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+# MSC3814: Dehydrated Devices with [SSSS]
+
+[MSC2697] introduces device
+dehydration -- a method for creating a device that can be stored in a user's
+account and receive [Megolm] sessions.  In this way, if a user has no other
+devices logged in, they can rehydrate the device on the next login and retrieve
+the [Megolm] sessions.
+
+However, the approach presented in that MSC has some downsides, making it
+tricky to implement in some clients, and presenting some UX difficulties.  For
+example, it requires that the device rehydration be done before any other API
+calls are made (in particular `/sync`), which may conflict with clients that
+currently assume that `/sync` can be called immediately after logging in.
+
+In addition, the user is required to enter a key or passphrase to create a
+dehydrated device.  In practice, this is usually the same as the [SSSS]
+key/passphrase, which means that the user loses the advantage of verifying
+their other devices via emoji or QR code: either they will still be required to
+enter their [SSSS] key/passphrase (or a separate one for device dehydration), or
+else that client will not be able to dehydrate a device.
+
+This proposal introduces another way to use the dehydrated device that solves
+these problems by storing the dehydration key in [SSSS], and by not changing the
+client's device ID.  Rather than changing its device ID when it rehydrates the
+device, it will keep its device ID and upload its own device keys. The client
+will separately rehydrate the device, fetch its to-device messages, and decrypt
+them to retrieve the Megolm sessions.
+
+## Proposal
+
+### Dehydrating a device
+
+The dehydration process is similar as in [MSC2697]. One important change is that
+the dehydrated device, the public device keys, and one-time keys are all
+uploaded in the same request. This change should prevent the creation of
+dehydrated devices which do not support end-to-end encryption.
+
+To upload a new dehydrated device, a client will use `PUT /dehydrated_device`.
+Each user has at most one dehydrated device; uploading a new dehydrated device
+will remove any previously-set dehydrated device.
+
+The dehydrated device *must* be cross-signed and have a fallback key.
+
+The client *must* use the public [Curve25519] [identity key] of the device,
+encoded as unpadded Base64, as the device ID.
+
+The `device_keys`, `one_time_keys`, and `fallback_keys` fields use the same
+structure as for the [`/keys/upload`] request.
+
+We add a new optional property to the device keys: `dehydrated`, which is set to
+`true` for dehydrated devices.  Defaults to `false`.  Clients that support
+dehydrated devices *must not* encrypt to devices marked as being a dehydrated
+device if they are not cross-signed.  Clients should also drop any to-device
+messages from a device marked as being a dehydrated device, since dehydrated
+device should not be sending messages.  Clients can use also this flag to for
+other purposes, such as:
+
+- Display dehydrated devices differently from normal devices, to avoid confusing
+  from users who do not expect to see another device.
+- Don't send key forwarding requests to the dehydrated device, since it will
+  not respond to them.
+- Don't send room keys to the dehydrated device if the user has a sufficient
+  number of other devices, with the assumption that if the user logs in to a
+  new device, they can get the room keys from one of their other devices and/or
+  from key backup.  This will reduce the chances that the dehydrated device
+  will run out of one-time keys, and reduce the number of events that the
+  dehydrated device will need to decrypt.
