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Re: [openpgp] Signer's User ID

Thijs van Dijk <schnabbel@inurbanus.nl> Thu, 15 September 2016 10:24 UTC

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References: <87mvj9mrs9.wl-neal@walfield.org>
From: Thijs van Dijk <schnabbel@inurbanus.nl>
Date: Thu, 15 Sep 2016 12:23:55 +0200
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To: "Neal H. Walfield" <neal@walfield.org>
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Subject: Re: [openpgp] Signer's User ID
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Good catch. I think being explicit about the expected contents of this
packet also makes the next sentence redundant (the one that prohibits
embedding a UAT).

-Thijs

--
Thijs van Dijk

6A94 F9A2 DFE5 40E3 067E  C282 2AFE 9EFA 718B 6165

On 15 September 2016 at 11:33, Neal H. Walfield <neal@walfield.org> wrote:

> RFC 4880 says:
>
>   5.2.3.22.  Signer's User ID
>
>    (String)
>
>    This subpacket allows a keyholder to state which User ID is
>    responsible for the signing.  Many keyholders use a single key for
>    different purposes, such as business communications as well as
>    personal communications.  This subpacket allows such a keyholder to
>    state which of their roles is making a signature.
>
>    This subpacket is not appropriate to use to refer to a User Attribute
>    packet.
>
> In GnuPG, we use the user id's email address.  Thus, if a key has the
> user id "Neal H. Walfield" <neal@walfield.org>, and the user runs:
>
>   echo data to sign | gpg --default-key neal@walfield.org -s
>
> or
>
>   echo data to sign | gpg --default-key 'Neal H. Walfield' -s
>
> Then, in both cases we include this subpacket in the signature packet
> with the string "neal@walfield.org".
>
>
> Note: we use data for locating the key via DANE or WKS as well as in
> our TOFU implementation to identify the intended binding.
>
>
> Since, IMO, the spec is unclear about the contents of this field and
> Werner pointed out that another implementation could reasonably use
> the hash of the user id here, I think it makes sense to clarify
> exactly what is expected to increase interoperability.  My proposal to
> standardize GnuPG's behavior is:
>
>   5.2.3.22.  Signer's User ID
>
>    (String)
>
>    This subpacket allows a keyholder to state which User ID is
>    responsible for the signing.  Many keyholders use a single key for
>    different purposes, such as business communications as well as
>    personal communications.  This subpacket allows such a keyholder to
> *  state which of their roles is making a signature.  The value of
> *  this subpacket is a byte-for-byte copy of the RFC 2822 mail name-addr
> *  portion of the User ID.
>
>    This subpacket is not appropriate to use to refer to a User Attribute
>    packet.
>
> Thanks,
>
> :) Neal
>
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