Adds a constant leeway (60 seconds) to the nbf and exp claim check to
account for clock skew between the registry servers and the
authentication server that generated the JWT.
The leeway of 60 seconds is a bit arbitrary but based on the RFC
recommendation and hub.docker.com logs/metrics where we don't see
drifts of more than a second on our servers running ntpd.
I didn't attempt to make the leeway configurable as it would add extra
complexity to the PR and I am not sure how Distribution prefer to
handle runtime flags like that.
Also, I am simplifying the exp and nbf check for readability as the
previous `NOT (A AND B)` with cmp operators was not very friendly.
Ref:
https://tools.ietf.org/html/rfc7519#section-4.1.5
Signed-off-by: Marcus Martins <marcus@docker.com>
Since RawMessage json receivers take a pointer type, the Header structure should use points in order to call the json.RawMessage marshal and unmarshal functions
Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
