We have setup a document library(DL) in SharePoint site hosed on-premise with no direct access from public zone/internet being an intranet solution. We have some external users who need to upload some files that needs to go in this document library. These files are quite confidential in nature so security is a prime requirement. Following are the couple of options I have in mind:
External users upload their file in their designated folder to secure FTP using their own credential set. And also there is a background process (kind of directory watcher) that keep looking at the all the folders and moves the files to document library
We host a single page application which has One time password (OTP) kind of feature. External user comes to the secure site to specify Vendor team name and mobile number. OTP is generated and user is asked to specify the password along with file they would like to upload. This page also maps the vendor name with respective folder in DL and moves the files to SharePoint there on the fly.
Pls. share your opinion about these approaches. Also welcome your thoughts if there is any other better option.