Seitokai Yakuindomo - Ch. 583

[MangaDex]

 

[ReaperCreeper]

That solution is so crazy it might just work.

 

[Taper]

It's based on what's considered the first use of steganography — Herodotus tells of a wealthy man needing to send a secret message through enemy lines, so he shaved a slave's head and tattooed the message on his scalp, then waited for the hair to regrow before sending the slave out. The slave didn't know the message under questioning, so he was let through, and the recipient shaved the guy's head again to read the message, which was apparently not very time-sensitive.