Where does it store the file with the secret key?

[vporton]

Where does it store the file with the secret key (on Linux)?

[Bilb]

Attachments are stored in ~/.config/Session/attachments.noindex.
The key used to encrypt/decrypt attachments is stored in the database itself which is encrypted too with your app password or a generated one if you don't have any set.

In the sqlite database, the attachment encrypting key is stored in the key local_attachment_encrypted_key of the table 'items'
This is a generated key, not user entered. Basically on start, if none are set we generate one (in data.ts generateAttachmentKeyIfEmpty()

That key is then used for encrypting attachments on save and decrypting them on read.
Is that all you need?