RFC 6376, "DomainKeys Identified Mail (DKIM) Signatures", September 2011Source of RFC: dkim (sec)
Errata ID: 5713
Reported By: Victor Shrubowich
Date Reported: 2019-04-30
Rejected by: Benjamin Kaduk
Date Rejected: 2019-04-30
Section 2.8 says:
FWS = [*WSP CRLF] 1*WSP
It should say:
FWS = [*WSP] CRLF 1*WSP
In the ABNF RFC ([RFC5234]), section 3.8 states "Square brackets enclose an optional element sequence".
CRLF is required for folding. However, the CRLF in the FWS rule is shown inside the square brackets, which would make it optional. It should not be inside the square brackets.
(See Errata ID 5712 for FWS in [RFC5322], which is referenced at the end of section 2.8.)
As noted by Dave Crocker, Folding White Space is a construct that permits a newline but does not require it -- only whitespace of some form is required to be present in order to match, not a specific kind of whitespace. The present construction, as corresponds to RFC 5322, is as intended.