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The forum sharing client is a [BSP client](https://code.briarproject.org/briar/briar-spec/blob/master/protocols/BSP.md) that allows users to share forums with their contacts, who may accept or decline the invitations. It is used in conjunction with the [forum client](forum client).
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### Identifier
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The client's identifier is `org.briarproject.briar.forum.sharing`. The major version is 0.
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### Groups
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The client uses a separate BSP group for communicating with each contact. The [group descriptor](https://code.briarproject.org/briar/briar-spec/blob/master/protocols/BSP.md#23-groups) is a [BDF list](https://code.briarproject.org/briar/briar-spec/blob/master/BDF.md) containing the unique IDs of the contacts' identities, sorted in ascending order as byte strings.
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### Roles
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All communication happens between two contacts, who have symmetrical roles.
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### Sessions
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The messages exchanged between two contacts referring to a given forum constitute a session. The forum's unique ID is used as the session ID.
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### Message types
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The protocol uses five message types.
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**0: INVITE** - Sent by either party to start or restart a session. The message body is a BDF list with four elements: `messageType` (int), `previousMessageId` (unique ID or null), `descriptor` (list), and `text` (string or null).
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`previousMessageId` is the ID of the previous message in this session, if any. `descriptor` is the [descriptor of the forum](Forum-Client#group-identifiers) being shared. The forum ID must be calculated from the descriptor, as it is used by subsequent messages in the session. `text` is an optional message from the inviter to the invitee explaining the invitation.
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The sender sets the forum's visibility to VISIBLE when sending an invite message.
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**1: ACCEPT** - Sent in response to an invite. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
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`previousMessageId` is the ID of the previous message in this session.
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The sender sets the forum's visibility to SHARED when sending an accept message. The recipient sets the forum's visibility to SHARED when receiving an accept message.
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**2: DECLINE** - Sent in response to an invite. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
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`previousMessageId` is the ID of the previous message in this session.
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The recipient sets the forum's visibility to INVISIBLE when receiving a decline message.
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**3: LEAVE** - Sent by either party when unsubscribing from the forum, if the forum's visibility is VISIBLE or SHARED. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
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`previousMessageId` is the ID of the previous message in this session.
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The sender sets the forum's visibility to INVISIBLE when sending a leave message. The recipient sets the forum's visibility to INVISIBLE when receiving a leave message.
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**4: ABORT** - Sent by either party when recieving a message that is valid but unexpected in the current state. The message body is a BDF list with three elements: `messageType` (int), `forumId` (unique ID), and `previousMessageId` (unique ID).
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`previousMessageId` is the ID of the previous message in this session.
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The sender sets the forum's visibility to INVISIBLE when sending an abort message. The recipient sets the forum's visibility to INVISIBLE when receiving an abort message.
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### State machine
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![state-machine-4](/uploads/373807b51d3ba73d6f49a874b931ef0f/state-machine-4.png)
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[state-machine-4.odg](/uploads/4606c2f4b9d7f3c00482bdbf46fec250/state-machine-4.odg)
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Aborting from any state returns the session to the START state.
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### Validity policy
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* A message is valid if it is well-formed and its previous message (if any) is a valid message in the same session.
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### Storage policy
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* All messages are stored.
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### Sharing policy
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* All local messages are shared. |
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Moved to https://code.briarproject.org/briar/briar-spec/blob/master/clients/Forum-Sharing-Client.md |
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\ No newline at end of file |