Incoming Box

A mechanism for Trusted Applications to write encrypted data for a given user into the Soledad Server, which will sync it to the client to be processed afterwards.

  • Version: 0.2.0
  • Date: 27 jun 2017
  • Authors: kali, drebs, vshyba


The Incoming Box is a new feature of Soledad, (from 0.10 forward), that aim at improving the mechanism by which external Trusted Applications can write data that will be delievered to a user’s database in the server side, from where it will be further processed by the Soledad Client. This processing includes decrypting the payload, since the incoming data is expected to be encrypted by the Trusted Application.

Design Goal

Use the particular story about MX delivery to guide the design of a general mechanism that makes sense in the context of Soledad as a generic Encrypted Database Solution. The final solution should still be able to be the backend for different types of applications, without introducing abstractions that are too tied to the encrypted email use case.


  1. Deliver data (expected to be encrypted with a public-key mechanism) to the Soledad Server, so that it is written into the data space for a particular user.
  2. Provide a mechanism by which Soledad Client, acting on behalf of an user, can download the data, process it in any needed way, and eventually store it again in the conventional main storage that Soledad provides. Since the data is expected to be encrypted by the delivery party, the client-side processing includes any decryption step if needed.
  3. Allow to purge a document that has been correctly processed
  4. Allow to mark a document that has failed to be decrypted, to avoid trying to decrypt it in every processing loop


The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.


  • Blob refers to an encrypted payload that is stored by soledad server as-is. It is assumed that the blob was end-to-end encrypted by an external component before reaching the server. (See the Blobs for more detail)
  • A BlobsBackend implementation is a particular backend setup by the Soledad Server that stores all the blobs that a given user owns. For now, only a filesystem backend is provided.
  • An Incoming Message makes reference to the representation of an abstract entity that matches exactly one message item, no matter how it is stored (ie, docs vs. blobs, single blob vs chunked, etc). It can represent one Email Message, one URL, an uploaded File, etc. For the purpose of the email use case, an Incoming Message refers to the encrypted message that MX has delivered to the incoming endpoint, which is pgp-encrypted, and can have been further obfuscated.
  • By Message Processing we understand the sequence of downloading an incoming message, decrypting it, transform it in any needed way, and deleting the original incoming message.
  • An Incoming Box is a subset of the Blobs stored for an user, together with the mechanisms that provide semantics to store, list, fetch and process incoming message data chronologically.


  • Soledad Server is the particular implementation that runs in the server (the twisted implementation is the only one to the moment). This exposes several endpoints (documents synchronization, blob storage, incoming message box).
  • BlobsBackend is a particular implementation of the IBlobsBackend interface, such as FilesystemBlobsBackend, that the server has been configured to use.
  • Soledad Client is the particular client that runs in different desktop replicas in the u1db sense (the twisted implementation is the only one to the moment). It executes a sync periodically, that syncs all metadata docs (soledad l2db json documents), and then downloads the blobs on demand. The blobs that need to be synced are both the encrypted blobs that are linked from metadata docs, and in this case the blobs stored in the space for a given Incoming Box.
  • BlobsManager is the component that orchestrates the uploads/downloads and storage in the client side. The client storage backend currently is SQLCipher, using the BLOB type.
  • A Trusted Application is any application that is authorized to write data into the user incoming box. Initially, LEAP’s Encrypting Remailer Proxy (MX, for short) is going to be the main trusted application that will drive the development of the Incoming Box.
  • Client Trusted Application Consumer is any implementation of Soledad’s client IIncomingBoxConsumer, which is the interface that the client counterpart of the Trusted Application needs to implement in order to receive messages. It provides implementation for processing and saving Incoming Messages. In the encrypted email case, this component is the Incoming Mail Service in Bitmask Mail.

The User Experience

  • From the end user perspective (ie, the user of Bitmask Mail in this case), the behaviour of the Incoming Box client in Soledad is completely transparent. Periodically, Soledad will call the Client Trusted Application Consumer with new “messages” of any particular type backed by the Soledad Client Storage, without any other intervention that introducing the master passphrase.
  • From the client API perspective (considering the user is a Client Trusted Application developer), all it needs to do is to implement IIncomingBoxConsumer interface and register this new consumer on Soledad API, telling the desired namespace it wants to consume messages from. Soledad will then take care of calling the consumer back for processing and saving of Incoming Messages.

Writing Data Into The Incoming Box

  • Any payload MUST arrive already encrypted to the endpoint of the Incoming Box. Soledad Server, at version 1 of this spec, will not add any encryption to the payloads.
  • The details of the encryption scheme used by the Trusted Application to encrypt the delivered payload (MX in this case) MUST be shared with the domain-specific application that processes the incoming message on the client side (Incoming Mail Service in Bitmask Mail, in this case). This means that the encryption schema MUST be communicated to the Incoming Box API in the moment of the delivery.
  • Incoming Boxes MUST NOT be writeable by any other user or any external applications.


  • The Trusted Application and the Soledad Server exposing the Incoming Box endpoint MUST share a secret, that is written into the configuration files of both services.
  • The Incoming Box MUST NOT be accessible as a public service from the outside.

Listing All Incoming Messages

  • Soledad server will list all the messages in the Incoming Box every time that a client requests it.
  • The server MUST return the number of pending messages.
  • The server SHOULD skip messages from the returned set beyond a given size limit, if the client requests it so.
  • The server MAY allow pagination.

