Re: [MLS] Use Cases for avoiding Forward Secrecy

Eric Rescorla <> Wed, 28 February 2018 23:19 UTC

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From: Eric Rescorla <>
Date: Wed, 28 Feb 2018 15:18:25 -0800
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To: Dave Cridland <>
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Subject: Re: [MLS] Use Cases for avoiding Forward Secrecy
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Hi Dave,

I certainly agree with you that there are use cases where applications
might want to archive communications.

Generally, I think the right answer to these, however, isn't to modify the
messaging protocol but rather to add that functionality on the messaging
app over the top. That also is a much easier way to do new device history


On Wed, Feb 28, 2018 at 9:14 AM, Dave Cridland <> wrote:

> Hi folks,
> While I'm really pleased to see MLS, and I generally like the idea of
> Forward Secrecy, there's a couple of use cases where it might be worth
> avoiding. Feel free to correct me if these are in fact possible with
> Forward Secrecy. Both these relate to archival access to past
> messages:
> * UX - Some users (actually all of them) would like to be able to
> install client software on a new device and have their historical
> messages available to them. Most "business" messaging systems seem to
> work this way, as well as a number of consumer-grade systems. The
> nature of Forward Secrecy means that an archive would need to be held
> on one device and re-sent to another through the network, which is
> trickier to manage, and is reliant on multiple devices being online at
> overlapping times. Alternately, the archival copy might be re-uploaded
> to the server using a static encryption key, I suppose, which would
> rather spoil the point.
> * Retention - Many business and government deployments have mandatory
> retention requirements. An example is MIKEY-SAKKE, promoted in part by
> the UK Government for its own communications, which uses mandatory key
> escrow to keep an archived copy of the messages accessible to the
> business units involved. An advantage of the SAKKE system is that it
> allows the key escrow to be offline, limiting attack opportunities.
> Given the latter, for example, I could not use an MLS-based system to
> discuss a tax problem with the authority, and since I'm unlikely to
> have a SAKKE-based messaging client, I'm unlikely to have encrypted
> messaging to my tax authority at all - which seems signficantly worse
> than merely having no Forward Secrecy.
> None of this is to say that Forward Secrecy should be avoided
> entirely, of course.
> Dave.
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