Cipher Negotiation

Data channel cipher negotiation is complicated. This wiki defines the expected behaviour between OpenVPN servers and clients.

Table of Contents

1. OpenVPN Directives:
2. Expected Behaviour:
   1. Server version 2.5
      1. Client version 2.5
      2. Client version 2.4
      3. Client version 2.3
      4. Client version 2.2

OpenVPN Directives:

--data-cipher ALG:ALG - Data channel ciphers.
--data-cipher-fallback ALG:ALG* - Fallback data channel cipher(s) (*List?).
--cipher ALG - Data channel cipher. To be deprecated.

Expected Behaviour:

Server version 2.5

Client version 2.5

--cipher		--data-cipher		-fallback	NCP		Expected
Client	Server	Client	Server		Client	Server
-	-	-	AES-256-GCM:AES-128-GCM	-	YES	YES	OK
BF-CBC	-	-	AES-256-GCM:AES-128-GCM	-	YES	YES	OK

Client version 2.4

--cipher		--data-cipher		-fallback	NCP		Expected
Client	Server	Client	Server		Client	Server
-	-	-	AES-256-GCM:AES-128-GCM	-	YES	YES	OK
AES-256-CBC	-	-	AES-256-GCM:AES-128-GCM	-	YES	YES	OK
BF-CBC	-	-	AES-256-GCM:AES-128-GCM	-	YES	YES	OK

Client version 2.3

--cipher		--data-cipher		-fallback	NCP		Expected
Client	Server	Client	Server		Client	Server
-	-	-	AES-256-GCM:AES-128-GCM	-	NO	YES	FAIL
-	-	-	AES-256-GCM:AES-128-GCM	BF-CBC	NO	YES	OK
AES-256-CBC	-	-	AES-256-GCM:AES-128-GCM	-	NO	YES	FAIL
AES-256-CBC	-	-	AES-256-GCM:AES-128-GCM	AES-256-CBC	NO	YES	OK
BF-CBC	-	-	AES-256-GCM:AES-128-GCM	-	NO	YES	FAIL
BF-CBC	-	-	AES-256-GCM:AES-128-GCM	BF-CBC	NO	YES	OK

Client version 2.2

--cipher		--data-cipher		-fallback	NCP		Expected
Client	Server	Client	Server		Client	Server
-	-	-	AES-256-GCM:AES-128-GCM	-	NO	YES	FAIL
-	-	-	AES-256-GCM:AES-128-GCM	BF-CBC	NO	YES	OK
BF-CBC	-	-	AES-256-GCM:AES-128-GCM	-	NO	YES	FAIL
BF-CBC	-	-	AES-256-GCM:AES-128-GCM	BF-CBC	NO	YES	OK

And some fun ;-)

--cipher		--data-cipher		-fallback	NCP		Expected
Client	Server	Client	Server		Client	Server
-	-	-	AES-256-GCM:AES-128-GCM		YES	YES	OK
Even ..