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

Ben Schwartz <> Sat, 02 May 2020 18:11 UTC

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From: Ben Schwartz <>
Date: Sat, 2 May 2020 14:11:02 -0400
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To: Alexey Melnikov <>
Cc: "Salz, Rich" <>, "" <>
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Subject: Re: [Acme] WG last call for draft-ietf-acme-email-smime-06
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On Sat, May 2, 2020 at 8:35 AM Alexey Melnikov <>

> Hi Ben,
> On 21/04/2020 01:12, Ben Schwartz wrote:
> On Wed, Apr 1, 2020 at 5:40 AM Alexey Melnikov <>
> 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

Mandating that the subject line be non-human-readable seems like an
unnecessary barrier to adoption.

> 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=
>> <>> wrote:
>>> This mail begins a one-week working group last call on
>>> If you have comments or issues, please post here.
>>> If anyone wants to be a document shepherd, please contact the chairs.
>> Best Regards,
> Alexey