
### Secrets

{{% added-in v="1.1" %}}

Clients may have secret information that they wish to be made available
to other authorised clients, but that the server should not be able to
see, so the information must be encrypted as it passes through the
server. This can be done either asynchronously, by storing encrypted
data on the server for later retrieval, or synchronously, by sending
messages to each other.

Each secret has an identifier that is used by clients to refer to the
secret when storing, fetching, requesting, or sharing the secret.
Secrets are plain strings; structured data can be stored by encoding it
as a string.

The mechanism described in this section is known as "secure secret storage and
sharing", "SSSS", or "4S".

#### Storage

When secrets are stored on the server, they are stored in the user's
[account-data](#client-config), using an event type equal to the
secret's identifier. The keys that secrets are encrypted with are
described by data that is also stored in the user's account-data. Users
can have multiple keys, allowing them to control what sets of secrets
clients can access, depending on what keys are given to them.

##### Key storage

Each key has an ID, and the description of the key is stored in the
user's account data using the event type
`m.secret_storage.key.[key ID]`. The contents of the account data for
the key will include an `algorithm` property, which indicates the
encryption algorithm used, as well as a `name` property, which is a
human-readable name. Key descriptions may also have a `passphrase`
property for generating the key from a user-entered passphrase, as
described in [deriving keys from
passphrases](#deriving-keys-from-passphrases).

`KeyDescription`

| Parameter  | Type      | Description
|------------|-----------|-------------------------------------------------------------------------------------------------------------------------------------|
| name       | string    | Optional. The name of the key. If not given, the client may use a generic name such as "Unnamed key", or "Default key" if the key is marked as the default key (see below). |
| algorithm  | string    | **Required.** The encryption algorithm to be used for this key. Currently, only `m.secret_storage.v1.aes-hmac-sha2` is supported.   |
| passphrase | string    | See [deriving keys from passphrases](#deriving-keys-from-passphrases) section for a description of this property.                   |

Other properties depend on the encryption algorithm, and are described
below.

A key can be marked as the "default" key by setting the user's
account data with event type `m.secret_storage.default_key` to an
object that has the ID of the key as its `key` property. The default key
will be used to encrypt all secrets that the user would expect to be
available on all their clients. Unless the user specifies otherwise,
clients will try to use the default key to decrypt secrets.

Clients that want to present a simplified interface to users by not supporting
multiple keys should use the default key if one is specified. If no default
key is specified, the client may behave as if no key is present at
all. When such a client creates a key, it should mark that key as being the
default key.

`DefaultKey`

| Parameter  | Type      | Description
|------------|-----------|------------------------------------------|
| key        | string    | **Required.** The ID of the default key. |


###### `m.secret_storage.v1.aes-hmac-sha2`

For the purposes of allowing clients to check whether a user has correctly
entered the key, keys for use with the `m.secret_storage.v1.aes-hmac-sha2`
algorithm are stored with some additional data.

When storing a key, clients SHOULD:

1.  Given the secret storage key, generate 64 bytes by performing an
    HKDF with SHA-256 as the hash, a salt of 32 bytes of 0, and the empty
    string as the info. The first 32 bytes are used as the AES key,
    and the next 32 bytes are used as the MAC key.

2.  Generate 16 random bytes, set bit 63 to 0 (in order to work around
    differences in AES-CTR implementations), and use this as the AES
    initialization vector (IV).

3.  Encrypt a message consisting of 32 bytes of 0, using AES-CTR-256 using the
    AES key and IV generated above.

4.  Pass the raw encrypted data through HMAC-SHA-256 using the MAC key
    generated above.

5.  Encode the IV from step 2, and the MAC from step 4, using [unpadded
    base64](/appendices/#unpadded-base64), and store the results in the `iv`
    and `mac` properties respectively in the `m.secret_storage.key.[key ID]`
    account-data. (The ciphertext from step 3 is discarded after passing
    through the MAC calculation.)

