SSH Agent keys >4096 bit?

Ali Lown ali at
Fri May 4 21:54:08 CEST 2012

>> Might I point out that discussion is with respect to an 8k RSA SSH key
>> for SSH authentication, not for email. A 2 second delay during the
>> initialization of an SSH connection is not a problem.
> And here is precisely something interesting: 8k RSA is discussed as a method
> to keep messages confidential for decades. I haven't looked into it, but I'm
> under the impression RSA is used purely for authentication in SSH, not for
> key exchange[1]. What are you protecting decades against here? A server
> reusing a random challenge? That seems quite far fetched.

I created the 8k keys prior to understanding the full effects
reasoning behind a 1k/2k key simply because it was't particularly
computationally expensive for me to do, and I saw no harm in being
overly cautious with a longer key than average.
I see no purpose though (at this stage, with my public key spread
around a variety of locations without issue) in generating a new
'smaller' key for the sole purpose of being able to use GPG's SSH
agent, requiring me to change the public key in every location.

> Oh, by the way, only the computational load for the client was discussed.
> There's also the server (although the public side of the computation is
> quicker than the private side). The server gets logins from potentially a
> lot of clients.

I think this is fairly irrelevant to the discussion. Yes there is an
overhead, but performing the calculations is not a significant
concern. (If a server is getting lots of fake logon attempts, you need
to sort out your firewall instead).

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