Today, we fold header names with strings.EqualFold when signing, but
strings.ToLower when verifying.
This should be harmless for valid messages (that have only ASCII
characters in the header names), but if one contains an (invalid) header
with a non-ASCII character, we could fail to verify the message we sign.
The inconsistency would only happen if a header has a non-ASCII
character that strings.EqualFold and strings.ToLower handle differently
(like "ſubject", with U+017F).
To make signing and verifying consistent, this patch makes signing use
strings.ToLower.
This bug was found by fuzzing the sign-verify cycle, which will be added
in a later patch.
Today, if a message does not have a blank line (which separates headers
and body), we also include all lines except the first as the body (even
the ones parsed as headers). This is incorrect.
This patch fixes the code so that in those messages, all the lines are
headers, and the body is empty.
Note that since SMTP messages have to end with a new line, that final
line will act as the separator if there is no body. So this bug only
impacts signing/verification of files on disk via chasquid-util.
When parsing a header that has a continuation line, today we append that
line to it, which copies the whole header each time.
On normal messages this is negligible, but a header with a very large
number of continuation lines would cause a DoS through work
amplification.
This patch fixes the problem by making header continuation handling
linear: we now accumulate the header lines in a list, and then join them
only once.
Thanks to John JEAN <john@wargan.com> who reported a similar issue
(fixed in a previous patch), which led to finding this bug.
Alberto Bertogli