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Showing posts from January, 2010

Salmon's 'Discoverable URIs' and PKI

One issue that's cropped up in Salmon is how to identify authors.  Mostly, feed syndication protocols hand-wave about this and at most specify a field for an email address.  Atom goes one step further and provides a way to specify a URI for an author, which could be a web site or profile page.


The main things that Salmon needs are (1) a stable, user-controlled identifier that can be used to correlate messages and (2) a way to jump from that identifier to a public key used for signing salmon.  The existing atom:author/uri element works fine for #1.  Webfinger+LRDD discovery can then take over for #2.  What comes out of that process is a public key suitable for verifying the provenance (authorship) of a salmon.

Now for the details.  It turns out that the flow for Webfinger is pretty stable, so any time you have an email like identifier for an author you can just slap "acct:" in front and things will Just Work.  But I don't want to limit authorship only to acct: URIs - …

A Web Wide Public Key Infrastructure

In my last post, I talked about Magic Signatures, the evolution of Salmon's Magic Security Pixie Dust into something concrete.  There's an important bit missing, which I'll talk about now:  Public key signatures are fairly useless without a way to discover and use other people's public keys in a secure, reliable way.  We need a distributed, easily deployable public key infrastructure.  Let's build one.

The basic plan for Salmon signature verification is fairly simple:  Use Webfinger discovery on the author of the Salmon to find their Magic Signature public key, if they have one.  To find this, you first get the Webfinger XRD file, then look for a "magickey" link:
<Link rel="http://salmon-protocol.org/ns/magickey" href="https://www.example.org/0B6A58300/publickey" />
Retrieve the magickey resource, parse it into a public key in your favorite library, and use that to verify the signature of the original Salmon you started with.

A few…

Magic Signatures for Salmon

In writing the spec for Salmon we soon discovered that what we really wanted was S/MIME signatures for the Web.  In other words, given a message, let you sign it with a private key, and let receivers verify the signature using the corresponding public key.  Signing and verifying are pretty well understood, but in practice canonicalizing data and signing is hard to get right.  Making sure that the mechanism adopted is really deployable and interoperable, even in restricted environments, is a top priority for Salmon.

I'm calling this the "Magic Signature" mechanism because it's not really Salmon-specific and you can analyze it without thinking much about Salmon at all.

One of the reasons why this is hard is because of the abstraction layers that we have in place in our software.  For example, encryption algorithms operate on byte sequences, but a given XML document can have many different byte sequence serialized forms.  Even JSON isn't immune to this, though mandati…

Fork in the Road for Salmon

Happy New Year!
Over the past several weeks, I've been doing a Salmon conference roadshow, talking and listening to people about the protocol and getting feedback.
At IIW, we had a "Magic Security Pixie Dust" Salmon session which kicked around the use cases and challenges in detail.  There and elsewhere I got basically two kinds of feedback:  (a) The specification, especially the signatures, was too complicated; and (b) the specification, especially the signatures, was not comprehensive enough.
There was a suggestion to drop signatures entirely and just rely on reputation of the salmon generators, who would be on the hook for vouching for the identity of their users.  This would simplify the protocol but at the cost of giving it effectively the same security characteristics as email.  This was initially attractive and I spent some time playing with what that would look like.  Unfortunately, I am pretty sure that it would just make operation more complex and adoption more pr…