Algorithm 2 TEE-enforced single-use self-parent.

Require: Event $e_n$, stored hash $h_{\mathrm{last}}$ Ensure: Signed event $(e_n,{\sigma }_n)$ or $\mathsf{Reject}$ 1:$h_{\mathrm{parent}}\leftarrow \mathrm{Hash}(e_n.\mathrm{self}\_\mathrm{parent})$ 2:if  $h_{\mathrm{parent}}=h_{\mathrm{last}}$  then 3:    ${\sigma }_n\leftarrow \mathrm{TEE}.\mathrm{Sign}\left(e_n\right)$ 4:    $h_{\mathrm{last}}\leftarrow \mathrm{Hash}\left(e_n\right)$               ▹ Update the hash for the next event 5:    return $(e_n,{\sigma }_n)$ 6:else 7:    return $\mathsf{Reject}$ 8:end if