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



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_64/
> 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-6517
> 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


Regards
 Eric