E2E: implement sharing

[tobiasKaminsky]

• sharing between user
• review PR: https://github.com/nextcloud/end_to_end_encryption_rfc/pull/40
• test first sharing
• outline migration together

@allexzander @marinofaggiana @tobiasKaminsky @artonge

[tobiasKaminsky]

First steps

• allow to share encrypted folder (only folder, not files) to user (no groups, no conversations, no federation)
  • postponed until decision of https://github.com/nextcloud/server/issues/36559
  • only on topmost folder, not possible for subfolders
• retrieve public key of sharee (postponed: https://github.com/nextcloud/server/issues/36559)
  • if none exists: do not share, show error message to user "not possible as sharee has no e2e setup"
  • if exists: store public key, use it, share folder
• allow sharee to access metadata file of specific folder (@artonge is this checked right now?)
• https://github.com/nextcloud/end_to_end_encryption_rfc/blob/b78aa9073b494588caf938d151b0bf5dd7103d2e/RFC.md#add-someone-to-an-end-to-end-encrypted-folder
• add recipient to recipients array with userId, certificate
• Add/replace the new metadata-key of every recipient in the recipients-list. The metadata-key is encrypted with the recipient's public key.
• The SHA-256 hash of the metadata-key is added to the keyChecksum array.
• last step: uploading encrypted file(s) using new created metadata key and verify it

[mpivchev]

Is this actually in progress? Nobody is assigned to this.

[marinofaggiana]

yes

[mpivchev]

Can we assign whoever is working on this? @tobiasKaminsky ?

[marinofaggiana]

look top, but since it is a job that touches so many of us, it is a little difficult to assign it.