matrix-spec/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 (MSC-1219) can store the decryption key for the backups on the server, or cross-signing (MSC-1756) can store the signing keys. This proposal presents a standardized way of storing such data.

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, it must be encoded as a string.

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. Each key has an ID, and a discription of the key is stored in the user's account_data using the 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. The contents will be signed as signed JSON using the user's master cross-signing key. Other properties depend on the encryption algorithm, and are described below.

Encrypted data can be stored using the account_data API. The type for the account_data is 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.recovery_key, or the self-signing key for cross-signing could be stored under the type m.cross_signing.self_signing.

Data will be stored using using the following format:

{
    "encrypted": {
      [key ID]: {
        "ciphertext": "base64+encoded+encrypted+data",
        "mac": "base64+encoded+mac"
      }
    }
}

The encrypted property is map from key ID to an object. The algorithm 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.

FIXME: the key format was chosen so that existing backups could be easily migrated by just copying the configuration from the backup config to the key description. However, we need a way of signalling that the key for the backup is the same as the key for decrypting the other bits. Maybe a special flag in the account data? Or special case backups somehow, say, have clients inspect the backup's auth_data to see of the key config is the same?

Encryption algorithms

m.secret_storage.v1.curve25519-aes-sha2

The public key is stored in the pubkey property of the m.secret_storage.key.[key ID] account_data.

The data is encrypted and MACed as follows:

1. Generate an ephemeral curve25519 key, and perform an ECDH with the ephemeral key and the public key to generate a shared secret. The public half of the ephemeral key, encoded using base64, becomes the ephemeral property.
2. Using the shared secret, generate 80 bytes by performing an HKDF using SHA-256 as the hash, with a salt of 32 bytes of 0, and with the empty string as the info. The first 32 bytes are used as the AES key, the next 32 bytes are used as the MAC key, and the last 16 bytes are used as the AES initialization vector.
3. Encrypt the data using AES-CBC-256 with PKCS#7 padding. 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 first 8 bytes of the resulting MAC are base64-encoded, and become the mac property.

(The key HKDF, AES, and HMAC steps are the same as what are used for encryption in olm and megolm.)

FIXME: add an example of m.secret_storage.key.*, and of encrypted data.

Keys

When a user is given a raw key for m.secret_storage.v1.curve25519-aes-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",
        "rounds": 100000
    },
    ...
}

m.pbkdf2

The key is generated using PBKDF2 using the salt given in the salt parameter, and the number of rounds given in the rounds parameter.

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 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.share event, encrypted using olm. When the original client obtains the secret, it sends a m.secret.request event with action set to cancel_request to all devices other than the one that it received the secret from.

Clients SHOULD ensure that they only share secrets with other devices that are allowed to see them. For example, clients SHOULD only share secrets with 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 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", "cancel_request"].
• 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.share

Sent by a client to share a secret with another device, in response to an m.secret.request event. Typically it is 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 the /sync. 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

Yes.

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.