Swapnote, aka Nintendo Letter Box in EUR, is an eShop application which allows you to send notes to friends via SpotPass/StreetPass.
Every note written and received besides the built-in notes are stored in Extdata. There's two extdata files per note, these are created when new notes are saved. Properly recovering cleartext via XOR for the *whole* note data is difficult, since these extdata files would already have the note data stored. The graphics data for these notes are stored via MPOs.
The ExtData File System for Swapnote is as follows:
root ├── icon ├── boss │ └── wTJ$.LlMB~!!+s2 └── user ├── letter.tmp ├── fruit.tbl ├── 000 │ └── XXX (where XXX is the result of sprintf(filename,"%03x",value); ) └── letterpaper └── XXX.apd (where XXX.apd is the result of sprintf(filename,"%03x.apd",value); )