[Ace] EST over CoAP: Randomness

Hannes Tschofenig <Hannes.Tschofenig@arm.com> Thu, 09 May 2019 14:42 UTC

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From: Hannes Tschofenig <Hannes.Tschofenig@arm.com>
To: "ace@ietf.org" <ace@ietf.org>
Thread-Topic: EST over CoAP: Randomness
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Date: Thu, 9 May 2019 14:42:35 +0000
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Subject: [Ace] EST over CoAP: Randomness
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Hi all,

I am still a bit unhappy about this paragraph:

"
   Constrained devices sometimes do not have the necessary hardware to
   generate statistically random numbers for private keys and DTLS
   ephemeral keys.  Past experience has also shown that low-resource
   endpoints sometimes generate numbers which could allow someone to
   decrypt the communication or guess the private key and impersonate as
   the device [PsQs] [RSAorig].  Additionally, random number key
   generation is costly, thus energy draining.
"

If you get hardware that does not have a hardware-based RNG then you are in trouble. The main security protocols we look into do not work without a source of randomness. Hence, getting the certificate & private key from the server will not get you too far.

I believe we should encourage developers to pick the correct hardware for the task rather than making them believe we have come up with solutions that allow them to get away without a hardware-based RNG.

I also do not believe the statement that random number key generation is costly. Can you give me some number?

The references to [PsQs] [RSAorig] are IMHO also not appropriate because they are conveying a different message (at least that's my understanding from reading them). The message is that you have to be careful with designing and using a random number generator on embedded systems because the sources of entropy may just not be there (like keyboards, harddisk drive, processing scheduling, etc.).

Ciao
Hannes

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