[Acme] Remove combinations array

Jacob Hoffman-Andrews <jsha@eff.org> Wed, 17 August 2016 18:22 UTC

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Subject: [Acme] Remove combinations array
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https://github.com/ietf-wg-acme/acme/pull/171

This is a fairly complicated part of the protocol, and not used in
practice. For instance, in Let's Encrypt's implementation, there are
always three challenges, any one of which may be fulfilled by the client.

After this change, all challenges are considered to be combined with an
"OR." That is, any challenge within an authorization may be completed to
make the authorization valid.

Authorizations within the new-application object are considered to be
combined with an "AND." That is, all of them must become valid before
the certificate will be issued. The combination of the two means that we
have similar expressiveness as before, even without the combinations array.

Thoughts?