Hi, On 08/12/17 20:07, 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> > --- > doc/tls-crypt-v2.txt | 164 > +++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 164 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..578b2f9 > --- /dev/null > +++ b/doc/tls-crypt-v2.txt > @@ -0,0 +1,164 @@ > +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 loosing > pre-shared
did you really mean "loosing"? or is it a misspelled "losing"? > +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 4 512-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`` > + > +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 > +3. Create a wrapped client key ``WKc``, using the same nonce-misuse-resistant > + SIV conruction we use for tls-crypt: > + > + ``T = HMAC-SHA256(Ka, Kc || metadata)`` > + > + ``IV = 128 most significant bits of T`` > + > + ``WKc = T || AES-256-CTR(Ke, IV, Kc || metadata)`` > + > +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``) For the server key you've said "using OpenVPN's ``--genkey``", while for the client key you haven't mentioned the ``--tls-crypt-v2-genkey`` introduced in patch 3/10. Any reason for that? > + > + > +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 without payload, 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 > + > + a. unwraps ``WKc`` and strips ``WKc`` from the message. > + b. uses unwrapped ``Kc`` to verify the remaining > + P_CONTROL_HARD_RESET_CLIENT_V3 message's authentication. > + > + The message is dropped and no error response is sent when either a or b > + fails (DoS protection). > + > +4. Server optionally checks metadata using a --tls-crypt-v2-verify script > + > + Metadata could for example contain the users certificate serial, such that > + the incoming connection can be verified against a CRL, or a notAfter > + timestamp that limits the key's validity period. > + > + 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.) just a thought here: the metadata is actually created by the server/provider and it is authenticated/encrypted with the server key. Isn't this enough to ensure that its content is not malicious and thus allow the server to parse it right after having received the HARD_RESET_V3 (instead of performing a "three-way handshake" first)? > + > + RFC: should the server send a 'key rejected' message if the key is e.g. > + revoked or expired? That allows for better client-side error reporting, > but > + also reduces the 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. RFC: should we add an option along the lines of > +--tls-crypt-v2-allow-insecure-fallback to allow admins to enable this anyway? > +It might help with transitioning. good question, but I think people are already fighting with this now while transitioning to tls-crypt, thus I would vote for not introducing more code complexity. > + > +``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 anymore 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.) > The rest looks good! Thanks for this exhaustive description! Cheers, -- Antonio Quartulli
signature.asc
Description: OpenPGP digital signature
------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________ Openvpn-devel mailing list Openvpn-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/openvpn-devel
