matrix-doc/proposals/1946-secure_server-side_storage.md

Secure Secret Storage and Sharing

Some features may require clients to store encrypted data on the server so that it can be shared securely between clients. Clients may also wish to securely send such data directly to each other. For example, key backups (MSC1219) can store the decryption key for the backups on the server, or cross-signing (MSC1756) can store the signing keys. This proposal presents a standardized way of storing such data.

Changes

• MSC2472 changed the encryption algorithm used from an asymmetric algorithm (Curve25519) to a symmetric algorithm (AES).

Proposal

Secrets are data that clients need to use and that are sent through or stored on the server, but should not be visible to server operators. Secrets are plain strings -- if clients need to use more complicated data, they must be encoded as a string, such as by encoding as JSON.

Storage

If secret data is stored on the server, it must be encrypted in order to prevent homeserver administrators from being able to read it. A user can have multiple keys used for encrypting data. This allows the user to selectively decrypt data on clients. For example, the user could have one key that can decrypt everything, and another key that can only decrypt their user-signing key for cross-signing.

Key descriptions and secret data are both stored in the user's account_data.

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

Example:

A key with ID abcdefg is stored in m.secret_storage.key.abcdefg

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

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.

Secret storage

Encrypted data is stored in the user's account_data using the event type defined by the feature that uses the data. For example, decryption keys for key backups could be stored under the type m.megolm_backup.v1, or the self-signing key for cross-signing could be stored under the type m.cross_signing.self_signing.

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.

Example:

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

org.example.some.secret:

{
  "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:

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

m.secret_storage.key.key_id_2:

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

Encryption algorithms

m.secret_storage.v1.aes-hmac-sha2

Secrets are encrypted using AES-CTR-256 and MACed using HMAC-SHA-256. The data is encrypted and MACed 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. This becomes the iv property, encoded using base64.
3. Encrypt the data using AES-CTR-256 using the AES key generated above. This encrypted data, encoded using base64, becomes the ciphertext property.
4. Pass the raw encrypted data (prior to base64 encoding) through HMAC-SHA-256 using the MAC key generated above. The resulting MAC is base64-encoded and becomes the mac property.

(We use AES-CTR to match file encryption and key exports.)

For the purposes of allowing clients to check whether a user has correctly entered the key, clients should:

1. encrypt and MAC a message consisting of 32 bytes of 0 as described above, using the empty string as the info parameter to the HKDF in step 1.
2. store the iv and mac in the m.secret_storage.key.[key ID] account-data.

For example, the m.secret_storage.key.key_id for a key using this algorithm could look like:

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

and data encrypted using this algorithm could look like this:

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

Keys

When a user is given a raw key for m.secret_storage.v1.aes-hmac-sha2, it will be encoded as follows (this is the same as what is proposed in MSC1703):

• prepend the two bytes 0x8b and 0x01 to the key
• compute a parity byte by XORing all bytes of the resulting string, and append the parity byte to the string
• base58-encode the resulting byte string with the alphabet '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'.
• format the resulting ASCII string into groups of 4 characters separated by spaces.

When decoding a raw key, the process should be reversed, with the exception that whitespace is insignificant in the user's ASCII input.

Passphrase

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:

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

m.pbkdf2

The key is generated using PBKDF2 using the salt given in the salt parameter, and the number of iterations given in the iterations parameter. The key size that is generated is given by the bits parameter, or 256 bits if no bits parameter is given.

Sharing

Rather than (or in addition to) storing secrets on the server encrypted by a shared key, devices can send secrets to each other, encrypted using olm.

To request a secret, a client sends a m.secret.request device event with action set to request to other devices, and name set to the name of the secret that it wishes to retrieve. A device that wishes to share the secret will reply with a m.secret.send event, encrypted using olm. When the original client obtains the secret, it sends a 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.

If a feature allows secrets to be stored or shared, then for consistency it SHOULD use the same name for both the account_data event type and the name in the m.secret.request.

Event definitions

m.secret.request

Sent by a client to request a secret from another device. It is sent as an unencrypted to-device event.

• name: (string) Required if action is request. The name of the secret that is being requested.
• action: (enum) Required. One of ["request", "request_cancellation"].
• requesting_device_id: (string) Required. ID of the device requesting the secret.
• request_id: (string) Required. A random string uniquely identifying the request for a secret. If the secret is requested multiple times, it should be reused. It should also reused in order to cancel a request.

m.secret.send

Sent by a client to share a secret with another device, in response to an m.secret.request event. It MUST be encrypted as an m.room.encrypted event, then sent as a to-device event.

• request_id: (string) Required. The ID of the request that this a response to.
• secret: (string) Required. The contents of the secret.

Tradeoffs

Currently, only a public/private key mechanism is defined. It may be useful to also define a secret key mechanism.

Potential issues

Keeping all the data and keys in account data means that it may clutter up /sync requests. However, clients can filter out the data that they are not interested in. One possibility for addressing this would be to add a flag to the account data to indicate whether it should come down the /sync or not.

Security considerations

By storing information encrypted on the server, this allows the server operator to read the information if they manage to get hold of the decryption keys. In particular, if the key is based on a passphrase and the passphrase can be guessed, then the secrets could be compromised. In order to help protect the secrets, clients should provide feedback to the user when their chosen passphrase is considered weak, and may also wish to prevent the user from reusing their login password.

Conclusion

This proposal presents a common way for bits of encrypted data to be stored on a user's homeserver for use by various features.