papers/lux-pq-migration/sections/05-phase-3-pure-post-quantum.tex

\section{Phase 3: Pure Post-Quantum}


\textbf{Status: Future.}

Phase 3 removes classical cryptography from the consensus path entirely.

\subsection{Trigger Conditions}

Phase 3 activates when:
\begin{enumerate}[leftmargin=2em]
\item CRQCs demonstrably break elliptic curve cryptography in practice.
\item OR: the classical signature adds no security value (all realistic adversaries have quantum access).
\item AND: $>$99\% of network participants have upgraded to Phase 2+ software.
\end{enumerate}

\subsection{Changes from Phase 2}

\begin{center}
\begin{tabular}{lll}
\toprule
\textbf{Component} & \textbf{Phase 2} & \textbf{Phase 3} \\
\midrule
Per-block consensus & ML-DSA primary + BLS fallback & ML-DSA only \\
EC keys & Required (EVM compat) & Optional (legacy EVM only) \\
BLS keys & Required (fallback) & Deprecated \\
Block header & $\sim$3,500 B & $\sim$3,400 B (no BLS) \\
EVM addresses & secp256k1-derived & ML-DSA-derived (new format) \\
\bottomrule
\end{tabular}
\end{center}

\subsection{EVM Address Migration}

The most disruptive change in Phase 3 is EVM address derivation. Classical EVM addresses are derived from secp256k1 public keys ($\text{keccak256}(\text{pubkey})[12:32]$). Post-quantum addresses must derive from ML-DSA public keys.

Migration path:
\begin{enumerate}[leftmargin=2em]
\item Users register their ML-DSA public key via a binding transaction signed by their secp256k1 key.
\item The binding creates a mapping: $\text{classicalAddress} \leftrightarrow \text{pqPublicKey}$.
\item During Phase 3, both address formats are accepted. Transactions may be signed by either key.
\item Eventually, new accounts use only PQ-derived addresses.
\end{enumerate}