I think the checksum is wrong in "ubuntu-touch/stable/hammerhead/index.json".
The "shasum /pool/keyring-f812366ff5018823acd3918f65837f7ade9679431e6e5ca6627a4e48f8dd643b.tar.xz -a 256" is not "3cde8858c4feea1b4f4055ee86800d3c500a5cfc7fc26805bf2bc6613cff5e9c".