RFC 8617, "The Authenticated Received Chain (ARC) Protocol", July 2019

Errata ID: 7910
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Steffen Nurpmeso
Date Reported: 2024-04-26

Section 4.1.2 says:

   arc-ams-info = instance [CFWS] ";" tag-list
   arc-message-signature = "ARC-Message-Signature:" [CFWS] arc-ams-info

It should say:

   arc-ams-info = instance [FWS] ";" tag-list
   arc-message-signature = "ARC-Message-Signature:" [FWS] arc-ams-info

Notes:

The RFC claims in 4.1.2

The AMS header field has the same syntax and semantics as the DKIM-
Signature field [RFC6376], with three (3) differences:

but the three differences do not denote the FWS->CFWS change.

CFWS is to be parsed very differently than FWS, given its potentially infinite recursion behaviour, and the possibility to use quoted-pair's, ie, "escapability", something which (like almost RFC 5322 as such in practice) the DKIM RFC circumvents by using VALCHAR, a corruption of VCHAR as of RFC 5234.
In effect neither of these standards adhere to neither of RFC 5322 (plain atext, quoted-string, quoted-pair) nor RFC 2045 (K=V without whitespace; quoted-printable or base64 for 7-bit clarity etc etc), making them very hard to parse, to mention my humble opinion.

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