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I used to build TDE in Live Build but haven't done so for a couple of years. Anyway you need to download the appropriate keyring for TDE from TDE itself, read the live build manual (it tells you exactly what to do) and place the relevant file(s) in the relevant place in the build environment. Live build will see the keyring, install it, read the repositories, and install the packages you tell it to install plus their dependencies and recommends if you allow installation of recommends.