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feat!: QR codes and symmetric encryption for broadcast channels (#7268)
Follow-up for https://github.com/chatmail/core/pull/7042, part of https://github.com/chatmail/core/issues/6884. This will make it possible to create invite-QR codes for broadcast channels, and make them symmetrically end-to-end encrypted. - [x] Go through all the changes in #7042, and check which ones I still need, and revert all other changes - [x] Use the classical Securejoin protocol, rather than the new 2-step protocol - [x] Make the Rust tests pass - [x] Make the Python tests pass - [x] Fix TODOs in the code - [x] Test it, and fix any bugs I find - [x] I found a bug when exporting all profiles at once fails sometimes, though this bug is unrelated to channels: https://github.com/chatmail/core/issues/7281 - [x] Do a self-review (i.e. read all changes, and check if I see some things that should be changed) - [x] Have this PR reviewed and merged - [ ] Open an issue for "TODO: There is a known bug in the securejoin protocol" - [ ] Create an issue that outlines how we can improve the Securejoin protocol in the future (I don't have the time to do this right now, but want to do it sometime in winter) - [ ] Write a guide for UIs how to adapt to the changes (see https://github.com/deltachat/deltachat-android/pull/3886) ## Backwards compatibility This is not very backwards compatible: - Trying to join a symmetrically-encrypted broadcast channel with an old device will fail - If you joined a symmetrically-encrypted broadcast channel with one device, and use an old core on the other device, then the other device will show a mostly empty chat (except for two device messages) - If you created a broadcast channel in the past, then you will get an error message when trying to send into the channel: > The up to now "experimental channels feature" is about to become an officially supported one. By that, privacy will be improved, it will become faster, and less traffic will be consumed. > > As we do not guarantee feature-stability for such experiments, this means, that you will need to create the channel again. > > Here is what to do: > • Create a new channel > • Tap on the channel name > • Tap on "QR Invite Code" > • Have all recipients scan the QR code, or send them the link > > If you have any questions, please send an email to delta@merlinux.eu or ask at https://support.delta.chat/. ## The symmetric encryption Symmetric encryption uses a shared secret. Currently, we use AES128 for encryption everywhere in Delta Chat, so, this is what I'm using for broadcast channels (though it wouldn't be hard to switch to AES256). The secret shared between all members of a broadcast channel has 258 bits of entropy (see `fn create_broadcast_shared_secret` in the code). Since the shared secrets have more entropy than the AES session keys, it's not necessary to have a hard-to-compute string2key algorithm, so, I'm using the string2key algorithm `salted`. This is fast enough that Delta Chat can just try out all known shared secrets. [^1] In order to prevent DOS attacks, Delta Chat will not attempt to decrypt with a string2key algorithm other than `salted` [^2]. ## The "Securejoin" protocol that adds members to the channel after they scanned a QR code This PR uses the classical securejoin protocol, the same that is also used for group and 1:1 invitations. The messages sent back and forth are called `vg-request`, `vg-auth-required`, `vg-request-with-auth`, and `vg-member-added`. I considered using the `vc-` prefix, because from a protocol-POV, the distinction between `vc-` and `vg-` isn't important (as @link2xt pointed out in an in-person discussion), but 1. it would be weird if groups used `vg-` while broadcasts and 1:1 chats used `vc-`, 2. we don't have a `vc-member-added` message yet, so, this would mean one more different kind of message 3. we anyways want to switch to a new securejoin protocol soon, which will be a backwards incompatible change with a transition phase. When we do this change, we can make everything `vc-`. [^1]: In a symmetrically encrypted message, it's not visible which secret was used to encrypt without trying out all secrets. If this does turn out to be too slow in the future, then we can remember which secret was used more recently, and and try the most recent secret first. If this is still too slow, then we can assign a short, non-unique (~2 characters) id to every shared secret, and send it in cleartext. The receiving Delta Chat will then only try out shared secrets with this id. Of course, this would leak a little bit of metadata in cleartext, so, I would like to avoid it. [^2]: A DOS attacker could send a message with a lot of encrypted session keys, all of which use a very hard-to-compute string2key algorithm. Delta Chat would then try to decrypt all of the encrypted session keys with all of the known shared secrets. In order to prevent this, as I said, Delta Chat will not attempt to decrypt with a string2key algorithm other than `salted` BREAKING CHANGE: A new QR type AskJoinBroadcast; cloning a broadcast channel is no longer possible; manually adding a member to a broadcast channel is no longer possible (only by having them scan a QR code)
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@@ -3,6 +3,7 @@ import logging
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import pytest
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from deltachat_rpc_client import Chat, EventType, SpecialContactId
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from deltachat_rpc_client.const import ChatType
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from deltachat_rpc_client.rpc import JsonRpcError
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@@ -109,6 +110,143 @@ def test_qr_securejoin(acfactory):
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fiona.wait_for_securejoin_joiner_success()
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@pytest.mark.parametrize("all_devices_online", [True, False])
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def test_qr_securejoin_broadcast(acfactory, all_devices_online):
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alice, bob, fiona = acfactory.get_online_accounts(3)
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alice2 = alice.clone()
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bob2 = bob.clone()
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if all_devices_online:
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alice2.start_io()
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bob2.start_io()
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logging.info("===================== Alice creates a broadcast =====================")
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alice_chat = alice.create_broadcast("Broadcast channel for everyone!")
