secret, visit to Abadan in June (ibid. p.211-217).
Where does code come in?
The crucial message revealing the real destiation as Abadan is said to have been a coded message (NIKKEIリスキリング). The conclusion of a contract with Iran had been reported in code (『ペルシャ湾上の日章丸』p.138). When the Nissho Maru was returning to Japan, the home office sent a coded message to tell her to lower speed so as to arrive at noon on Saturday, 9 May 1953, because then even if AIOC requested seizure of cargo, a provisional disposition of the court would not be issued at least until Monday.
A businessman, Takeshi Hotoku (宝徳健), notes on his blog that it was his mother who actually operated sending of the coded messages. It appears the coded messages are preserved in Idemitsu Kosan.
I have not found detailed description of the code.
The reporting of the contract may have been sent by telegram. Telegrams were commonly sent by using a codebook. It was also common for businesses to use their proprietary codebooks. Radio messages to/from ships at sea were commonly sent by Morse code until about 1978 (Idemitsu Tanker). The radio messages to/from the Nissho Maru could have been encrypted with some special codebook or cipher.
I first learned of this episode in relation to a best selling novel, A Man Called Pirate (海賊と呼ばれた男) by Naoki Hyakuta (百田尚樹) (Wikipedia). Although his writing tends to have many historical inaccuracies, his description about coded messages in the novel may be of some interest. (The names in the novel are pseudonyms.)
『海賊とよばれた男』下 より
p.130
「翌日は帰国予定だったが，数日ずらすことにし，本社にその旨を電報で送った．今回，正明らと本社のやりとりは，通信の秘匿のためにすべて暗号を使用していた．暗号文を知らされているのは限られた者だけだった．この暗号を作ったのは武知と元ラジオ部の部長である藤本壮平である．旧中野学校の教官であった武知にとって暗号はお手のものだった．かつてラジオ修理を持ち込んで国岡商店に入った藤本も元海軍大佐である．二人は専門家でなくても使える便利な暗号を作った．」
イラン行きの文脈．
p.172
四月五日正午，日章丸がセイロンの南，コロンボ沖にさしかかったとき，国岡本社から無電が入った．日本を出て十三日目のことだった．
「SAKUR NXRQT LPRDX BFNOW TXKPJ」
通信長は意味のわからない無電に首を傾げながら，船長の新田に電文を持ってきた．
新田は「とうとう来たな」と思った．これは事前に打ち合わせておいた暗号電だった．彼はすぐに手帳の暗号解読表を見て電文を読み解いた．
そこにはこう書かれていた．
「アバダンへ行け」
p.205
日章丸は川崎に到着すると聞かされていた徳山の従業員たちは驚いた．慌てて，九州からも助っ人を要請してタンクの清掃に取り掛かった．
にわかに徳山が日本中の注目を浴びた．報道記者たちも続々と徳山に集まった．
しかしこれは鐡造の陽動作戦だった．彼は前もって，日章丸に向けて，暗号電文で次の指令を送っていたのだ．
「川崎において差し押さえの懸念あるにつき，九日午後より揚荷開始の予定．九日正午，検疫錨地に入港するよう適宜考慮して航海せよ」
初めから鐡造は九日に日章丸を川崎港に入港させるつもりだった．九日にこだわったのは，その日が土曜日だったからだ．
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