[CentOS] SSH Weak Ciphers

[Leon Fauster]

Am 19.10.2016 um 00:58 schrieb Gordon Messmer <gordon.messmer at gmail.com>:
> On 10/18/2016 03:28 PM, Clint Dilks wrote:
>> So first
>> question is are people generally modifying the list of ciphers supported by
>> the ssh client and sshd?
> 
> I suspect that "generally" people are not.  I do, because I can, and so that I can offer at least some advice to people who aim to do so.
> 
>> On CentOS 6 currently it looks like if I remove all the ciphers they are
>> concerned about then I am left with Ciphers
>> aes128-ctr,aes192-ctr,aes256-ctr for both /etc/ssh/sshd_config and
>> /etc/ssh/ssh_config.
> 
> If you're going to go down this road, you should probably look at key exchanges and HMACs as well.  On CentOS 7, I use:
> 
> KexAlgorithms curve25519-sha256 at libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
> Ciphers chacha20-poly1305 at openssh.com,aes256-gcm at openssh.com,aes128-gcm at openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
> MACs hmac-sha2-512-etm at openssh.com,hmac-sha2-256-etm at openssh.com,umac-128-etm at openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128 at openssh.com
> 
> On CentOS 6, I believe you'd have to drop all of the @openssh.com items.


Is there any command to find the supported list of KeyAlgos, MACs and Ciphers for 
the particular system (e.g. EL{5,6,7})? Similar to  $ openssl ciphers -v ...



>> Is just using these three ciphers like to cause me
>> any problems?  Could having so few ciphers be creating a security concern
>> itself?
> 
> I don't think it'd be a security concern, just compatibility issues.  So far, I've had minimal problems with restricted algorithms.  I do have to make an exception for a slightly old WD MyBook World edition.

--
LF