Hi, one little typ0 and one addition below:
On 22/10/18 19:45, Steffan Karger wrote: > This is a preliminary description of tls-crypt-v2. It should give a good > impression about the reasoning and design behind tls-crypt-v2, but might > need some polishing and updating. > > Signed-off-by: Steffan Karger <steffan.kar...@fox-it.com> > --- > v3: Include length in WKc > v4: Clarify metadata handling > v5: Typo fixes (thanks tincanteksup) > v6: Change WKc length to ciphertext length (was plaintext), textual fixes > v7: Textual fixes irt review > > doc/tls-crypt-v2.txt | 189 > +++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 189 insertions(+) > create mode 100644 doc/tls-crypt-v2.txt > > diff --git a/doc/tls-crypt-v2.txt b/doc/tls-crypt-v2.txt > new file mode 100644 > index 0000000..582a2b8 > --- /dev/null > +++ b/doc/tls-crypt-v2.txt > @@ -0,0 +1,189 @@ > +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 typ0: ``--genkey`` should be ``--tls-crypt-v2-genkey server`` > + 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`` I would add something like the paragraph about the server key: ... using OpenVPN's ``--tls-crypt-v2-genkey client`` > + > +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.) > I hope they can be fixed on the fly. Other than that: Acked-by: Antonio Quartulli <anto...@openvpn.net> -- Antonio Quartulli
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