Hi, On Mon, Sep 1, 2014 at 4:15 PM, Jan Just Keijser <janj...@nikhef.nl> wrote: > On 01/09/14 00:55, Jason Haar wrote: >> So am I correct in >> saying that an openvpn network using tls-auth plus client certs should >> be effectively immune to MiTM attacks, thereby making it OK to leave as >> much decision making as possible to the server? > the 'tls-cipher' negotiation is the first part of any OpenVPN > connection. It is used to initialize the contol channel authentication. > The control channel init phase can be strengthened with the 'tls-auth' > option, which causes openvpn to drop any packets that are not signed > with the right key.
Indeed. This means that tls-auth protects you against mitm attacks on the TLS handshake and the TLS implementation. It does not protect you when the TLS handshake can be broken passively (e.g. when the peers autonomously decide to use a weak cipher suite). > In my opinion the *clients* do not need to specify a stronger list of > tls-ciphers, but the server should. If the openvpn server only offers > strong ciphers for negotiating the control channel setup then any > clients that want to use weak ciphers are automatically excluded. I agree. Layered security is nice, but in this case it is probably not worth the management burden. Though if you are only in control of the client, you can still choose to ensure strong cipher suites if you really want to. But if you don't trust the server to choose a proper cipher, you probably should not trust it with your plaintext either... > If a weak tls-cipher is chosen then a MitM attack is theoretically > possible , or at least someone might be able to decipher the control > channel part too quickly. The data channel part is not affected by this. I don't agree here. The data channel keys are exchanged over the control channel. If the control channel is compromised, an attacker could obtain of influence the keys for the data channel. Hence, the control channel security is crucial to the data channel security. -Steffan ------------------------------------------------------------------------------ Slashdot TV. Video for Nerds. Stuff that matters. http://tv.slashdot.org/ _______________________________________________ Openvpn-users mailing list Openvpn-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/openvpn-users
