[Ach] OpenSSH settings

[Aaron Zauner]

On 03 Jan 2014, at 23:24, Andy Wenk <andy at nms.de> wrote:

> I did so and it works. I now have this:
> 
> Ciphers aes256-ctr,aes128-ctr
> 
> MACs hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
> 
> KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1 
> 
Looks correct for your distribution and version!

> As another user already asked in another thread I am wondering, if this is still more secure than the defaults regarding the documentation here:
> 
> http://www.openssh.org/cgi-bin/man.cgi?query=sshd_config
> 
> It reads:
> 
> Ciphers
>              Specifies the ciphers allowed for protocol version 2.  Multiple
>              ciphers must be comma-separated.  The supported ciphers are:
> 
>              ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
>              ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'',
>              ``aes128-gcm at openssh.com'', ``aes256-gcm at openssh.com'',
>              ``arcfour128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'',
>              ``cast128-cbc'', and ``chacha20-poly1305 at openssh.com’’.
[…]

> A short explanation is highly appreciated but I assume that only these ciphers should be choosen because of the key length and type and to revoke other, less secure ciphers.
- We do not support 3DES in our recommendations (112bit security vs. 128bit security minimum)
- We removed RC4 (arcfour) since there are a lot of known attacks out there
- We removed blowfish since it’s author (bruce schneier) says it’s deprecated
- We do not support CAST in our recommendations

You can pretty much find all the reasoning in the theory part of our paper.

Furthermore distributions might opt to change defaults and parameters. I hope that GCM mode, UMAC and ChaCha20 will soon find widespread adoption to distributions - they are fast and secure.

Thank you,
Aaron
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