Re: [Acme] various issues with the spec

Stefan Bühler <acme@stbuehler.de> Sun, 14 June 2015 10:31 UTC

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Date: Sun, 14 Jun 2015 12:31:21 +0200
From: Stefan =?UTF-8?B?QsO8aGxlcg==?= <acme@stbuehler.de>
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Subject: Re: [Acme] various issues with the spec
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On Sun, 14 Jun 2015 17:01:37 +1000
Fraser Tweedale <frase@frase.id.au>; wrote:

> On Sun, Jun 14, 2015 at 08:06:46AM +0200, Stefan Bühler wrote:
> > On Sun, 14 Jun 2015 11:17:38 +1000
> > Fraser Tweedale <frase@frase.id.au>; wrote:
> > 
> > > On Sun, Jun 14, 2015 at 12:24:32AM +0200, Stefan Bühler wrote:
> > > > * dvsni: Please don't require the domain name which is being
> > > > validated to be part of subjectAltName; configuring such
> > > > certificate might break a working setup in production, when it
> > > > "wins" over an already present and valid certificate for the
> > > > domain.
> > > > 
> > > No, this certificate is only presented for the host
> > > `<nonce>.acme.invalid'.
> > 
> > You are thinking of a setup where you configure explicitly which
> > certificate is used for which SNI value. But gnutls for example has
> > a nice feature where you can just give it all your certificates,
> > and it will pick the matching one automatically.
> > 
> Do you propose that the certificate *not* bear the domain name being
> validated in *either* the Subject DN or subjectAltName extension?

Yes, neither set in the common name nor in subjectAltName; I think
gnutls would find both.

Just for the record, I'd be fine with adding a subjectAltName like
`DNS:<dnsname>.acme.invalid', so the domain name is at least present in
some form (although I don't know what to gain from it).

> This probably does not affect the protocol, but I think is nice to
> include it anyway for the sake of being explicit.  Can you identify
> any existing server software which would is incompatible with ACME
> dvsni due to the validation certificate bearing the name being
> validated?

I happen to know such webserver: lighttpd 2 (not released yet); see
http://doc.lighttpd.net/lighttpd2/mod_gnutls.html

> > > > * dvsni: `The public key is the public key for the key pair
> > > > being authorized`. I hope this was just an accident, this would
> > > > be *really* wrong to require.
> > > > 
> > > Why would this be wrong?  Remember that this certificate is
> > > generated as part of, and intented for use only as part of the
> > > authorization workflow.  It has no bearing on certificates
> > > eventually issued for the domain name being authorized.
> > 
> > I don't want my webserver to see my account private key, ever. Am I
> > really the first guy to have a problem with that?
> > 
> > > > * dvsni: Don't require it to be a self-signed certificate - what
> > > > does it matter who signed it?
> > > > 
> > > It must be signed by the account key as evidence that the entity
> > > performing the authorization controls the account key.
> > 
> > What exactly is the attack scenario here if this is not checked?
> > Person A playing MITM to give control over domain B to account C,
> > and account C started the authorization but didn't actually want it
> > to succeed?
> > 
> > If you really require such evidence, perhaps it could be required to
> > have the certificate signed by the account key instead of being
> > self-signed.
> 
> You've convinced me on these latter points - the certificate should
> be signed by the account key but the key could be a different key
> (i.e. not a self-signed cert) - and this means that the web server
> need not have access to the account private key.

Ok.

regards,
Stefan