Move 'jwk' and 'alg' fields to protected header. (certbot#4677)

* Move 'jwk' and 'alg' fields to protected header.

Previously, these were in the unprotected JWS header, which Boulder currently
allows. However, the next version of the spec doesn't allow anything in the
unprotected header. Moving these fields now allows server implementers who are
implementing the Certbot/Boulder version of ACME
(https://github.com/letsencrypt/boulder/blob/master/docs/acme-divergences.md) to
use JOSE libraries that don't support unprotected headers.

Fixes certbot#4417.

* Only protect existing headers.

acme/acme/jose/jws.py

@@ -222,7 +222,8 @@ def sign(cls, payload, key, alg, include_jwk=True,
 
         protected_params = {}
         for header in protect:
-            protected_params[header] = header_params.pop(header)
+            if header in header_params:
+                protected_params[header] = header_params.pop(header)
         if protected_params:
             # pylint: disable=star-args
             protected = cls.header_cls(**protected_params).json_dumps()

acme/acme/jws.py

@@ -49,6 +49,6 @@ def sign(cls, payload, key, alg, nonce, url=None, kid=None):
         # jwk field if kid is not provided.
         include_jwk = kid is None
         return super(JWS, cls).sign(payload, key=key, alg=alg,
-                                    protect=frozenset(['nonce', 'url', 'kid']),
+                                    protect=frozenset(['nonce', 'url', 'kid', 'jwk', 'alg']),
                                     nonce=nonce, url=url, kid=kid,
                                     include_jwk=include_jwk)