Re: [Acme] [Technical Errata Reported] RFC8555 (6950)

[Lloyd W <[e-mail]>]

I _think_, two years ago, I was very interested in entropy, and how this Base64 specification seemed to us to be quite sloppy and open to misinterpretation, with expectations of entropy not being met, resulting weaknesses in protocols relying on it, etc.

But that was two years ago, when I was interested in entropy.

I'm frankly now far more interested in whether errata response and resolution is clearly defined in process and outcomes, with expected response times, whether waiting two years for a response to a raised erratum is reasonable, how the response baton gets passed along to ensure someone knowledgeable responds in a timely fashion, etc.

so, cc'ing IESG for their view on that.

best

Lloyd Wood 

[e-mail]

still jetlagged!

On 5 Jan 2024, at 23:49, Deb Cooley <[e-mail]> wrote:



My question to you would be:  which RFC will make the most impact?  Seems like your comments weren't really targeted at ACME per se, but at how to measure entropy using Base64 encoding?  I have not read RFC 4086 closely, but an RFC like that would impact all of them.

Just my thoughts... 

Deb Cooley

On Fri, Jan 5, 2024 at 7:43 AM <[e-mail]> wrote:

Thanks for that.

Well, I stand by my detailed comments of (checks dates) two years ago. Which I had forgotten I ever even made!

I know, covid and all, but lately in the past few years I’ve been sensing a vibe of why-even-bother-submitting errata anyway. (and sitting in a Tokyo airport transit lounge, heavily jetlagged, is really not the place for me to do any detailed analysis.)

Perhaps reframing it as some security concern, since entropy, might prompt wider review? security types are quite keen on this stuff, and do tend to like rigour in specifications to close off implementation misunderstandings loopholes.

best

Lloyd Wood

[e-mail]

On Friday, January 5, 2024, 21:14, Deb Cooley <[e-mail]> wrote:

I did some reading, some consulting, and some pondering.  I want to reject this errata.

1.  The original paragraph: 

token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy.
      It MUST NOT contain any characters outside the base64url alphabet
      and MUST NOT include base64 padding characters ("=").  See
      [RFC4086] for additional information on randomness requirements.

In my opinion this is sufficient.  It specifies how much entropy (w/ a ref), and specifies what cannot be included (padding, non-base54 characters).

Let me know if you have other thoughts.

Deb

On Thu, Jan 4, 2024 at 7:41 AM Deb Cooley <[e-mail]> wrote:

opinions?  Does entropy have to be measured as a base64 encoded value? 

Deb

On Mon, May 2, 2022 at 4:31 AM RFC Errata System <[e-mail]> wrote:

The following errata report has been submitted for RFC8555,
"Automatic Certificate Management Environment (ACME)".

--------------------------------------
You may review the report below and at:
https://www.rfc-editor.org/errata/eid6950" target="_blank">https://www.rfc-editor.org/errata/eid6950

--------------------------------------
Type: Technical
Reported by: Lloyd Wood <[e-mail]>

Section: GLOBAL

Original Text
-------------
token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy.

Corrected Text
--------------
token (required, string):  A random value that uniquely identifies
      the challenge.  This value MUST have at least 128 bits of entropy, which in the
      base64url alphabet means a minimum string length of 22 characters if the full
      scope of the base64url alphabet is in use in the token, by:
                        log2(64^22) = 132 bits of entropy



Notes
-----
This standards-track document doesn't specify the string ramifications for entropy; I'd expect it to be called out to implementers, just the once, and then referred to later at other tokens.

If entropy is log2 the number of possible characters (64 if full base64url set of chars is in use) then
log2 (64^21) = 126
log2 (64^22) = 132

so a minimum of 22 characters are needed to get a minimum of 128 bits of entropy in the token.

But, if the random value is specified using a subset of the base64url, say because the implementer doesn't like or use CAPITALS or (most likely) the punctuation symbols, then the token must necessarily be longer to meet the local implementer entropy requirement (though just losing only the punctuation marks means you're still good and meet the requirement with 22 characters). Not sure that matters so much on the wire.

I also have editing nits about base64url being defined clearly in ABNF just for Replay-Nonce:, but then both 'base64 alphabet' and 'base64url alphabet' are in use in the document, and base64url references are to RFC4648 via RFC7515, but those are to Base64url, not to base64url... it all seems a bit inconsistent editingwise. So all the references to 'base64 alphabet' should be to 'base64url alphabet' as defined in the doc, but it should really be 'Base64url alphabet' to be consistent with references?

(I really think that it should have been called 'Base-64_url alphabet' way back when to enphasise the punctuation use, but that ship has sailed.)

To me, 'base64 alphabet' is the a-zA-Z subset of base64... I think the document could be much clearer in this regard, and I hope any doc revisions taking into account all the other errata raised consider this too.

My thanks to Lee Maguire for pointing much of this out.

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-------------
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--------------------------------------
RFC8555 (draft-ietf-acme-acme-18)
--------------------------------------
Title               : Automatic Certificate Management Environment (ACME)
Publication Date    : March 2019
Author(s)           : R. Barnes, J. Hoffman-Andrews, D. McCarney, J. Kasten
Category            : PROPOSED STANDARD
Source              : Automated Certificate Management Environment
Area                : Security
Stream              : IETF
Verifying Party     : IESG