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Re: When to use pair-wise unique DIDs vs. just individual unique DIDs

From: =Drummond Reed <drummond.reed@evernym.com>
Date: Sun, 15 Apr 2018 19:50:25 -0700
Message-ID: <CAAjunna1HpVgOZa68H1Sz5mQkOWSKfHWJKhPmee+YQX+B42h7w@mail.gmail.com>
To: Chris Boscolo <chris@boscolo.net>
Cc: Carlos Bruguera <carlos@selfkey.org>, "W3C Credentials CG (Public List)" <public-credentials@w3.org>
On Sun, Apr 15, 2018 at 9:42 AM, Chris Boscolo <chris@boscolo.net> wrote:

> Drummond,
> I must admit, this answer caught me by surprise...
> How is what you are describing any different than our current federated ID
> nightmare?

Chris, I must have mis-communicated something. Keep reading.

> In the age of the Internet and hackers being mostly smarter than those
> building infrastructure, the term "private" is meaningless.
> "Private" just means hidden from the public until at some point in the
> future it is no longer hidden.

Yes, of course, the "privacy" of a microledger is not just that it's a
public ledger. The DIDs and DID documents on a microledger should still be
as privacy-protecting as they would be on a public ledger, i.e., they
shouldn't leak any information about the identity owner or relationship.
They just are not publicly searchable or viewable.

> Furthermore, I thought the whole point of the DID approach is that the DID
> owner can update the DDO if, for example, they want to update the keys.
> How would this work if as you say "and that each can maintain a copy of the
> other's DID document".

That may have been the phrase that tripped you up. The "copy" of the
shared DID document on the microledger is just like the "copy" maintained
by all the nodes on a distributed public ledger. In other words, only
the identity owner has the ability (or can delegate the ability) to make
any chances to the DID document.

To be clear, if Alice maintains a microledger with Bob, then Bob's node on
the microledger has a copy of the pairwise pseudonymous DID and DID
document that Alice has created for Bob, and Alice's node has a copy of the
pairwise pseudonymous DID and DID document that Bob has created for Alice.
Since both DID documents contain the service endpoints for their respective
agents, the two agents are the only "nodes" that need to keep
the microledger in sync (i.e., "consensus" is pretty easy). But it's still
an actual ledger, i.e., all transactions are signed, hashed together into a
chain, and replicated to all nodes.

> Looks like I either need to re-evaluate my support for this approach or
> hopefully some can help clarify this a bit more.

Hopefully I've done that; let me know if it's still not clear.


>   -chrisb
> On Sun, Apr 15, 2018 at 3:50 AM, Carlos Bruguera <carlos@selfkey.org>
> wrote:
>> Hey Drummond,
>> The approach of "private" pairwise DID seems totally reasonable as it
>> fits the very purpose of pairwise identifiers which is (in my
>> understanding) to establish a *private* channel for authentication and
>> authenticated comunication between entities. Also, leaving the ledger for
>> the data that is making purposedly public works not only as an anti-spam
>> measure on the ledger but also solves multiple privacy and anonymity issues.
>> I'm guessing this approach can also be used in cases where correlativity
>> is desired or at least tolerated (by using the same DID or "facet" for
>> authenticating to multiple (possibly related) services, even if generated
>> locally and exchanged privately)?... On a different line, is there any
>> level of "anchoring" for these "private DIDs" against the public ledger? Or
>> it's not necessary at all?
>> On Sun, Apr 15, 2018 at 9:38 AM, =Drummond Reed <
>> drummond.reed@evernym.com> wrote:
>>> On Sat, Apr 14, 2018 at 9:46 AM, Chris Boscolo <chris@boscolo.net>
>>> wrote:
>>>> First, Adam, thanks for posting the "WebAuthn & DID" presentation that
>>>> surfaced the discussion of using pair-wise unique DIDs.  And thank
>>>> you, Drummond, for linking to the discussion taking place at Sovrin on the
>>>> subject. (https://forum.sovrin.org/t/the-benefit-of-pairwise-dids/628/3)
>>>> I decided to pull this one question out into its own thread to get
>>>> clarification and to help inform how the WebAuthn protocol might be
>>>> modified to support DIDs.
>>>> I think the community would benefit if we had a clear understanding of
>>>> when pair-wise unique DIDs should be used vs. when a per-user unique DIDs
>>>> will suffice.
>>>> In the example, where a user is creating a new account on a popular
>>>> website it is clear to me that the user will want to use a unique DID for
>>>> only that site.  But, I question whether it is a good idea for the website
>>>> to create a unique DID to communicate with that one user.  In fact, I
>>>> wonder if doing so will open the door to other unintended ways of
>>>> correlating users with the site. (When these DIDs are in public ledgers.)
>>> Chris, I just wanted to point out why your final parenthetical is
>>> important to this discussion. In Sovrin architecture, pairwise
>>> pseudonymous DIDs *are not written to the public ledger*.
>>> It's true that a year ago, even as we started to use pairwise
>>> pseudonymous DIDs, we assumed they were all being written to the Sovrin
>>> public ledger because: a) they did not provide any correlate-able data, and
>>> b) we didn't have an alternative.
>>> We subsequently realized that, since the whole point of pairwise
>>> pseudonymous DIDs is that they are only needed by the two parties
>>> involved—and that each can maintain a copy of the other's DID
>>> document—there was no reason to write them to a public ledger. Rather the
>>> two parties could maintain them on their own private microledger.
>>> This has several significant advantages:
>>>    1. It is even better from a privacy perspective since neither the
>>>    pairwise pseudonymous DIDs nor their DID documents needed to be public.
>>>    2. It is wonderful from a scalability perspective since
>>>    the microledgers add almost no load to the public ledger.
>>>    3. It means the Sovrin public ledger can be optimized for public
>>>    DIDs and other SSI infrastructure data that needs to be fully public and
>>>    widely shared.
>>> Should these considerations be added to the DID spec?
>>> That's a very good question. I don't think the DID spec (or any other
>>> spec) should be weighed down with lots of implementation guidelines and
>>> advice, but we should probably mention the basic option that DIDs can be
>>> registered on public ledgers, private ledgers, or microledgers.
>>> What do you think?
Received on Monday, 16 April 2018 02:50:59 UTC

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