This process can be repeated by a client checking if the key is correct: the
MAC should match if the key is correct. Note, however, that these properties
are **optional**. If they are not present, clients must assume that the key is
valid.

Note also, that although clients SHOULD use unpadded base64 as specified above,
some existing implementations use standard [RFC4648-compliant
base64](https://datatracker.ietf.org/doc/html/rfc4648#section-4) with padding,
so clients must accept either encoding.

The structure of a `m.secret_storage.key.[key ID]` account data object for use
with this algorithm is therefore as follows:

`AesHmacSha2KeyDescription`

| Parameter   | Type   | Description                                                                                          |
|-------------|--------|------------------------------------------------------------------------------------------------------|
| name        | string | Optional. The name of the key.                                                                       |
| algorithm   | string | **Required.** The encryption algorithm to be used for this key: `m.secret_storage.v1.aes-hmac-sha2`. |
| passphrase  | object | See [deriving keys from passphrases](#deriving-keys-from-passphrases) section for a description of this property. |
| iv          | string | Optional. The 16-byte initialization vector for the validation check, encoded as base64.             |
| mac         | string | Optional. The MAC of the result of encrypting 32 bytes of 0, encoded as base64.                      |

For example, it could look like:

```json
{
  "name": "m.default",
  "algorithm": "m.secret_storage.v1.aes-hmac-sha2",
  "iv": "random+data",
  "mac": "mac+of+encrypted+zeros"
}
```

##### Secret storage

Encrypted data is stored in the user's account data using the event
type defined by the feature that uses the data. The account data will
have an `encrypted` property that is a map from key ID to an object. The
algorithm from the `m.secret_storage.key.[key ID]` data for the given
key defines how the other properties are interpreted, though it's
expected that most encryption schemes would have `ciphertext` and `mac`
properties, where the `ciphertext` property is the unpadded
base64-encoded ciphertext, and the `mac` is used to ensure the integrity
of the data.

`Secret`

| Parameter | Type             | Description |
|-----------|------------------|-------------|
| encrypted | {string: object} | **Required.** Map from key ID the encrypted data. The exact format for the encrypted data is dependent on the key algorithm. See the definition of `AesHmacSha2EncryptedData` in the [m.secret_storage.v1.aes-hmac-sha2](#msecret_storagev1aes-hmac-sha2-1) section. |

Example:

Some secret is encrypted using keys with ID `key_id_1` and `key_id_2`:

`org.example.some.secret`:

```nohighlight
{
  "encrypted": {
    "key_id_1": {
      "ciphertext": "base64+encoded+encrypted+data",
      "mac": "base64+encoded+mac",
      // ... other properties according to algorithm property in
      // m.secret_storage.key.key_id_1
    },
    "key_id_2": {
      // ...
    }
  }
}
```

and the key descriptions for the keys would be:

`m.secret_storage.key.key_id_1`:

```nohighlight
{
  "name": "Some key",
  "algorithm": "m.secret_storage.v1.aes-hmac-sha2",
  // ... other properties according to algorithm
}
```

`m.secret_storage.key.key_id_2`:

```nohighlight
{
  "name": "Some other key",
  "algorithm": "m.secret_storage.v1.aes-hmac-sha2",
  // ... other properties according to algorithm
}
```

If `key_id_1` is the default key, then we also have:

`m.secret_storage.default_key`:

```nohighlight
{
  "key": "key_id_1"
}
```

###### `m.secret_storage.v1.aes-hmac-sha2`

Secrets encrypted using the `m.secret_storage.v1.aes-hmac-sha2`
algorithm are encrypted using AES-CTR-256, and authenticated using
HMAC-SHA-256. The secret is encrypted as follows:

1.  Given the secret storage key, generate 64 bytes by performing an
    HKDF with SHA-256 as the hash, a salt of 32 bytes of 0, and with the
    secret name as the info. The first 32 bytes are used as the AES key,
    and the next 32 bytes are used as the MAC key.

