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Re: [Acme] Remove the hyphen from the acceptable character set for _acme-challenge TXT records.

Michael Richardson <mcr+ietf@sandelman.ca> Thu, 26 November 2020 01:34 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: "Manger, James" <James.H.Manger@team.telstra.com>, Mark Tomlin <Mark@mimocad.io>, "acme@ietf.org" <acme@ietf.org>
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Date: Wed, 25 Nov 2020 20:34:31 -0500
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Subject: Re: [Acme] Remove the hyphen from the acceptable character set for _acme-challenge TXT records.
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Manger, James <James.H.Manger@team.telstra.com> wrote:
    > An Acme CA may choose the dns-01 challenge token, but what goes in the
    > DNS TXT record is base64url(SHA-256(f(token and account key))).

I don't see this anywhere in RFC8555.
I see section 8.1, which deals with a keyAuthorization.

section 8.4 says that the token must have 128-bits of entropy,
and that it should restrict itself to the base64url alphabet.

Beyond that, I don't see any formula like above: an ACME server could, as you
say below, notice when needed "-" in the result and try again.
I think we are agreeing, but you are just telling me what current code does?

    > So to avoid hyphens in that result the CA would need to: pick a random
    > token; do the hash & base64url calculations; then repeat with a new
    > random token if any hyphens are present. It will only take 2 tries on
    > average ; - )

--
Michael Richardson <mcr+IETF@sandelman.ca>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide

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