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

Alexey Melnikov <> Sat, 02 May 2020 12:35 UTC

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From: Alexey Melnikov <>
To: Ben Schwartz <>
Cc: "Salz, Rich" <>, "" <>
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Subject: Re: [Acme] WG last call for draft-ietf-acme-email-smime-06
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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.
>>     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?

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.

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
>>     <
>>     <>> 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,