2.  Generate 16 random bytes, set bit 63 to 0 (in order to work around
    differences in AES-CTR implementations), and use this as the AES
    initialization vector (IV).

3.  Encrypt the data using AES-CTR-256 using the AES key and IV generated
    above.

4.  Pass the raw encrypted data through HMAC-SHA-256 using the MAC key
    generated above.

5.  Encode the IV from step 2, the ciphertext from step 3, and MAC from step 4,
    using [unpadded base64](/appendices/#unpadded-base64), and store them as
    the `iv`, `ciphertext`, and `mac` properties respectively in the account
    data object.

    **Note**: some existing implementations encode these properties using
    standard [RFC4648-compliant
    base64](https://datatracker.ietf.org/doc/html/rfc4648#section-4) with
    padding, so clients must accept either encoding.

The structure of the `encrypted` property of an account data object encrypted
with this algorithm is therefore as follows:

`AesHmacSha2EncryptedData`

| Parameter  | Type    | Description
|------------|---------|------------------------------------------------------------------------|
| iv         | string  |  **Required.** The 16-byte initialization vector, encoded as base64.  |
| ciphertext | string  |  **Required.** The AES-CTR-encrypted data, encoded as base64.          |
| mac        | string  |  **Required.** The MAC, encoded as base64.                             |


For example, data encrypted using this algorithm could look like this:

```json
{
  "encrypted": {
      "key_id": {
        "iv": "16+bytes+base64",
        "ciphertext": "base64+encoded+encrypted+data",
        "mac": "base64+encoded+mac"
      }
  }
}
```

##### Key representation

When a user is given a raw key for `m.secret_storage.v1.aes-hmac-sha2`,
the key should be presented as a string using the common [cryptographic key
representation](/appendices/#cryptographic-key-representation).

##### Deriving keys from passphrases

A user may wish to use a chosen passphrase rather than a randomly
generated key. In this case, information on how to generate the key from
a passphrase will be stored in the `passphrase` property of the
`m.secret_storage.key.[key ID]` account-data. The `passphrase` property
has an `algorithm` property that indicates how to generate the key from
the passphrase. Other properties of the `passphrase` property are
defined by the `algorithm` specified.

Currently, the only algorithm defined is `m.pbkdf2`. For the `m.pbkdf2` algorithm, the `passphrase` property has the
following properties:

| Parameter  | Type    | Description                                                            |
|------------|---------|------------------------------------------------------------------------|
| algorithm  | string  | **Required.** Must be `m.pbkdf2`                                       |
| salt       | string  | **Required.** The salt used in PBKDF2.                                 |
| iterations | integer | **Required.** The number of iterations to use in PBKDF2.               |
| bits       | integer | Optional. The number of bits to generate for the key. Defaults to 256. |

The key is generated using PBKDF2 with SHA-512 as the hash, using the
salt given in the `salt` parameter, and the number of iterations given
in the `iterations` parameter.

Example:

```nohighlight
{
    "passphrase": {
        "algorithm": "m.pbkdf2",
        "salt": "MmMsAlty",
        "iterations": 100000,
        "bits": 256
    },
    ...
}
```

#### Sharing

To request a secret from other devices, a client sends an
`m.secret.request` device event with `action` set to `request` and
`name` set to the identifier of the secret. A device that wishes to
share the secret will reply with an `m.secret.send` event, encrypted
using olm. When the original client obtains the secret, it sends an
`m.secret.request` event with `action` set to `request_cancellation` to
all devices other than the one that it received the secret from. Clients
should ignore `m.secret.send` events received from devices that it did
not send an `m.secret.request` event to.

Clients must ensure that they only share secrets with other devices that
are allowed to see them. For example, clients should only share secrets
with the user’s own devices that are verified and may prompt the user to
confirm sharing the secret.

##### Event definitions

{{% event event="m.secret.request" %}}

{{% event event="m.secret.send" %}}