+
+`PUT /dehydrated_device`
+
+```jsonc
+{
+  "device_id": "dehydrated_device_id",
+  "device_data": {
+    "algorithm": "m.dehydration.v2"
+    "other_fields": "other_values"
+  },
+  "initial_device_display_name": "foo bar", // optional
+  "device_keys": {
+    "user_id": "<user_id>",
+    "device_id": "<device_id>",
+    "valid_until_ts": <millisecond_timestamp>,
+    "dehydrated": true,
+    "algorithms": [
+        "m.olm.curve25519-aes-sha2",
+    ]
+    "keys": {
+        "<algorithm>:<device_id>": "<key_base64>",
+    },
+    "signatures": {
+        "<user_id>": {
+            "<algorithm>:<device_id>": "<signature_base64>"
+        }
+    }
+  },
+  "fallback_keys": {
+    "<algorithm>:<device_id>": "<key_base64>",
+    "signed_<algorithm>:<device_id>": {
+      "fallback": true,
+      "key": "<key_base64>",
+      "signatures": {
+        "<user_id>": {
+          "<algorithm>:<device_id>": "<key_base64>"
+        }
+      }
+    }
+  },
+  "one_time_keys": {
+    "<algorithm>:<key_id>": "<key_base64>"
+  }
+}
+```
+
+Result:
+
+```json
+{
+  "device_id": "dehydrated device's ID"
+}
+```
+
+### Rehydrating a device
+
+To rehydrate a device, a client first calls `GET /dehydrated_device` to see if
+a dehydrated device is available.  If a device is available, the server will
+respond with the dehydrated device's device ID and the dehydrated device data.
+
+`GET /dehydrated_device`
+
+Response:
+
+```json
+{
+  "device_id": "dehydrated device's ID",
+  "device_data": {
+    "algorithm": "m.dehydration.v2",
+    "other_fields": "other_values"
+  }
+}
+```
+
+If no dehydrated device is available, the server responds with an error code of
+`M_NOT_FOUND`, HTTP code 404.
+
+If the client is able to decrypt the data and wants to use the dehydrated
+device, the client retrieves the to-device messages sent to the dehydrated
+device by calling `POST /dehydrated_device/{device_id}/events`, where
+`{device_id}` is the ID of the dehydrated device.  Since there may be many
+messages, the response can be sent in batches: the response must include a
+`next_batch` parameter, which can be used in a subsequent call to `POST
+/dehydrated_device/{device_id}/events` to obtain the next batch.
+
+```
+POST /dehydrated_device/{device_id}/events
+{
+  "next_batch": "token from previous call" // (optional)
+}
+```
+
+Response:
+
+```jsonc
+{
+  "events": [
+    // array of to-device messages, in the same format as in
+    // https://spec.matrix.org/unstable/client-server-api/#extensions-to-sync
+  ],
+  "next_batch": "token to obtain next events"
+}
+```
+
+Once a client calls `POST /dehydrated_device/{device_id}/events` with a
+`next_batch` token, unlike the `/sync` endpoint, the server should *not* delete
+any to-device messages delivered in previous batches. This should prevent the
+loss of messages in case the device performing the rehydration gets deleted. In
+the case the rehydration process gets aborted, another device will be able to
+restart the process.
+
+For the last batch of messages, the server will still send a
+`next_batch` token, and return an empty `events` array when called with that
+token, this signals to the client that it has received all to-device events and
+it can delete the dehydrated device and create a new one.
+
+If the given `device_id` is not the dehydrated device ID, the server responds
+with an error code of `M_FORBIDDEN`, HTTP code 403.
+
+### Deleting a dehydrated device
+
+A dehydrated device will get replaced whenever a new device gets uploaded using
+the `PUT /dehydrated_device`, this makes a `DELETE /dehydrated_device`
+unnecessary, though for completeness sake and to give client authors to get back
+to a state where no dehydrated device exists for a given user we will introduce
+one.
+
+`DELETE /dehydrated_device`
+
+Response:
+
+```json
+{
+  "device_id": "dehydrated device's ID"
+}
+```
+
+### Device Dehydration Format
+
+We define the following format for storing the dehydrated device (based on the
+libolm pickle format):
+
+We store the device as a concatenation of binary values.  Multi-byte numbers are
+stored in big-endian format.  The version is set to 0x80000000.  (Setting the
+high bit to 1 is to avoid confusion with the libolm pickle format for accounts,
+which was used in a previous version of this MSC.)
