Re: [TLS] The future of external PSK in TLS 1.3

Watson Ladd <> Wed, 30 September 2020 12:50 UTC

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From: Watson Ladd <>
Date: Wed, 30 Sep 2020 08:50:31 -0400
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To: Hannes Tschofenig <>
Cc: "Blumenthal, Uri - 0553 - MITLL" <>, "" <>
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Subject: Re: [TLS] The future of external PSK in TLS 1.3
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On Wed, Sep 30, 2020 at 3:32 AM Hannes Tschofenig
<> wrote:
> Hi Watson,
> through Arm I deal with customers who use microcontrollers that have all sorts of limitations. So, the question is not so much whether these limitations exist but rather whether you care and what exactly these limitations are (CPU processing, RAM, flash memory, energy, networking bandwidth, cost, physical size limitations, limitations caused by the environment these devices operate in, etc.).
> PSK is the most efficient mechanism we have. Not only does it perform extremely well when it comes to CPU performance it also reduces the size of code size, and RAM utilization. Also the bandwidth requirements are minimal.
> Of course, regular 32-bit microcontrollers are all able to do public key crypto operations (see a presentation I did a while ago in the LWIG group -- There are, however, some environments where you just cannot wait multiple seconds for a handshake to complete.

This presentation doesn't mention many well known optimization
opportunities, and seems to show an M3+ device having ~100ms for an
operation. Obviously this is more expensive than the M0, but in some
cases it's worth paying. Then of course one can optimize the handshake
through extensions. RSA verification (not signing) is very cheap.

Recommended N doesn't stop people from using PSK when appropriate if
other constraints make it the most appropriate choice.

> In discussions in the IETF I notice for some the IoT computing world starts with Cortex A-class devices, as they are found in Raspberry Pis, tablets and phones. Those are high performance processors where crypto is lightning fast. But don't forget the other family of processors, of which there are probably more than a 80 billion out in the wild already.
> Ciao
> Hannes
> -----Original Message-----
> From: TLS <> On Behalf Of Watson Ladd
> Sent: Wednesday, September 30, 2020 2:29 AM
> To: Blumenthal, Uri - 0553 - MITLL <>
> Cc:
> Subject: Re: [TLS] The future of external PSK in TLS 1.3
> On Tue, Sep 29, 2020 at 12:49 PM Blumenthal, Uri - 0553 - MITLL <> wrote:
> >
> > I share Achim's concerns.
> >
> > But I believe the explanations will turn out mostly useless in the real world, as the "lawyers" of the industry are guaranteed to steer away from something "not recommended".
> >
> > In one word: bad.
> Why is PSK so necessary? There are very few devices that can't handle the occasional ECC operation.  The key management and forward secrecy issues with TLS-PSK are real. Steering applications that can afford the CPU away from PSK and toward hybrid modes is a good thing and why this registry exists imho.
> --
> Astra mortemque praestare gradatim
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