Re: [Acme] WG last call for draft-ietf-acme-email-smime-06

Ben Schwartz <bemasc@google.com> Mon, 04 May 2020 17:49 UTC

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From: Ben Schwartz <bemasc@google.com>
Date: Mon, 4 May 2020 13:48:26 -0400
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To: Ryan Sleevi <ryan-ietf@sleevi.com>
Cc: Alexey Melnikov <alexey.melnikov@isode.com>, "Salz, Rich" <rsalz@akamai.com>, "acme@ietf.org" <acme@ietf.org>
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Subject: Re: [Acme] WG last call for draft-ietf-acme-email-smime-06
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On Sun, May 3, 2020 at 1:36 AM Ryan Sleevi <ryan-ietf@sleevi.com> wrote:

>
>
> On Sat, May 2, 2020 at 2:11 PM Ben Schwartz <bemasc=
> 40google.com@dmarc.ietf.org> wrote:
>
>>
>>
>> On Sat, May 2, 2020 at 8:35 AM Alexey Melnikov <alexey.melnikov@isode.com>
>> wrote:
>>
>>> Hi Ben,
>>> On 21/04/2020 01:12, Ben Schwartz wrote:
>>>
>>>
>>> On Wed, Apr 1, 2020 at 5:40 AM Alexey Melnikov <
>>> alexey.melnikov@isode.com> wrote:
>>>
>>>> Hi Ben,
>>>>
>>>> My apologies for missing your email in March:
>>>>
>>>
>>> And mine for this delayed response.
>>>
>>>> On 12/03/2020 20:42, Ben Schwartz wrote:
>>>>
>>>> Section 3 says token-part1 "contains at least 64 bit of entropy", but
>>>> Section 3.1 says token-part1 "MUST be at least 64 octet long after
>>>> decoding".  Is this difference deliberate?
>>>>
>>>> No, I obviously made a typo when saying octets. I will fix.
>>>>
>>> Fixed.
>>>
>>> Also 64 octets of entropy is a _lot_.  RFC 8555 says "the token is
>>>> required to contain at least 128 bits of entropy".
>>>>
>>>> The draft seems to be oriented entirely toward use with e-mail clients
>>>> that have a built-in ACME-S/MIME client.  I'm a bit disappointed that the
>>>> draft doesn't accommodate users with "naive" email clients very well, e.g.
>>>> by allowing customized subject lines.
>>>>
>>>> Actually, I was trying to accommodate naive email clients, but it was a
>>>> fine balance trying to specify minimal requirements.
>>>>
>>>> Can you suggest some specific text to change and then we can discuss
>>>> whether or not it should be done? My thinking about the Subject header
>>>> field was that I wanted to have a unique subject (so that ACME email
>>>> messages are easily findable). I also wanted to allow the token in the
>>>> subject for APIs that can easily access Subject and not other header fields.
>>>>
>>> In that case, I would suggest "... subject ending with "(ACME:
>>> <token-part1>)", where ...".  That would allow the first part of the
>>> subject (most likely to be seen by a human) to be human-readable.
>>>
>>> After thinking a bit more about this:
>>>
>>> As ACME servers are generating ACME challenge emails, the requirement on
>>> them is stricter (they create the first message in an email thread). I am
>>> tempted to leave this as is. Can you think of a case where ACME servers
>>> would be unable to comply with this restriction?
>>>
>> My concern is that users will not know what to do if they receive an
>> email whose subject line is "ACME: awlkNSdpijawrfz...".  Users are used to
>> seeing emails whose subject line is "Please verify your email address" or
>> "Confirm your email".  (My inbox is full of them.)  I see no reason to
>> disallow that here.
>>
>> Mandating that the subject line be non-human-readable seems like an
>> unnecessary barrier to adoption.
>>
>
> Are you expecting humans to be the primary interaction point?
>

Ideally, ACME-aware mail clients will handle these messages in some
beautiful, mostly-invisible way, but mail clients don't update as fast as
browsers.  Even many years after most users have ACME-aware clients, I
think a sizable fraction will have non-aware clients.

It almost seems that ensuring human unfriendliness is a feature, not a bug,
> towards the goal of automation.
>

That was my expectation after reading draft-06, but Alexey explained that
human-friendliness was in-scope, hence my suggestions.

This structure seems especially important if it has a chance to be adopted
> by publicly trusted CAs. One of the big concerns with existing validation
> approaches is bodies that are rich-text with links used for approval, and
> for which anti-spam or scanning engines inspect (“click”) and cause
> improper authorizations.
>

According to this draft, authorization requires sending a specially formed
reply email, so I think the risk of a scanning engine accidentally
authorizing is low.

The more structure, the better, towards preventing accidental
> authorizations.
>
> ACME responses already allow arbitrary prefix to accommodate naive clients.
>>>
>>> Similarly, for Section 3.2. Point 6, I would relax the requirement to
>>> state that this block must appear somewhere in the body.  That way, if the
>>> user sees the response message, it can provide some explanation of what is
>>> going on.
>>>
>>> Good idea. Changed.
>>>
>>> For Section 3.1 Point 5, I don't understand why the body is restricted
>>> to text/plain.  In particular, I think hyperlinks to explanations and
>>> instructions are likely to be helpful.  I also wonder whether support for
>>> multipart/multilingual could be useful.
>>>
>>> The body is irrelevant to ACME-aware clients, so it seems like there
>>> could be a lot of freedom in how this is constructed.
>>>
>>> This is true for the challenge email.
>>>
>> Yes, that's what I was referring to.
>>
>>> There is a requirement on S/MIME (if used) to provide header protection,
>>> but I agree that otherwise the body structure can be pretty flexible.
>>>
>>> Most email clients automatically convert HTTPS URLs to hyperlinks, which
>>> should make the silly schemes I'm imagining possible, but not very
>>> attractive, for ordinary users.
>>>
>>>> Best Regards,
>>>>
>>>> Alexey
>>>>
>>>> I assume this is deliberate, perhaps because of a desire to use
>>>> short-TTL S/MIME certificates that would be impractical to provision
>>>> manually, but the draft doesn't mention a rationale.
>>>>
>>>> On Thu, Mar 12, 2020 at 2:52 PM Salz, Rich <rsalz=
>>>> 40akamai.com@dmarc.ietf...org <40akamai.com@dmarc..ietf.org>> wrote:
>>>>
>>>>> This mail begins a one-week working group last call on
>>>>> https://datatracker.ietf.org/doc/draft-ietf-acme-email-smime/?include_text=1
>>>>>
>>>>>
>>>>>
>>>>> If you have comments or issues, please post here.
>>>>>
>>>>>
>>>>>
>>>>> If anyone wants to be a document shepherd, please contact the chairs.
>>>>>
>>>> Best Regards,
>>>
>>> Alexey
>>>
>>>
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>