On Thu, June 16, 2016 14:23, Valeri Galtsev wrote:
>
> On Thu, June 16, 2016 1:09 pm, Gordon Messmer wrote:
>>
>> I doubt that most users check the dates on SSL certificates,
>> unless they are familiar enough with TLS to understand that
>> a shorter validity period is better for security.
>
> Oh, this is what he meant: Cert validity period. Though I agree
> with you in general (shorter period public key is exposed smaller
> chance secret key brute-force discovered),
Like many things that appear to be common-sense these assumptions have
no empirical basis. A properly generated RSA certificate and key of
sufficient strength -- RSA k>=2048bits -- should provide protection
from brute force attacks for decades if not centuries. The usual way a
private key gets compromised is by theft or by tampering with its
generation. Putting yourself on a hamster wheel of constant
certificate generation and distribution simply increases the
opportunities for key theft and tampering.
Keys issued to individuals certainly should have short time limits on
them. In the same way that user accounts on systems should always
have a near term expiry date set. People are careless. And their
motivations are subject to change. So having a guillotine date on a
personal certificate makes sense from an administrative standpoint.
One wants to fail safe. But modifying certificates on sealed
servers?. Really, unless one has evidence of penetration and theft of
the key store, what possible benefit accrues from changing secured
device keys on a frequent basis?
We mainly use 4096bit keys which will be secure from brute force until
the advent of Quantum computing. At which point brute force attacks
will become a pointless worry. Not because the existing RSA
certificates and keys will withstand those attacks but because the
encryption process itself will move onto quantum devices. That
development, if and when it occurs, will prove more than the code
breakers will ever be able to handle. Of course then one must worry
about the people who build the devices. But we all have to do that
already. Bought any USB devices from China recently?
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