Aijirushi (相印) are identifying marks used to gain entry into a military encampment or as a mark of comradeship. According to the Shoninki shinobi manual, when two or more shinobi are working together, passwords or aijirushi (相印) identification marks should be utilized.
When scouting ahead, a shinobi should listen carefully to secure the enemy's password. Natori, the author, then goes on to list how people used to counteract this technique:
- Tachisuguri Isuguri (立ちすぐり居すぐり): This is the method of standing up or sitting down when a password is given to add security and to identify enemy spies
- Aijirushi (相印): Markings, emblems or signals
- Aikotoba (相言葉): The addition of supporting combination passwords
After the Ichigun Ichimi (one district one band) pact between the Iga and Koka provinces, a specific way of constructing a torch was agreed upon, this torch was used as an aijirushi for Iga and Koka shinobi to identify each other as allies.