Re: [MLS] Use Cases for avoiding Forward Secrecy

Richard Barnes <> Thu, 01 March 2018 02:21 UTC

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From: Richard Barnes <>
Date: Wed, 28 Feb 2018 21:21:53 -0500
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To: Dave Cridland <>
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Subject: Re: [MLS] Use Cases for avoiding Forward Secrecy
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Re-adding the list.

On Feb 28, 2018 18:21, "Richard Barnes" <> wrote:

> Hey Dave,
> Thanks for the feedback.  This is a good point to clarify.  It had also
> crossed my mind, as the product I work with most in my day job also has
> retention features.
> However, I kind of chose not to worry about it, under the following
> theory: You can always build a non-forward-secret system out of a
> forward-secret one.  For example, you could use the FS keys to encrypt a
> long-term key, passing it forward across FS boundaries.
> So kind of like Raphael was saying, I figured an application with
> history-sharing or retention needs could wrap see extra stuff around MLS to
> dial back the appropriate knobs.
> --Richard
> On Feb 28, 2018 12:14, "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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>> MLS mailing list