[ntp:questions] SSL time request

Chris Albertson albertson.chris at gmail.com
Sun May 22 01:04:41 UTC 2011

On Sat, May 21, 2011 at 3:18 PM, Danny Mayer <mayer at ntp.org> wrote:
> On 5/19/2011 12:06 PM, Kevin Coulombe wrote:
>> Hi,
>> I'm looking into producing a 15 days demonstration version of an application
>> for a client. I'm considering the use of an internet clock to validate this
>> demo's duration, but a simple http request would be too easy to hack.
>> Is there any way to request time in a secure manner such as an SSL
>> connection. What I need is to validate that we are actually communicating
>> with the "real" time server and that the message isn't tampered with.
> The proper way to do this with NTP is to use the autokey protocol which
> will authenticate the servers. Note that the basic way that NTP works is
> to use 3 or more NTP servers which allows it to select only consistent
> truechimers and that are close enough to each other timewise that it can
> choose one of them to synch to. It is actually quite hard to tamper with
> an NTP packet and have it not be noticed, mainly because it does not
> rely on a single NTP server. Add autokey for each of the servers and
> just about nothing can be tampered with.
> Note that SSL is *not* secure when it comes to time since the SSL
> certificate is in turn dependent on time. The autokey protocol deals
> with that issue.

Let me add one more thing.  It should be obvious.  You need to do the
NTP protocol from within your program.  Letting NTP sync the system
time then using that is to easy to work around.

But a simpler method is to use SSL to "tunnel" to your server and use
something very simple like the "time" protocol.  NTP will give time to
the milisecond level but the time protocol is faster and good enough
for a second or so, more than you need.

None of this will work if the person trying to break it knows about
software, he'd just binary patch your software with an unconditional

Chris Albertson
Redondo Beach, California

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