Indeed, osc signkey returns a different key ID.
It appears to want this key: https://download.opensuse.org/repositories/openSUSE:/Backports:/SLE-15-SP3:/Update/standard/repodata/repomd.xml.key

I wrote a small bash function:

# rpmimportkey() {
> sudo rpm --import - << _END_KEY  
> $(curl "$1")
> _END_KEY
> }

then called it:
# rpmimportkey https://download.opensuse.org/repositories/openSUSE:/Backports:/SLE-15-SP3:/Update/standard/repodata/repomd.xml.key

they key is now in RPM's database:
# rpm -qa | grep 65176565
gpg-pubkey-65176565-5d94a381


Erico Mendonca
Premium Services
SUSE

From: eric.schirra@localdomain.dehaem <eric.schirra@localdomain.dehaem> on behalf of Eric Schirra <ecsos@opensuse.org>
Sent: Monday, March 22, 2021 11:54 AM
To: opensuse-packaging@opensuse.org <opensuse-packaging@opensuse.org>
Subject: Re: Public Key for Leap 15.3
 
Same error.

When i use "osc -A https://api.opensuse.org signkey openSUSE:Backports:SLE-15-
SP3" separat on console it say:

openSUSE:Backports:SLE-15-SP3 has no key, trying openSUSE:Backports
openSUSE:Backports has no key, trying openSUSE
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: GnuPG v1.4.5 (GNU/Linux)
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=R8Ym
-----END PGP PUBLIC KEY BLOCK-----


What i wonder:

openSUSE:Backports:SLE-15-SP3 has no key
openSUSE:Backports has no key

So the error from osc is right:

openSUSE:Backports:SLE-15-SP3 public key not available.


So. From where ca i get the key?
Or. Where is the key?

Regards
 Eric

PS: I'am under Leap 15.2. I i do such build since many years.



Am Montag, 22. März 2021, 15:35:49 CET schrieb Erico Mendonca:
> You can ask OSC directly for a project key and import it:
>
> # sudo rpm --import - <<_END_KEY
> $(osc -A https://api.opensuse.org signkey openSUSE:Backports:SLE-15-SP3)
> _END_KEY
>
>
> [cid:bcfc4406-ba65-4406-a808-379bb84f038b]
> Erico Mendonca
> Premium Services
> SUSE
>
>
> ________________________________
> From: eric.schirra@localdomain.dehaem <eric.schirra@localdomain.dehaem> on
> behalf of Eric Schirra <ecsos@opensuse.org> Sent: Monday, March 22, 2021
> 11:26 AM
> To: opensuse-packaging@opensuse.org <opensuse-packaging@opensuse.org>
> Subject: Re: Public Key for Leap 15.3
>
> Am Montag, 22. März 2021, 14:45:44 CET schrieb Erico Mendonca:
> > ________________________________
> > From: eric.schirra@localdomain.dehaem <eric.schirra@localdomain.dehaem> on
> > behalf of Eric Schirra <ecsos@opensuse.org> Sent: Monday, March 22, 2021
> > 9:41 AM
> > To: opensuse-packaging@opensuse.org <opensuse-packaging@opensuse.org>
> > Subject: Public Key for Leap 15.3
> >
> > Hello,
> >
> > can someone tell me where i can get public keys for Backports:SLE-15-SP3
> > to
> > build Leap 15.3 packages under Leap 15.2?
> >
> > Example for my error:
> > Verifying integrity of cached packages
> > using keys from SUSE:SLE-15:Update, SUSE:SLE-15:GA, SUSE:SLE-15-SP3:GA,
> > SUSE:SLE-15-SP2:Update, SUSE:SLE-15-SP2:GA, SUSE:SLE-15-SP1:Update, games,
> > SUSE:SLE-15-SP1:GA, openSUSE:Leap, openSUSE
> > warning:
> > /var/tmp/osbuild-packagecache/openSUSE:Backports:SLE-15-SP3/standard/
> > x86_64/libminiupnpc-devel-2.0.20171102-bp153.1.33.x86_64.rpm: Header V3
> > RSA/ SHA256 Signature, key ID 65176565: NOKEY
> > /var/tmp/osbuild-packagecache/openSUSE:Backports:SLE-15-SP3/standard/x86_6
> > 4/ libminiupnpc-devel-2.0.20171102-bp153.1.33.x86_64.rpm : public key not
> > available
> >
> > Is this the one you're looking for?
> >
> > http://download.opensuse.org/distribution/leap/15.3/repo/oss/gpg-pubkey-65
> > 17 6565-59787af5.asc
> >
> >
> > [cid:c2a2e1f4-bc6f-4abe-b63a-64eb2a2ed3f5]
> > Erico Mendonca
> > Premium Services
> > SUSE
>
> No it isn't.
> I use "osc build openSUSE_Leap_15.3" on my console under Leap 15.2
>
> And it begins to download needed packages.
> But break at the end with error:
>    public key not available
> from
>   openSUSE:Backports:SLE-15-SP3
>
> When all downloaded and i start osc build again than it does not build and
> rise up the error:
>
> Verifying integrity of cached packages
> using keys from SUSE:SLE-15:Update, SUSE:SLE-15:GA, SUSE:SLE-15-SP3:GA,
> SUSE:SLE-15-SP2:Update, SUSE:SLE-15-SP2:GA, home:ecsos, SUSE:SLE-15-
> SP1:Update, SUSE:SLE-15-SP1:GA, openSUSE:Leap, openSUSE
> warning:
> /var/tmp/osbuild-packagecache/openSUSE:Backports:SLE-15-SP3/standard/
> noarch/golang-packaging-15.0.12-bp153.1.21.noarch.rpm: Header V3 RSA/SHA256
> Signature, key ID 65176565: NOKEY
> /var/tmp/osbuild-packagecache/openSUSE:Backports:SLE-15-SP3/standard/noarch/
> <my_downloaded packages>.noarch.rpm : public key not available
>