Hi Sebastian,

On 01/12/16 16:09, Sebastian Rubenstein wrote:
> Hi,
>
> The contents of the following config file is provided by my VPN provider. I 
> have redacted it to remove confidential information:
>
> client
> dev tun
> proto tcp
> remote 1.2.3.4 443
> cipher BF-CBC
> tun-ipv6
> redirect-gateway ipv6
> resolv-retry infinite
> nobind
> persist-key
> persist-tun
> comp-lzo
> verb 3
> remote-cert-tls server
> ping-restart 60
> script-security 2
> up /etc/openvpn/update-resolv-conf
> down /etc/openvpn/update-resolv-conf
> ping 10
>
> <ca>
> Large chunks of alphanumeric text
> </ca>
>
> <cert>
> Large chunks of alphanumeric text
> </cert>
>
> <key>
> Large chunks of alphanumeric text
> </key>
>
> # Limit range of possible TLS cipher-suites
> tls-cipher 
> TLS-DHE-RSA-WITH-AES-256-CBC-SHA:TLS-DHE-RSA-WITH-CAMELLIA-256-CBC-SHA:TLS-DHE-RSA-WITH-3DES-EDE-CBC-SHA:TLS-DHE-RSA-WITH-AES-128-CBC-SHA:TLS-DHE-RSA-WITH-SEED-CBC-SHA:TLS-DHE-RSA-WITH-CAMELLIA-128-CBC-SHA
>
>
> I use OpenVPN 2.3.13 on Ubuntu in a terminal to connect to the VPN server. 
> During the process of connecting, a warning appeared:
>
>
> WARNING: INSECURE cipher with block size less than 128 bit (64 bit).  This 
> allows attacks like SWEET32.  Mitigate by using a --cipher with a larger 
> block size (e.g. AES-256-CBC).
>
>
> When queried by me, the technical support staff of my VPN provider answered 
> as follows:
>
>
> We are aware of the SWEET32 attack, however we do mitigate it by setting the 
> reneg to 64MB, which means after each 64MB of data or the minimum time for a 
> key renegotiation, it will renegotiate the keys.
>
> You can read about it here: "https://sweet32.info";
>
> We have plans to add a AES TCP port, however I can't say how long time that 
> will take, currently we are in the process of adding another UDP AES port.
>
>
> Guys, if you look at the contents of the config file above, I do not see a 
> reneg value of 64MB. Is the technical support person telling the truth or is 
> he just bullsh**ing me?
>
>


the setting they mention is the intended mitigation for this attack. The 
beauty of this hack is that it can be set on the server without changing 
anything on the clients; you can find out whether they are actually 
doing this by launching OpenVPN with "verb 5 " set and watching the 
output for any renegotiation parameters. Another way to test it is by 
downloading a file larger than 64 MB - that should trigger a 
renegotiation, which you should also be able to find in the client-side 
log file.

HTH,

JJK


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