The cipher Shikamaru set up is a clumsy variation of an
autokey cipher based on the passkey konohagakure. Using it is simple but a pain in the ass quite time-consuming, based on the
tabula recta he gave everyone. It's easier to crack than a properly made autokey, but it'd still take multiple messages. A character familiar with cryptography will recognize the form of cipher and probably be able to tell that it was built by someone with no formal knowledge of the subject.
The simple tool is
here. Characters with the key: Shikamaru, Sai, Kakashi, Naruto
first use
gs swnhz
wrye reffsz skpklrxo if hhkk soxrxsjr jpar fbid xrsvf woikon
usxh tsa skdknvh
[decrypts into]
we fight
when nurses separate us take sedative vial from their pocket
dont get sedated