\chapter{The Pyramid: From Field to Signature, and Where You Come In}
\label{ch:pyramid}

\section{The view from the field layer}

Chapter~\ref{ch:field} left us holding a verified field. A signature scheme
is still three stories up. This closing chapter walks the remaining layers
--- what each one \emph{states}, what makes each one \emph{hard}, and where
the campaign stands as this book goes to press --- then hands you the map
and the keys.

\begin{center}
\begin{tikzpicture}[
  lay/.style={draw=ink2,thick,rounded corners=2pt,align=center,minimum height=1.0cm},
  st/.style={font=\footnotesize\color{ink2},anchor=west,align=left}
]
  \node[lay,fill=accentsoft,minimum width=3.0cm]  (sig) at (0,3.75) {\textbf{Signature}};
  \node[lay,fill=warnsoft,minimum width=5.4cm]    (sca) at (0,2.5)  {\textbf{Scalars mod $\boldsymbol{\ell}$}};
  \node[lay,fill=provensoft,minimum width=7.8cm]  (grp) at (0,1.25) {\textbf{Group law}};
  \node[lay,fill=codebg,minimum width=10.2cm]     (fld) at (0,0)    {\textbf{Field $\Fp$}};
  \node[st] at (5.7,0)    {\textbf{done}: certificates in 4 repos, axiom-clean};
  \node[st] at (5.7,1.25) {\textbf{done}: complete addition, all 4 forks};
  \node[st] at (5.7,2.5)  {\textbf{in progress}: foundations proven,\\ mul at the kernel frontier};
  \node[st] at (5.7,3.75) {\textbf{ahead}: awaits scalars;\\ hash axiomatized by design};
\end{tikzpicture}
\end{center}

\section{The group law: geometry becomes algebra}

An elliptic curve is a set of points $(x,y)$ satisfying an equation; for
Ed25519 it is the \emph{twisted Edwards} curve
$-x^2 + y^2 = 1 + d\,x^2 y^2$ over $\Fp$. The miracle: these points form a
\emph{group} under the addition law
\[
(x_1,y_1) + (x_2,y_2) \;=\;
\left(
\frac{x_1 y_2 + x_2 y_1}{1 + d\,x_1 x_2 y_1 y_2},\;
\frac{y_1 y_2 + x_1 x_2}{1 - d\,x_1 x_2 y_1 y_2}
\right).
\]
Two facts make this law a verifier's dream, and both carry Edwards-curve
signatures for exactly this reason. First, it is \textbf{complete}: for the
Ed25519 parameters those denominators are \emph{never zero} --- no special
cases for doubling, no branch for the identity, hence constant-time-friendly
code with no rarely-taken paths for bugs to hide in. (The proof, due to
Bernstein and Lange, is a jewel of quiet algebra: if a denominator vanished,
$d$ would have to be a square in $\Fp$ --- and it is not, which is a
\lean{decide}-scale fact away from primality.) Second, the implementation
represents points \emph{projectively} (extended coordinates $(X:Y:Z:T)$,
avoiding division entirely) --- so the layer has its own denotation,
$(X:Y:Z:T) \mapsto (X/Z, Y/Z)$, and its own commuting squares built on the
field layer's specs. Same movie, one floor up: the verified group law in the
companion repos is precisely the statement that projective point addition
implements the rational formula above, all bounds included, for each fork's
own extraction.

\section{Scalars: a second field, and a frontier}

The group of curve points has order $8\ell$ with
$\ell = 2^{252} + 27742\ldots$ prime. Signature arithmetic happens in
exponents --- multiples of points --- so it is arithmetic mod $\ell$: a
\emph{second} finite field, with its own Rust implementation (radix-52
limbs, Montgomery multiplication) and its own denotation bridge. Nothing
conceptually new --- which is itself the lesson: the method \emph{scales
sideways} without new ideas.

The engineering, however, has a frontier, and this book has told you enough
truth to locate it precisely. Scalar Montgomery multiplication mixes
$2^{256}$-scale coefficients into single certificate steps; this is the
kernel-capacity wall of Chapter~\ref{ch:field}, and it marks the current
working edge of the campaign: additions and the foundational constants are
certified (including the pleasing theorem that the code's constant
\code{L} \emph{is} $\ell$); the multiplication path is a construction site
with scaffolding --- decomposed lemmas, isolated carry steps ---
mid-assembly, honestly labeled in-repo.

