The Mexican Army Cipher Disk was used during the time of conflict between Mexico and the USA shortly before World War I.
You can turn the disks individually (or not). You can lock screen orientation and turn on vibrations.
You can cipher and decipher a message and display the frequency analysis for each disk.
You can break a message searching some keywords.

Note for teachers : in order that your students familiarize themselves with the disk and, initially, only use it (on a tablet available at their disposal, for example), you can first disable the other activities by setting a password from the main activity. Then, you can enable the other activities later.