Checksums on files / reliable up/download

To have reliable up- and downloads, generating a checksum on server is needed.

Upload

• client generates checksum
• client uploads file with checksum
• server checks checksum and stores it after successfull upload

Download

• client retrieves file and checksum
• client checks checksum

On backward compatibility:

Upload:

• if no checksum is provided, e.g. legacy apps / 3rd party apps -> generate checksum on server

Download:

• legacy clients can simply ignore checksum

Additional for Android Files we can check if files are already uploaded (despite on relying on file names).

(this was shortly discussed in NC15 planning meeting)

Additionally, the ability to request checksum of file via endpoint from https://github.com/nextcloud/server/issues/25949:

• client detects that file with same name already exists
• we do not know if it is the same file
• client requests only for this file via new(?) endpoint a checksum
• client compares it
• only if this differs, it presents conflict resolution dialog

Start by reviewing the upload and download flows described in the issue, along with the proposed endpoint for retrieving a file checksum. The issue requires a design covering checksum generation, validation, storage, retrieval, backward compatibility, and Android file checks; done means these paths reliably detect transfers or conflicts without breaking legacy clients.