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snapshot = alice_chat.get_basic_snapshot()
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assert not snapshot.is_unpromoted # Broadcast channels are never unpromoted
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logging.info("===================== Bob joins the broadcast =====================")
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qr_code = alice_chat.get_qr_code()
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bob.secure_join(qr_code)
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alice.wait_for_securejoin_inviter_success()
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bob.wait_for_securejoin_joiner_success()
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alice_chat.send_text("Hello everyone!")
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def get_broadcast(ac):
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chat = ac.get_chatlist(query="Broadcast channel for everyone!")[0]
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assert chat.get_basic_snapshot().name == "Broadcast channel for everyone!"
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return chat
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def wait_for_broadcast_messages(ac):
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chat = get_broadcast(ac)
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snapshot = ac.wait_for_incoming_msg().get_snapshot()
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assert snapshot.text == "You joined the channel."
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assert snapshot.chat_id == chat.id
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snapshot = ac.wait_for_incoming_msg().get_snapshot()
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assert snapshot.text == "Hello everyone!"
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assert snapshot.chat_id == chat.id
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def check_account(ac, contact, inviter_side, please_wait_info_msg=False):
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# Check that the chat partner is verified.
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contact_snapshot = contact.get_snapshot()
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assert contact_snapshot.is_verified
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chat = get_broadcast(ac)
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chat_msgs = chat.get_messages()
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if please_wait_info_msg:
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first_msg = chat_msgs.pop(0).get_snapshot()
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assert first_msg.text == "Establishing guaranteed end-to-end encryption, please wait…"
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assert first_msg.is_info
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encrypted_msg = chat_msgs[0].get_snapshot()
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assert encrypted_msg.text == "Messages are end-to-end encrypted."
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assert encrypted_msg.is_info
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member_added_msg = chat_msgs[1].get_snapshot()
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if inviter_side:
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assert member_added_msg.text == f"Member {contact_snapshot.display_name} added."
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else:
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assert member_added_msg.text == "You joined the channel."
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assert member_added_msg.is_info
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hello_msg = chat_msgs[2].get_snapshot()
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assert hello_msg.text == "Hello everyone!"
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assert not hello_msg.is_info
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assert hello_msg.show_padlock
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assert hello_msg.error is None
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assert len(chat_msgs) == 3
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chat_snapshot = chat.get_full_snapshot()
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assert chat_snapshot.is_encrypted
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assert chat_snapshot.name == "Broadcast channel for everyone!"
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if inviter_side:
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assert chat_snapshot.chat_type == ChatType.OUT_BROADCAST
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else:
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assert chat_snapshot.chat_type == ChatType.IN_BROADCAST
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assert chat_snapshot.can_send == inviter_side
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chat_contacts = chat_snapshot.contact_ids
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assert contact.id in chat_contacts
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if inviter_side:
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assert len(chat_contacts) == 1
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else:
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assert len(chat_contacts) == 2
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assert SpecialContactId.SELF in chat_contacts
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assert chat_snapshot.self_in_group
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wait_for_broadcast_messages(bob)
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check_account(alice, alice.create_contact(bob), inviter_side=True)
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check_account(bob, bob.create_contact(alice), inviter_side=False, please_wait_info_msg=True)
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logging.info("===================== Test Alice's second device =====================")
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# Start second Alice device, if it wasn't started already.
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alice2.start_io()
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while True:
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msg_id = alice2.wait_for_msgs_changed_event().msg_id
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if msg_id:
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snapshot = alice2.get_message_by_id(msg_id).get_snapshot()
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if snapshot.text == "Hello everyone!":
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break
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check_account(alice2, alice2.create_contact(bob), inviter_side=True)
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logging.info("===================== Test Bob's second device =====================")
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# Start second Bob device, if it wasn't started already.
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bob2.start_io()
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bob2.wait_for_securejoin_joiner_success()
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wait_for_broadcast_messages(bob2)
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check_account(bob2, bob2.create_contact(alice), inviter_side=False)
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# The QR code token is synced, so alice2 must be able to handle join requests.
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logging.info("===================== Fiona joins the group via alice2 =====================")
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alice.stop_io()
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fiona.secure_join(qr_code)
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alice2.wait_for_securejoin_inviter_success()
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fiona.wait_for_securejoin_joiner_success()
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snapshot = fiona.wait_for_incoming_msg().get_snapshot()
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assert snapshot.text == "You joined the channel."
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get_broadcast(alice2).get_messages()[2].resend()
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snapshot = fiona.wait_for_incoming_msg().get_snapshot()
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assert snapshot.text == "Hello everyone!"
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check_account(fiona, fiona.create_contact(alice), inviter_side=False, please_wait_info_msg=True)
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# For Bob, the channel must not have changed:
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check_account(bob, bob.create_contact(alice), inviter_side=False, please_wait_info_msg=True)
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def test_qr_securejoin_contact_request(acfactory) -> None:
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"""Alice invites Bob to a group when Bob's chat with Alice is in a contact request mode."""
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alice, bob = acfactory.get_online_accounts(2)
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