Manifesto Printed in Secret for Uprising against the Shogunate

Righteous indignation against the corrupt shogunate made Oshio Heihachiro (Wikipedia), government official in Osaka himself, to organize an uprising in 1837. In the previous year, he had drafted a 2000-character manifesto and had it printed secretly. In order to prevent the woodcutters from reading the content, the woodcut was prepared in 32 divisions, so that each piece looked like nothing but a cipher. In the image below, divisions between woodcut blocks are clearly seen as horizontal spacing. (I learned of the episode in an epic manga by Minamoto Taro, Fuunji-tachi (Wikipedia, Wide edition, vol.14, p.212).) Although the prematurely started uprising was suppressed in a day, the manifesto of Oshio, who was also a scholar, was hand-copied and circulated widely.

References:
Here is an image of the manifesto preserved in Joshoji (成正寺), the temple also housing Oshio's tomb and a memorial, as well as the woodcut reproduced in 2006 (関西・大阪２１世紀協会「なにわ大坂をつくった100人　第20話 大塩平八郎（1793－1837年）」).
The copy shown in the photo on an auction site is in the form of a scroll and appears to be a different copy from the same woodcut (なにわ歴博ブログ　～等身大の学芸員生活～).
The image below is taken from the National Diet Library (image, catalogue, 大塩中斎先生天保救民告文).
The image at another auction site seems to be a contemporary handwritten copy (『委託HK◇大塩平八郎檄文 巻子装 天保八（1837）年（思想家 陽明学 大塩中斎 近世古文書 大坂 天保の大飢饉 書状 幕臣 大塩平八郎の乱）』).
Other copies (early printing? in different layouts) are found here (大塩平八郎檄文) and here (大塩平八郎檄文).

A transcription is found at 本山美彦「土佐堀川が育んだ改革者たちの言葉（6）」 (with modern translation and annotations) and also at 資料385　大塩平八郎の檄文.

Hidden Message from a Scholar Inprisoned by the Shogunate

A scholar of western science inprisoned by the shogunate regime sent out poems disgused as a blank sheet. I learned the episode in an epic manga by Minamoto Taro, Fuunji-tachi (Wikipedia, Wide edition, vol.18, p.42).
In the period when a small island in Nagasaki was the only place of contact with the western world for Japan, Takano Choei (高野長英, Wikipedia) studied there under a German physician, Franz von Siebold. In 1839, during the suppression of scholars of western studies in the aftermath of the Morrison Incident (Wikipedia), Takano was inprisoned for criticizing the shogunate's handling of the Incident. In June 1844, he fleed during a fire of the prison.
In January of that year, he had sent a letter to Mogi Kyoichiro (茂木恭一郎), in which a seemingly blank sheet was enclosed (called 獄中角筆詩文 or 獄中爪書). On the sheet are written five Chinese poems, of which the fifth is said to imply his intention of prison break (Choei Takano Memorial Hall (高野長英記念館); a catalogue entry).
Altough it is sometimes called 爪書 (nail writing), it was written with a stylus. The deciphering of the text expressed as indent on the seemingly blank sheet is described by Kobayashi Yoshinori in 小林芳規 (2014), "角筆のひらく文化史 : 見えない文字を読み解く" (岩波書店) (catalogue entry; sample pdf including the deciphered text).
The author is the leading authority in stylus writing. According to his interview (「角筆」の世界　文化功労者の小林芳規名誉教授に聞く), stylus writing (writing by indent) was widely used. The first instance was found in 1961. Many found thereafter were used to indicate reading of Chinese characters. More were found from all over Japan. He further found similar stylus writing in China and Korea. He says his preliminary search also found specimens in the manuscript bible from the 11th to 12th century.

Embedding Text in Text in Japanese Poetry

There was no systematic cryptography in Japan before she opened the country to the Western world in the nineteenth century. But some techniques in poetry are somewhat similar to concealment cipher.
Acrostics, called oriku in the field of Japanese poetry, are well-known (Wikipedia).

The other day, I learned of a more ingenious technique called 物名 (mononona or butsumei, "a thing's name") or 隠し題 (kakushi-dai, "a hidden theme") in an essay in a newspaper (The Asahi Shimbun, 22 December 2023).

One poem (tanka) is
茎も葉もみな緑なる深芹は
　洗ふ根のみや白く見ゆらむ
(Kuki mo ha mo, mina midori naru fukaseri ha,
arafu ne nomi ya shiroku miyu ramu.)
The latter part ("arafu ne nomi ya shiroku miyu ramu") means "is it only the washed root that looks white?" but a part of it can be parsed in a different way: "Arafune no miyashiro", which means something like "the sacred shrine of Arafune."

The tanka which hides the longest hidden text found by 小林祥次朗, "発掘　日本のことば遊び" (2002-10-28, JapanKnowledge) is:
時雨する川辺の紅葉出でてまづ
　橋の上の木濡れつつぞ見る
(Shigure suru kawabe no momiji ide te matsu
hashi no uhe no ki nure tsutsu zo miru.)
The text in bold is part of a phrase meaning "first look upon a tree while getting wet", but can be parsed "matsuhashi no uhe no kinu" (a kind of ancient court attire).