[Acme] Predictability of http-01 challenge response

[Hugo Landau <[e-mail]>]

Someone pointed out to me that the http-01 challenge can be satisfied
using no knowledge other than the account fingerprint.

For example, as an nginx config:

  location ~ "^/\.well-known/acme-challenge/(-_a-zA-Z0-9]+)$" {
    default_type text/plain;
    return 200 "$1.ACCOUNT_THUMBPRINT";
  }

Was this an intended design feature of the http-01 challenge?

Perhaps more pertinently I should ask, is this property desirable?

This reduces the http-01 challenge to what is essentially an account key
nomination, alebit with a random token (probably needed to satisfy CA/B
forum requirements). I've previously suggested that e.g. the DNS
challenge be changed similarly. In that regard it's quite convenient.
I'm just mildly worried that this property doesn't seem to have been
particularly noticed or discussed, in terms of its security properties.

Musing a little, if this property was deemed desirable, the possibility
of multiple account keys could be better accommodated by including say,
a hash of the account key thumbprint in the URL. Reasoning: The response
value shouldn't match the request value to guard against webservers
which for some reason echo everything they see. A hash of an account key
thumbprint is sufficient to identify an account amongst a list of known
accounts, for the purposes of determining which account an ACME server
is soliciting an endorsement for.

OTOH if this property was deemed undesirable the token in the URL could
be hashed; the response would then prove the server knew the token.

Hugo Landau