[Openvpn-devel,v6,2/7] tls-crypt-v2: add specification to doc/

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+Client-specific tls-crypt keys (--tls-crypt-v2)
+===============================================
+
+This document describes the ``--tls-crypt-v2`` option, which enables OpenVPN
+to use client-specific ``--tls-crypt`` keys.
+
+Rationale
+---------
+
+``--tls-auth`` and ``tls-crypt`` use a pre-shared group key, which is shared
+among all clients and servers in an OpenVPN deployment.  If any client or
+server is compromised, the attacker will have access to this shared key, and it
+will no longer provide any security.  To reduce the risk of losing pre-shared
+keys, ``tls-crypt-v2`` adds the ability to supply each client with a unique
+tls-crypt key.  This allows large organisations and VPN providers to profit
+from the same DoS and TLS stack protection that small deployments can already
+achieve using ``tls-auth`` or ``tls-crypt``.
+
+Also, for ``tls-crypt``, even if all these peers succeed in keeping the key
+secret, the key lifetime is limited to roughly 8000 years, divided by the
+number of clients (see the ``--tls-crypt`` section of the man page).  Using
+client-specific keys, we lift this lifetime requirement to roughly 8000 years
+for each client key (which "Should Be Enough For Everybody (tm)").
+
+
+Introduction
+------------
+
+``tls-crypt-v2`` uses an encrypted cookie mechanism to introduce
+client-specific tls-crypt keys without introducing a lot of server-side state.
+The client-specific key is encrypted using a server key.  The server key is the
+same for all servers in a group.  When a client connects, it first sends the
+encrypted key to the server, such that the server can decrypt the key and all
+messages can thereafter be encrypted using the client-specific key.
+
+A wrapped (encrypted and authenticated) client-specific key can also contain
+metadata.  The metadata is wrapped together with the key, and can be used to
+allow servers to identify clients and/or key validity.  This allows the server
+to abort the connection immediately after receiving the first packet, rather
+than performing an entire TLS handshake.  Aborting the connection this early
+greatly improves the DoS resilience and reduces attack service against
+malicious clients that have the ``tls-crypt`` or ``tls-auth`` key.  This is
+particularly relevant for large deployments (think lost key or disgruntled
+employee) and VPN providers (clients are not trusted).
+
+To allow for a smooth transition, ``tls-crypt-v2`` is designed such that a
+server can enable both ``tls-crypt-v2`` and either ``tls-crypt`` or
+``tls-auth``.  This is achieved by introducing a P_CONTROL_HARD_RESET_CLIENT_V3
+opcode, that indicates that the client wants to use ``tls-crypt-v2`` for the
+current connection.
+
+For an exact specification and more details, read the Implementation section.
+
+
+Implementation
+--------------
+
+When setting up a tls-crypt-v2 group (similar to generating a tls-crypt or
+tls-auth key previously):
+
+1. Generate a tls-crypt-v2 server key using OpenVPN's ``--genkey``.  This key
+   contains 2 512-bit keys, of which we use:
+
+   * the first 256 bits of key 1 as AES-256-CTR encryption key ``Ke``
+   * the first 256 bits of key 2 as HMAC-SHA-256 authentication key ``Ka``
+
+   This fomat is similar to the format for regular ``tls-crypt``/``tls-auth``
+   and data channel keys, which allows us to reuse code.
+
+2. Add the tls-crypt-v2 server key to all server configs
+   (``tls-crypt-v2 /path/to/server.key``)
+
+
+When provisioning a client, create a client-specific tls-crypt key:
+
+1. Generate 2048 bits client-specific key ``Kc``
+
+2. Optionally generate metadata
+
+   The first byte of the metadata determines the type.  The initial
+   implementation supports the following types:
+
+   0x00 (USER):         User-defined free-form data.
+   0x01 (TIMESTAMP):    64-bit network order unix timestamp of key generation.
+
+   The timestamp can be used to reject too-old tls-crypt-v2 client keys.
+
+   User metadata could for example contain the users certificate serial, such
+   that the incoming connection can be verified against a CRL.
+
+   If no metadata is supplied during key generation, openvpn defaults to the
+   TIMESTAMP metadata type.
