diff --git a/CHANGELOG.md b/CHANGELOG.md
index c22f2cc..c2d2639 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,12 @@ Pre-1.0, breaking changes bump the MINOR version.
 ## [Unreleased]
 
 ### Added
+- **Optional spored signature verification** (#26) — an `EC2Provider`
+  `sporedSigningPublicKey` (PEM) makes the bootstrap verify the downloaded
+  `spored`'s detached signature (`openssl`, fail-closed) against a launcher-held
+  key before install, proving authenticity — not just the SHA256 checksum
+  (integrity). Ports the Go bootstrap's `SPORED_SIG_VERIFY` path. Default stays
+  checksum-only, matching the Go tool when no key is compiled in.
 - **Lifecycle-hook tags** (#25) — emit the `spawn:*` tags for daemon-enforced
   hooks so an instance spawn-ts launches is honored by a real spored (spawn-ts,
   a browser launcher, can't run them itself): `pre-stop` (+timeout),
diff --git a/docs/architecture.md b/docs/architecture.md
index c66e735..53d1eb9 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -44,7 +44,13 @@ tool; the GUI and terminal are two consumers of that library.
 - `ec2.ts` — `EC2Provider` over `@aws-sdk/client-ec2` v3; targets real AWS or a
   substrate emulator by endpoint.
 - `userdata.ts` — the instance bootstrap that installs `spored`, so instances
-  self-terminate even with the browser closed.
+  self-terminate even with the browser closed. It downloads the arch-appropriate
+  `spored` from the regional S3 bucket and **always verifies a SHA256 checksum**
+  (guards corruption). Optionally, when an `EC2Provider` is given a
+  `sporedSigningPublicKey`, it also **verifies a publisher signature** (`openssl`
+  against a key carried by the launcher, not the binary's bucket — fail-closed),
+  proving authenticity. Checksum-only is the default, matching the Go tool when
+  no signing key is compiled in.
 
 ### `src/cli`
 
diff --git a/src/aws/ec2.test.ts b/src/aws/ec2.test.ts
index 0872c98..832fc2c 100644
--- a/src/aws/ec2.test.ts
+++ b/src/aws/ec2.test.ts
@@ -190,6 +190,24 @@ describe("EC2Provider.launch", () => {
     await provider().launch(baseSpec, T0);
     expect(sent.some((c) => c instanceof DescribeImagesCommand)).toBe(false);
   });
+
+  it("threads the signing key into the bootstrap user-data when configured", async () => {
+    handler = () => ({ Instances: [{ InstanceId: "i-1", State: { Name: "pending" } }] });
+    const pem = "-----BEGIN PUBLIC KEY-----\nMFkwEwYHkeymaterial==\n-----END PUBLIC KEY-----";
+    await provider({ sporedSigningPublicKey: pem }).launch(baseSpec, T0);
+    const b64 = lastOf(RunInstancesCommand).input.UserData!;
+    const script = Buffer.from(b64, "base64").toString("utf8");
+    expect(script).toContain("spored-signing-key.pem");
+    expect(script).toContain("MFkwEwYHkeymaterial==");
+    expect(script).toContain("openssl dgst -sha256 -verify");
+  });
+
+  it("omits signature verification when no key is configured", async () => {
+    handler = () => ({ Instances: [{ InstanceId: "i-1", State: { Name: "pending" } }] });
+    await provider().launch(baseSpec, T0);
+    const script = Buffer.from(lastOf(RunInstancesCommand).input.UserData!, "base64").toString("utf8");
+    expect(script).not.toContain("spored-signing-key.pem");
+  });
 });
 
 describe("archForInstanceType", () => {
diff --git a/src/aws/ec2.ts b/src/aws/ec2.ts
index 97e85f8..0c1c2e0 100644
--- a/src/aws/ec2.ts
+++ b/src/aws/ec2.ts
@@ -46,6 +46,12 @@ export interface EC2ProviderOptions {
   iamInstanceProfile?: string;
   /** Login username for bootstrap (default ec2-user). */
   username?: string;
+  /**
+   * PEM spore.host signing public key. When set, launched instances verify the
+   * spored binary's signature before running it (fail-closed). Absent = the
+   * bootstrap relies on the SHA256 checksum only. See userdata.ts.
+   */
+  sporedSigningPublicKey?: string;
 }
 
 export class EC2Provider implements Provider {
@@ -79,6 +85,7 @@ export class EC2Provider implements Provider {
         publicKey: this.opts.publicKey,
         command: spec.onComplete ? undefined : undefined, // workload wiring is a later feature
         sessionTimeoutMs: spec.sessionTimeoutMs,
+        sporedSigningPublicKey: this.opts.sporedSigningPublicKey,
       }),
     );
 
