papers/chain-first-architecture/sections/04-worm-audit-trail.tex

\section{WORM Audit Trail}

\label{sec:worm}
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The blockchain is a natural WORM (Write Once, Read Many) storage
medium: once a transaction is included in a finalized block, it cannot
be modified or deleted without forging digital signatures or controlling
a supermajority of validators.

\subsection{Hash Chain Integrity}

Each block contains a hash of the previous block, forming a hash
chain from genesis to the current head:

\[
  H_h = \mathsf{Hash}(H_{h-1} \| \mathsf{TxRoot}_h \|
  \mathsf{StateRoot}_h \| \mathsf{Timestamp}_h)
\]

\begin{theorem}[Hash Chain Tamper Detection]
\label{thm:hashchain}
Any modification to any block $h' \leq h$ in the chain is detectable
by any party that knows the current block hash $H_h$.
\end{theorem}

\begin{proof}
Suppose block $h'$ is modified, producing a different hash
$H'_{h'} \neq H_{h'}$.  Block $h'+1$ includes $H_{h'}$ in its
preimage.  Modifying block $h'$ without modifying block $h'+1$ causes
the hash chain to break: $\mathsf{Hash}(H'_{h'} \| \cdots) \neq
H_{h'+1}$.  Therefore the adversary must also modify block $h'+1$,
which in turn requires modifying block $h'+2$, and so on up to block
$h$.  This cascade requires modifying every block from $h'$ to $h$,
which requires forging the digital signatures of the block producers
for each of those blocks.  Under the EUF-CMA security of the signature
scheme, this is infeasible.

Any party that stores $H_h$ (the current head hash) can verify the
integrity of any earlier block by recomputing the hash chain.  If
the recomputed $H_h$ differs from the stored $H_h$, a modification
has occurred.
\end{proof}

\subsection{Compliance with Rule 17a-4}

SEC Rule 17a-4(f) requires that records be preserved in a format that:
\begin{enumerate}[nosep]
  \item Preserves records exclusively in a non-rewritable,
        non-erasable format (WORM).
  \item Verifies automatically the quality and accuracy of the
        recording process.
  \item Serializes the original and duplicate units of storage media
        and provides a time-date for the required period of retention.
\end{enumerate}

The chain-first architecture satisfies all three requirements:
\begin{enumerate}[nosep]
  \item The blockchain is WORM by construction (hash chain + digital
        signatures).
  \item The indexer deterministically replays chain events; any
        discrepancy between chain and cache is detected automatically.
  \item Every block carries a timestamp, and the chain provides a
        total ordering of all events with cryptographic serialization
        (block height + transaction index).
\end{enumerate}

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