The secret writing system used by women in China is discussed at World of Nushu. This site is by Orie Endo, a sociolinguistic in Japan. Read about how she learned of 女書 in the 1990s and made trips to interview users of 女書. It is an endangered language mode. The existence of this language was kept a secret until 1983 when it was reported to the authorities in China; see the Wikipedia article for additional information.