Subject: [Acme] external-account-binding signature

From: Josh Soref <[email-masked]>
Date: Sun, 19 Feb 2017 17:41:38 -0500

> ... The protected header of the JWS MUST meet the following criteria:
>
> * The "alg" field MUST indicate a MAC-based algorithm
> * The "kid" field MUST contain the key identifier provided by the CA
> * The "nonce" field MUST NOT be present
> * The "url" field MUST be set to the same value as the outer JWS
>
> The "signature" field of the JWS will contain the MAC value computed with the
> MAC key provided by the CA.

Why isn't this last sentence part of the preceding bulleted list?

I think the answer is that something is trying to tell me about the
double layers of signatures.

But, if that's the case, I don't think this is clear enough for me to
figure out.

<blockquote>
{
  "protected": base64url({
...
  }),
  "payload": base64url({
...
    "external-account-binding": {
      "protected": base64url({
...
      }),
      "payload": base64url(/* same as in "jwk" above */),
      "signature": /* MAC using MAC key from CA */
    }
  }),
  "signature": "5TWiqIYQfIDfALQv...x9C2mg8JGPxl5bI4"
}
</blockquote>