Hello Bjoern,
No, the FolderKey.bch file cannot be used to gain access to your encrypted files. This file serves three purposes:
1) Marking a folder as "encrypted"
A folder which contains a FolderKey.bch file is recognized as an encrypted folder by Boxcryptor and thus be marked "green" in the Boxcryptor drive. All files created in such an encrypted folder are automatically encrypted.
2) Permission to access a folder
In order to be allowed to open a folder in Boxcryptor, a user must be able to successfully decrypt the key stored in the FolderKey.bch file. If he cannot, Boxcryptor will deny access to this folder. (Note: This is not a strong access restriction but rather a convenience feature. Anybody with access to the source folder could easily open the folder outside of Boxcryptor. Strong access restriction is only available for files whose content is encrypted by Boxcryptor - folders itself (not their files) cannot be "encrypted" as they're just containers for files)
3) Permission inheritance
Most importantly, the FolderKey.bch of a folder is used as a permission "template" for new files and folders created within this folder. When you create a new file, Boxcryptor will apply the same permissions of its parent folder (instead of just you). So if users A, B and C have access to a folder, new files will also be accessible for the users A, B and C. This information needs to be stored somewhere and we're using the FolderKey.bch file for it.
Best regards,
Robert
PS: If the FolderKey.bch file is deleted, no damage is done. All information required to decrypt a file is directly stored in the encrypted file itself. When the FolderKey.bch file is missing, the parent folder will simply not be recognized as an encrypted folder anymore (but it's encrypted content will be).