+
+```text
+   ┌───────────────────────────────────────────────────────────┐
+   │                        Pickle                             │
+   ├───────────────────────────────────────────────────────────┤
+   │Name                    │ Type          │ Size (bytes)     │
+   ├────────────────────────┼───────────────┼──────────────────┤
+   │Version                 │ u32           │ 4                │
+   │Curve25519 private key  │ [u8; 32]      │ 32               │
+   │Ed25519 private key     │ [u8; 32]      │ 32               │
+   │Number of one-time keys │ u32           │ 4                │
+   │One-time keys           │ [OneTimeKey]  │ N * 32           │
+   │Fallback key            │ OptFallback   │ 1 or 33          │
+   └────────────────────────┴───────────────┴──────────────────┘
+
+   ┌───────────────────────────────────────────────────────────┐
+   │                      OneTimeKey                           │
+   ├────────────────────────┬───────────────┬──────────────────┤
+   │Name                    │ Type          │ Size (bytes)     │
+   ├────────────────────────┼───────────────┼──────────────────┤
+   │Curve25519 private key  │ [u8; 32]      │ 32               │
+   └────────────────────────┴───────────────┴──────────────────┘
+
+   ┌───────────────────────────────────────────────────────────┐
+   │                      OptFallback                          │
+   ├────────────────────────┬───────────────┬──────────────────┤
+   │Name                    │ Type          │ Size (bytes)     │
+   ├────────────────────────┼───────────────┼──────────────────┤
+   │Fallback present        │ boolean       │ 1                │
+   │Fallback private key    │ [u8; 32]      │ 0 or 32          │
+   └────────────────────────┴───────────────┴──────────────────┘
+```
+
+The data is then encrypted and encoded as follows.
+
+#### Encryption key
+
+The encryption key used for the dehydrated device will be randomly generated
+and stored/shared via SSSS using the name `m.dehydrated_device`.
+
+A 96-bit (12-byte) nonce is randomly generated; each time a device is
+dehydrated, a new nonce must be generated.
+
+The plain-text is encrypted with ChaCha20-Poly1305 as defined in
+[RFC8439](https://datatracker.ietf.org/doc/html/rfc8439) using the encryption
+key and nonce.
+
+The ciphertext and nonce are then encoded as [unpadded
+Base64](https://spec.matrix.org/v1.12/appendices/#unpadded-base64) and inserted
+into the `device_pickle` and `nonce` properties, respectively, of the
+`device_data` JSON message.  The `algorithm` property is set to `m.dehydration.v2`.
+
+ ```json
+{
+  "device_data": {
+    "algorithm": "m.dehydration.v2",
+    "device_pickle": "encrypted dehydrated device"
+    "nonce": "random nonce"
+  }
+}
+```
+
+#### Test vectors
+
+TODO: put a test vector here
+
+## Potential issues
+
+The same issues as in
+[MSC2697](https://github.com/matrix-org/matrix-doc/pull/2697) are present for
+this proposal.  For completeness, they are repeated here:
+
+### One-time key exhaustion
+
+The dehydrated device may run out of one-time keys, since it is not backed by
+an active client that can replenish them.  Once a device has run out of
+one-time keys, no new olm sessions can be established with it, which means that
+devices that have not already shared megolm keys with the dehydrated device
+will not be able to share megolm keys.  This issue is not unique to dehydrated
+devices; this also occurs when devices are offline for an extended period of
+time.
+
+This may be addressed by using [fallback keys](https://spec.matrix.org/v1.9/client-server-api/#one-time-and-fallback-keys).
+
+To reduce the chances of one-time key exhaustion, if the user has an active
+client, it can periodically replace the dehydrated device with a new dehydrated
+device with new one-time keys.  If a client does this, then it runs the risk of
+losing any megolm keys that were sent to the dehydrated device, but the client
+would likely have received those megolm keys itself.
+
+Alternatively, the client could perform a `/sync` for the dehydrated device,
+dehydrate the olm sessions, and upload new one-time keys.  By doing this
+instead of overwriting the dehydrated device, the device can receive megolm
+keys from more devices.  However, this would require additional server-side
+changes above what this proposal provides, so this approach is not possible for
+the moment.