Processing Incoming Messages

  • The Blobs containing the Incoming Messages need the capability to be marked as in one of the following states: PENDING, PROCESSING, PROCESSED, FAILED.
  • The default state for a new message in the Incoming Box is PENDING.
  • Before delivering a Message to a client for processing, the server MUST mark the blob that contains it as PROCESSING, reserving the message for this client so other replicas don’t try to repeat the processing.
  • The server MAY expire the PROCESSING flag if the defined PROCESSING_THRESHOLD is passed, to avoid data left unusable by stalled clients.
  • A message marked as PROCESSING MUST only be marked as PROCESSED by the server when it receives a confirmation by the replica that initiated the download request. This confirmation signals that the message is ready to be deleted.
  • A Client MUST request to the server to mark an incoming message as PROCESSED only when there are guarantees that the incoming message has been processed without errors, and the parts resulting of its processing are acknowleged to have been uploaded successfully to the central replica in the server.

Marking a Message as Failed

  • A Soledad Client MUST be able to mark a given message as FAILED. This covers the case in which a given message failed to be decrypted by a implementation-related reason (for instance: uncatched exceptions related to encoding, wrong format in serialization). The rationale is that we don’t want to increase overhead by retrying decryption on every syncing loop, but we don’t want to discard a particular payload. Future versions of the client might implement bugfixes or workarounds to try succeed in the processing.
  • Therefore, a Soledad Client SHOULD be able to add its own version when it marks a message as temporarily failed.
  • After some versions, a message SHOULD be able to be marked as permanently failed.
  • Reservation MUST be released, as any other replica MUST be able to pick the message to retry. This covers the case of a new replica being added with an updated version, which can be able to handle the failure.

Deleting Incoming Messagges

  • Any message in the Incoming Box marked as PROCESSED MAY be deleted by the server.
  • Any message in the Incoming Box marked as PERMANENTLY FAILED MAY be deleted by the server.

Implementation Details

Server Blob Backend

In the Server Side, the implementation of the Incoming Box MUST be done exclusively at the level of the BlobStorage. The Blobs implementation in both Soledad Server and Client have enough knowledge of the incoming box semantics to allow its processing to be done without resorting to writing documents in the main soledad json storage.

For simplicity, the IncomingBox endpoint is assumed to be running under the same process space than the rest of the Soledad Server.

Preffix Namespaces

The Trusted Application code responsible for delivering messages into Soledad Server Incoming Box endpoint MUST specify as a request parameter the dedicated namespace that it desires to use and be authorized to write into it. This is done so Soledad can list, filter, flag, fetch and reserve only messages known to the Trusted Application, avoiding to mix operations with blobs from the global namespace or messages from another Trusted Application.

LIST commands

The server MUST reply to several LIST commands, qualified by namespace and by other query parameters. Some of these commands are optional, but the server SHOULD reply to them signaling that they are not supported by the implementation.

The Server MUST return a list with the UIDs of the messages.

Supported listing features are: * Filter by namespace, which selects the namespace to do the list operation. * Filter by flag, which lists only messages/blobs marked with the provided flag. * Ordering, which changes the order of listing, such as older or newer first. * Count, which returns the total list size after filtering, instead of the list itself.

Client side Processing

It is assumed, for simplicity, that the Trusted Application consumer implementation shares the process memory space with the soledad client, but this doesn’t have to hold true in the future.

The class responsible for client side processing on Soledad Client is named IncomingBoxProcessingLoop. Its role is to orchestrate processing with the Incoming Box features on server side, so it can deliver messages to it’s registered Trusted Application Consumers.

  • To begin a processing round, the client starts by asking a list of the pending messages.
  • To avoid potentially costly traversals, the client limits the query to the most recent N blobs flagged as PENDING.
  • To avoid downloading bulky messages in the incoming queue (for example, messages with very big attachments), the client MAY limit the query on a first pass to all pending blobs smaller than X Kb.
  • After getting the list of Incoming Messages in the PENDING set, the client MUST start downloading the blobs according to the uuids returned.
  • Download SHOULD happen in chronological order, from the list. Download may happen in several modalities: concurrently, or sequentially.
  • The Soledad Client MUST provide a mechanism so that any clientside counterpart of the Trusted Application (ie: Bitmask Mail) can register a consumer for processing and saving each downloaded message.
  • In the reference implementation, since the consumers that the client registers are executed in the common event loop of the Soledad Client process, attention SHOULD be payed to the callbacks not blocking the main event loop.

Example of a Trusted Application Client Consumer:

class MyConsumer(object):
    def __init__(self): = 'My Consumer'

    def process(self, item, item_id, encrypted=True):
        cleartext = my_custom_decrypt(item) if encrypted else item
        processed_parts = my_custom_processing(item)
        return defer.succeed(processed_parts)

    def save(self, parts, item_id):
        return defer.gatherResults([ for part in parts])

Future Features

Still subject to discussion, but some features that are desired for future iterations are:

  • Provide a mechanism to retry documents marked as failed by previous revisions.
  • Internalizing public key infrastructure (using ECC).
  • ACLs to allow other users to push documents to an user Incoming Box.
  • Provide alternative implementations of the Incoming Box endopoint (for example, in Rust)

Writing Data When User Quota is Exceeeded

  • The server SHOULD move the payload to the permanent storage in the user storage space only after checking that the size of the storage currently occupied by the user data, plus the payload size does not exceed the allowed quota, if any, plus a given tolerance limit.
  • The Trusted Application SHOULD receive an error message as a response to its storage request, so that it can register the failure to store the data, or inform the sender in the case in which it is acting as a delegate to deliver a message.


In order to improve performance and responsiveness, a list request MAY be qualified by the following parameters that the server SHOULD satisfy. The responses are, in any case, a list of the uuids of the Blobs.

  • Pagination.
  • Include messages with PROCESSING flag.


  • SIZE_LIMIT: skips messages bigger than a given size limit, to avoid downloading payloads too big when client is interested in a quick list of incoming messages.


IncomingBox.list('mx', size_limit=10MB)