\section{The apex: what ``verified signature'' will say}

EdDSA verification accepts $(R, s)$ on message $m$ under key $A$ iff
\[
8 s B \;=\; 8 R + 8\,H(R, A, m)\,A
\]
in the curve group ($B$ the base point, $H$ = SHA-512, the $8$s absorbing
the cofactor). The apex certificate will state: \emph{the extracted
verification routine returns true exactly when this equation holds} ---
given the two declared trusted-base entries you can already predict:
SHA-512 as an ideal hash (axiomatized by design --- hash function
correctness is a different mathematical universe), and the SIMD
point-multiplication backends (untranslatable, documented). Everything
between those declared boundaries and the field bedrock: kernel-checked,
axiom-clean, per fork.

Read that sentence again with Chapter~\ref{ch:honesty} eyes: it is a
\emph{smaller} claim than ``Ed25519 is verified!'' --- and that is exactly
why you can believe it.

\section{What you now know, and where to take it}

Take inventory. You can read a goal state and drive a proof; you know which
decision procedure owns which arithmetic fragment; you can build a
denotation bridge and state a two-clause spec; you can certify a prime with
a witness tree; you can audit anyone's certificate in one command and four
questions. That skill set is not Ed25519-specific --- it is the working
method of machine-checked mathematics applied to systems, and elliptic
curves were merely your first campaign.

Where to go from here, in increasing order of ambition:

\begin{itemize}[leftmargin=1.4em]
\item \textbf{Read a real proof end-to-end.} \code{FieldSpec.lean} in
  \code{dalek-ed25519-verified}, top to bottom, with this book as the
  decoder ring. Budget an afternoon; expect the odd hour of humility.
\item \textbf{Extend the pyramid.} The scalar layer's open lemmas are
  decomposed, labeled, and waiting; the repos' \code{CONTRIBUTING} notes
  state exactly what a finished brick looks like (spec shape, axiom
  audit, check-script entry). Frontier work, undergraduate-accessible.
\item \textbf{Verify something of yours.} Pick a 200-line pure function you
  actually use --- a parser, a checksum, a data structure --- write its
  denotation (what does it \emph{mean}?), state the square, prove it.
  The first solo bridge is the moment this stops being a course.
\item \textbf{Go deeper into the theory.} \emph{Theorem Proving in Lean 4}
  (the official text), \emph{Mathematics in Lean} (Mathlib's course), and
  the Lean Zulip --- an unusually welcoming expert community --- are the
  standard next doors.
\end{itemize}

\begin{aha}
One last reframe, the one this book was secretly about. ``Formal
verification'' sounds like bureaucracy --- forms, stamps, compliance. What
you actually practiced is closer to \emph{engineering's version of the
scientific method}: make the claim precise enough to be falsifiable, then
let an incorruptible referee try to falsify it, then publish the referee's
report with the assumptions itemized. Cryptography needed that discipline
first because its failures are silent and adversarial. It will not need it
last.
\end{aha}

\begin{tryit}
The graduation exercise. In the mini-system from
\code{exercises/Ch09.lean}, the file \code{exercises/Ch12.lean} plants a
\emph{deliberate off-by-one carry bug} in a variant \lean{add'} --- of
exactly the species from Chapter~\ref{ch:why}: correct on all limb pairs
except a thin boundary slice. Your final tasks: (1) write the spec ---
watch it \emph{refuse to prove}; (2) extract the counterexample from the
stuck goal state; (3) confirm by \lean{\#eval}; (4) fix the code and finish
the proof. That arc --- spec, refusal, counterexample, fix, certificate ---
is the entire profession in miniature. Welcome to it.
\end{tryit}

\begin{checkpoint}
The book's ending is a beginning, so the final checkpoint is prospective:
you should be able to (1) state what each pyramid layer claims and which
denotation it rides on; (2) explain to a security engineer why completeness
of the Edwards law matters to \emph{code}; (3) locate the current frontier
and say precisely why it is hard; and (4) name the next proof \emph{you}
intend to write. The authors of the companion repositories left the
scaffolding up on purpose.
\end{checkpoint}