[Evolution] Encrypting with GnuPG: `no imported public key` if key is not valid (not ultimate trusted)

[Tim Rausch <mail.gnome.org@tim.rausch.family>]

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Hello,

I'm running Evolution 3.26.1-1 on Debian stretch (amd64) and would like
to use GnuPG to secure my mails. My GPG private key is stored on a
YubiKey (but that shouldn't matter).

I imported my own key and the public keys of the people I want to write
to GPG and selected my key in the preferences of Evolution. Now,
decrypting mails sent to me works as well as sending signed (not
encrypted messages). Sending encrypted messages to myself also works as
the ownertrust of my own key is set to "ultimate".

The issues now is that I can not send mails to people whos keys are not
valid, that means they don't have ultimate ownertrust and are not
signed by me. Sending an encrypted mail to such a key fails with
Evolution showing the following error:

    Could not create message.
    You may need to select different mail options.

    Detailed error: Failed to encrypt: Invalid recipient 
    <recipients@mail.address>  specified. A common issue is that the
    gpg2 doesn’t have imported public key for this recipient. 

But GPG really has the recipient's key imported (`echo "foobar" | gpg
--encrypt -r recipients@mail.address` works). 

The output of `strace -p $(pidof evolution) -f -e trace=execve` shows
the command executed by Evolution to encrypt a mail:

    [pid  4537] execve("/usr/bin/gpg2", ["gpg2", "--verbose",
    "--no-secmem-warning", "--no-greeting", "--no-tty", "--batch",
    "--yes", "--status-fd=67", "--encrypt", "--armor", "-u",
    "my@mail.address", "-r", "<recipients@mail.address>", "--output",
    "-"], [/* 34 vars */]) = 0

When I try to run this command manually on shell (replacing "--status-
fd=67" with "--status-fd=1"), I get the following:

    [GNUPG:] KEY_CONSIDERED <recipient's key fingerprint> 0
    gpg: using pgp trust model
    gpg: using subkey <recipient's encryption subkey id> instead of
    primary key <recipient's primary key id>
    [GNUPG:] KEY_CONSIDERED <recipient's key fingerprint> 0
    gpg: automatically retrieved 'recipients@mail.address' via Local
    gpg: <recipient's encryption subkey id>: There is no assurance
    this key belongs to the named user
    [GNUPG:] INV_RECP 10 recipients@mail.address
    [GNUPG:] FAILURE encrypt 53
    gpg: [stdin]: encryption failed: Unusable public key

I think the issue is that the recipient's public key is listed as:

    [ unknown] (1). Pecipient's Name <recipients@mail.address>

in `gpg -k`. `[ unknown]` is AFAIK the key's validity. If it is `[
unkonwn]` this causes GPG to ask interactively whether you really want
to use this key:

    ➜  ~ echo "foo" | gpg --encrypt -r recipients@mail.address --armor
    gpg: automatically retrieved 'recipients@mail.address' via Local
    gpg: <recipient's encryption subkey id>: There is no assurance
    this key belongs to the named user
    sub  rsa4096/<recipient's encryption subkey id> 2015-10-14
    Recipient's Name <recipients@mail.address>
     Primary key fingerprint: <recipient's key fingerprint>
          Subkey fingerprint: <recipient's encryption subkey
                               fingerprint>

    It is NOT certain that the key belongs to the person named
    in the user ID.  If you *really* know what you are doing,
    you may answer the next question with yes.

    Use this key anyway? (y/N) y
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    [...]
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I think this behaviour of GPG causes the encryption in Evolution to
fail. 

Resetting Evolution and GnuPG didn't fix the problem. When running
Evolution as another user, the issue also appeared.

Is this issue already known or could this problem also be caused by
something "on my side"? What can I do to fix this problem?

Thank you very much and best regards,
Tim Rausch

P.S. I asked that question on StackExchange (https://unix.stackexchange
.com/questions/401920/), there might be some further details.

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