+
+3. Create a wrapped client key ``WKc``, using the same nonce-misuse-resistant
+   SIV construction we use for tls-crypt:
+
+   ``len = len(WKc)`` (16 bit, network byte order)
+
+   ``T = HMAC-SHA256(Ka, len || Kc || metadata)``
+
+   ``IV = 128 most significant bits of T``
+
+   ``WKc = T || AES-256-CTR(Ke, IV, Kc || metadata) || len``
+
+   Note that the length of ``WKc`` can be computed before composing ``WKc``,
+   because the length of each component is known (and AES-256-CTR does not add
+   any padding).
+
+4. Create a tls-crypt-v2 client key: PEM-encode ``Kc || WKc`` and store in a
+   file, using the header ``-----BEGIN OpenVPN tls-crypt-v2 client key-----``
+   and the footer ``-----END OpenVPN tls-crypt-v2 client key-----``.  (The PEM
+   format is simple, and following PEM allows us to use the crypto lib function
+   for en/decoding.)
+
+5. Add the tls-crypt-v2 client key to the client config
+   (``tls-crypt-v2 /path/to/client-specific.key``)
+
+
+When setting up the openvpn connection:
+
+1. The client reads the tls-crypt-v2 key from its config, and:
+
+   1. loads ``Kc`` as its tls-crypt key,
+   2. stores ``WKc`` in memory for sending to the server.
+
+2. To start the connection, the client creates a P_CONTROL_HARD_RESET_CLIENT_V3
+   message, wraps it with tls-crypt using ``Kc`` as the key, and appends
+   ``WKc``.  (``WKc`` must not be encrypted, to prevent a chicken-and-egg
+   problem.)
+
+3. The server receives the P_CONTROL_HARD_RESET_CLIENT_V3 message, and
+
+   1. reads the WKc length field from the end of the message, and extracts WKc
+      from the message
+   2. unwraps ``WKc``
+   3. uses unwrapped ``Kc`` to verify the remaining
+      P_CONTROL_HARD_RESET_CLIENT_V3 message's (encryption and) authentication.
+
+   The message is dropped and no error response is sent when either 3.1, 3.2 or
+   3.3 fails (DoS protection).
+
+4. Server optionally checks metadata using a --tls-crypt-v2-verify script
+
+   This allows early abort of connection, *before* we expose any of the
+   notoriously dangerous TLS, X.509 and ASN.1 parsers and thereby reduces the
+   attack surface of the server.
+
+   The metadata is checked *after* the OpenVPN three-way handshake has
+   completed, to prevent DoS attacks.  (That is, once the client has proved to
+   the server that it possesses Kc, by authenticating a packet that contains the
+   session ID picked by the server.)
+
+   A server should not send back any error messages if metadata verification
+   fails, to reduce attack surface and maximize DoS resilience.
+
+6. Client and server use ``Kc`` for (un)wrapping any following control channel
+   messages.
+
+
+Considerations
+--------------
+
+To allow for a smooth transition, the server implementation allows
+``tls-crypt`` or ``tls-auth`` to be used simultaneously with ``tls-crypt-v2``.
+This specification does not allow simultaneously using ``tls-crypt-v2`` and
+connections without any control channel wrapping, because that would break DoS
+resilience.
+
+WKc includes a length field, so we leave the option for future extension of the
+P_CONTROL_HEAD_RESET_CLIENT_V3 message open.  (E.g. add payload to the reset to
+indicate low-level protocol features.)
+
+``tls-crypt-v2`` uses fixed crypto algorithms, because:
+
+ * The crypto is used before we can do any negotiation, so the algorithms have
+   to be predefined.
+ * The crypto primitives are chosen conservatively, making problems with these
+   primitives unlikely.
+ * Making anything configurable adds complexity, both in implementation and
+   usage.  We should not add any more complexity than is absolutely necessary.
+
+Potential ``tls-crypt-v2`` risks:
+
+ * Slightly more work on first connection (``WKc`` unwrap + hard reset unwrap)
+   than with ``tls-crypt`` (hard reset unwrap) or ``tls-auth`` (hard reset auth).
+ * Flexible metadata allow mistakes
+   (So we should make it easy to do it right.  Provide tooling to create client
+   keys based on cert serial + CA fingerprint, provide script that uses CRL (if
+   available) to drop revoked keys.)