3

I was reading and tls-crypt and was curious would that increase security and reduce the chance of keys being compromised during handshakes and that it offers better security over tls-auth?

Maybe someone could better explain tls-auth and tls-crypt and how they improve security?

my current client file:

client
tls-client
dev tun
proto udp
remote 1.2.3.4 9999
<ca>
</ca>
<cert>
</cert>
<key>
</key>
pull
auth-nocache 
cipher AES-256-CBC
keysize 256
compress lz4-v2
reneg-sec 36000
keepalive 30 120
6

TLS Handshake can be more or less broken down into following steps:

  1. Clients sends "client hello" to server, along with client's random value and supported cipher suite.
  2. Server responds "server hello" to client, along with server's random value and chosen ciper suite.
  3. Server sends its certificate to client for authentication.
  4. Client verifies server identity.
  5. Client creates a random pre-master secret and encrypts it with the public key from the server's certificate.
  6. Client sends the encrypted pre-master secret to server.
  7. Server MAY request certificate from client if required.
  8. Both server and client generate the session key based on the pre-master secret.
  9. The server and client can now exchange encrypted messages using the session key.

The difference between tls-auth and tls-crypt is that starting from step 1, tls-crypt will encrypt all messages with a pre-shared key.

This provides several benefits:

  1. It hides the initialization of a TLS handshake with a OpenVPN server. This is helpful in some situations when OpenVPN protocol signature is detected and blocked.
  2. It prevents TLS denial of service attacks. With tls-auth the attacker can open thousands of TLS connections simultaneously but not provide a valid certificate, jamming the available ports. With tls-crypt the server would reject the connection up-front at step 1.
  3. Data is encrypted twice, once by tls-crypt and once by the TLS session.
| improve this answer | |
  • Is it safe to assume that the double encryption adds additional overhead to CPU on end-user devices like cell phones because everything has to be decrypted twice? – Jason Sep 5 '18 at 14:10
  • To be honest I can't say for sure, but encryption is only a single part of the entire communication process. So actual CPU usage will not really double and I doubt there will be a significant difference (Unfortunately I have no experimental data to support this, but it is easy to test for yourself, and I would love to know the result). Plus compared to assymetric encryption, symmetric encryption is orders of magnitude faster and less power hungry. – Victor Wong Sep 5 '18 at 14:45
  • 3
    TLS is only used to establish the tunnel; it's only a tiny amount of metadata about the tunnel that gets doubly-encrypted, the bulk data being sent through the tunnel wouldn't be affected. – dlitz Apr 9 '19 at 5:15

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