There is no such thing as a “package”. It is a repository of binary data with references to data in it (ala git). The whole repo and all data is gpg signed.
> ostree show flathub:runtime/org.kde.Platform/x86_64/6.6
commit a7443e846cf67d007fcecda5c9dc27844001cfb8929064395cfc25c6d71d9474
Parent: 23107550082daf3b2892a4a0db2543838578ca882340a756b988bc5c1614540c
ContentChecksum: 607ba9475d32a24c51509bc7919f5a93d401f8f7198c30ad93ad74051d966c41
Date: 2024-01-30 13:55:08 +0000
build of org.kde.Sdk, Tue Jan 30 11:23:00 UTC 2024 (5998d2f3ef21414d14f066ab91fa44e5aef65b90)
Name: org.kde.Platform
Arch: x86_64
Branch: 6.6
Built with: Flatpak 1.14.4
Found 1 signature:
Signature made Tue 30 Jan 2024 12:21:18 PM CST using RSA key ID 562702E9E3ED7EE8
Good signature from "Flathub Repo Signing Key <flathub@flathub.org>"
Primary key ID 4184DD4D907A7CAE
Key expires Mon 14 Jun 2027 08:19:40 AM CDT
Primary key expires Mon 14 Jun 2027 08:18:56 AM CDT