Hangul is an artificial alphabet created in the fifteenth century as a script for Korean. It is interesting because graphic units for consonants and vowels are combined horizontally or vertically to form syllable symbols. Because of this system, I think substitution cipher is not possible in Hangul script.
Once characters are encoded into digits or roman letters, encryption methods including substitution and transposition are applicable. Today's computer can of course handle Hangul characters. But in the early years of telegraphy, telegrams in Korea had to be in Chinese characters or Latin letters.
So, a telegraph codebook for Chinese characters were used in Korea.
I have seen a Korean version (『漢電』) of a Chinese telegraph codebook.
Even after WWII, it appears a telegraph codebook with similar content was used, in view of an edition adapted for use by those who could not read Chinese characters (Korean Telegraphic Code Book, with characters arranged by sounds in English alphabetic order according to the McCune-Reischauer system of transliteration).
These are already covered in 電碼――中国の文字コード, dealing with Chinese telegraph codes, in which I now made small corrections.
(By the way, I wrote the above ten days ago, but I couldn't upload it because my smartphone failed and I couldn't pass the two-factor authentication for logging into the blog.)