+
+### Accumulated to-device messages
+
+If a dehydrated device is not rehydrated for a long time, then it may
+accumulate many to-device messages from other clients sending it Megolm
+sessions.  This may result in a slower initial sync when the device eventually
+does get rehydrated, due to the number of messages that it will retrieve.
+Again, this can be addressed by periodically replacing the dehydrated device,
+or by performing a `/sync` for the dehydrated device and updating it.
+
+## Alternatives
+
+As mentioned above,
+[MSC2697](https://github.com/matrix-org/matrix-doc/pull/2697) tries to solve
+the same problem in a similar manner, but has several disadvantages that are
+fixed in this proposal.
+
+Since the device ID is used in URL path parameters, we could use URL-safe base64
+to derive the device ID.  However, this would result in the identity key being
+represented in two similar-but-different ways (URL-safe base64 in the device ID,
+and regular base64 in the device keys structure), which could lead to confusion.
+
+Rather than keep the name "dehydrated device", we could change the name to
+something like "shrivelled sessions", so that the full expansion of this MSC
+title would be "Shrivelled Sessions with Secure Secret Storage and Sharing", or
+SSSSSS.  However, despite the alliterative property, the term "shrivelled
+sessions" is less pleasant, and "dehydrated device" is already commonly used to
+refer to this feature.
+
+The alternatives discussed in MSC2697 are also alternatives here.
+
+
+## Security considerations
+
+### Weak SSSS passphrase/key
+
+A similar security consideration to the one in MSC2697 also applies to this
+proposal: if SSSS is encrypted using a weak passphrase or key, an attacker
+could access it and rehydrate the device to read the user's encrypted
+messages.
+
+### Display of dehydrated devices
+
+As mentioned earlier, clients may wish to display dehydrated devices differently
+from normal devices by checking the `dehydrated` flag in the device's keys.
+Clients must exercise care when doing so, as this may allow a attacker to hide a
+malicious device.  Clients *must not* encrypt messages to a dehydrated device
+that is not cross-signed.  Clients should indicate the presence of the
+dehydrated device, even if it is not listed along with the normal devices.  For
+example, a client could hide the dehydrated device from the device list, but
+indicate that "The dehydrated device feature is enabled".  A user can only have
+one dehydrated device available at a time, so if more than one device is marked
+as `dehydrated: true`, the client should display them all as normal devices.
+Clients can also display a warning in such a situation.
+
+## Unstable prefix
+
+While this MSC is in development, the `/dehydrated_device` endpoints will be
+reached at `/unstable/org.matrix.msc3814.v1/dehydrated_device`, and the
+`/dehydrated_device/{device_id}/events` endpoint will be reached at
+`/unstable/org.matrix.msc3814.v1/dehydrated_device/{device_id}/events`.  The
+dehydration algorithm `m.dehydration.v2` will be called
+`org.matrix.msc3814.v2`.  The SSSS name for the dehydration key will be
+`org.matrix.msc3814` instead of `m.dehydrated_device`.
+
+## Dependencies
+
+None
+
+[RFC5869]: https://datatracker.ietf.org/doc/html/rfc5869
+[AES-256]: http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf
+[CBC]: http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
+[PKCS#7]: https://tools.ietf.org/html/rfc2315
+[Curve25519]: http://cr.yp.to/ecdh.html
+[identity key]: https://gitlab.matrix.org/matrix-org/olm/-/blob/master/docs/olm.md#initial-setup
+[Megolm]: https://gitlab.matrix.org/matrix-org/olm/blob/master/docs/megolm.md
+[SSSS]: https://spec.matrix.org/v1.7/client-server-api/#storage
+[MSC2697]: https://github.com/matrix-org/matrix-doc/pull/2697
+[`/keys/upload`]: https://spec.matrix.org/v1.7/client-server-api/#post_matrixclientv3keysupload
+[device keys]: https://spec.matrix.org/v1.7/client-server-api/#device-keys
+[HMAC-SHA-256]: https://datatracker.ietf.org/doc/html/rfc2104