diff --git a/src/aws/userdata.test.ts b/src/aws/userdata.test.ts
index cefe47c..474685e 100644
--- a/src/aws/userdata.test.ts
+++ b/src/aws/userdata.test.ts
@@ -79,6 +79,33 @@ describe("buildLinuxBootstrap", () => {
     expect(s).not.toContain("releases/latest/download");
     expect(s).not.toContain("github.com/spore-host/spawn/releases");
   });
+
+  it("omits signature verification by default (checksum only)", () => {
+    const s = buildLinuxBootstrap({ username: "ec2-user" });
+    expect(s).not.toContain("spored-signing-key.pem");
+    expect(s).not.toContain("openssl dgst");
+    expect(s).toContain("sha256sum"); // checksum still present
+  });
+
+  it("adds a fail-closed signature-verify block when a signing key is supplied", () => {
+    const pem = "-----BEGIN PUBLIC KEY-----\nMFkwEwYHtestkey==\n-----END PUBLIC KEY-----";
+    const s = buildLinuxBootstrap({ username: "ec2-user", sporedSigningPublicKey: pem });
+    expect(s).toContain("/etc/spawn/spored-signing-key.pem");
+    expect(s).toContain("MFkwEwYHtestkey==");
+    expect(s).toContain('SIG_URL="${CHECKSUM_URL%.sha256}.sig"');
+    expect(s).toContain("openssl dgst -sha256 -verify");
+    // Fail-closed: refuses on a missing sig and on a bad sig.
+    expect(s).toContain("refusing to run an unsigned binary");
+    expect(s).toContain("signature verification FAILED");
+    // Runs before the atomic install (verify then mv).
+    expect(s.indexOf("openssl dgst")).toBeLessThan(s.indexOf("/usr/local/bin/spored"));
+  });
+
+  it("treats a blank signing key as disabled", () => {
+    expect(buildLinuxBootstrap({ username: "ec2-user", sporedSigningPublicKey: "   " })).not.toContain(
+      "spored-signing-key.pem",
+    );
+  });
 });
 
 describe("encodeUserData", () => {
diff --git a/src/aws/userdata.ts b/src/aws/userdata.ts
index e341745..f7fa77a 100644
--- a/src/aws/userdata.ts
+++ b/src/aws/userdata.ts
@@ -33,6 +33,16 @@ export interface BootstrapOptions {
    * does NOT stop/terminate the instance (that's the idle-instance lifecycle).
    */
   sessionTimeoutMs?: number;
+  /**
+   * PEM-encoded spore.host signing PUBLIC key. When set, the bootstrap verifies
+   * the downloaded spored's detached signature against this key (fail-closed)
+   * before installing — proving authenticity, not just integrity. The key is
+   * carried by the launcher (trusted), NOT served from the binary's S3 bucket,
+   * so a bucket compromise can't forge it (spore-host#440). Absent = checksum
+   * only (guards corruption), matching the Go tool's default when no key is
+   * compiled in.
+   */
+  sporedSigningPublicKey?: string;
 }
 
 /**
@@ -64,6 +74,11 @@ chmod 600 /etc/spawn/command
   // shells. Seconds are computed here so the script needs no duration parser.
   const sessionBlock = buildSessionTimeoutBlock(opts.sessionTimeoutMs ?? 0);
 
+  // Optional publisher-signature verification of the spored binary. Empty when
+  // no signing key is supplied (checksum-only, the default). Runs after the
+  // SHA256 check and before the atomic install; fail-closed on any mismatch.
+  const sigVerifyBlock = buildSigVerifyBlock(opts.sporedSigningPublicKey);
+
   return `#!/bin/bash
 set -e
 
@@ -122,7 +137,7 @@ if curl -f -s -o /tmp/spored.sha256 "$CHECKSUM_URL" 2>/dev/null; then
     rm -f "$SPORED_TMP"; exit 1
   fi
 fi
-
+${sigVerifyBlock}
 # Atomic install — rename works even if an old spored is executing (#27).
 chmod +x "$SPORED_TMP"
 mv -f "$SPORED_TMP" /usr/local/bin/spored
@@ -158,6 +173,42 @@ systemctl start spored
 `;
 }
 
+/**
+ * Bootstrap fragment that verifies the spored binary's detached signature
+ * against a supplied signing PUBLIC key before install. Empty when no key is
+ * given (checksum-only default). Ports the Go bootstrap's SPORED_SIG_VERIFY
+ * path (pkg/launcher/bootstrap.go): download `<binary>.sig` next to the
+ * checksum URL, base64-decode to DER, `openssl dgst -sha256 -verify`,
+ * fail-closed on missing/invalid signature.
+ */
+function buildSigVerifyBlock(publicKeyPem?: string): string {
+  if (!publicKeyPem || !publicKeyPem.trim()) return "";
+  return `
+# Publisher-signature verification (spore-host#440). The checksum above only
+# proves the download wasn't corrupted (it's served from the same bucket as the
+# binary); this proves authenticity against a key carried by the launcher, not
+# the bucket. Fail-closed.
+mkdir -p /etc/spawn
+cat > /etc/spawn/spored-signing-key.pem <<'EOFSPOREDPUBKEY'
+${publicKeyPem.trim()}
+EOFSPOREDPUBKEY
+SIG_URL="\${CHECKSUM_URL%.sha256}.sig"
+if ! curl -f -s -o /tmp/spored.sig "$SIG_URL"; then
+  echo "spored signature not found at $SIG_URL — refusing to run an unsigned binary" >&2
+  rm -f "$SPORED_TMP"; exit 1
+fi
+# The .sig is base64-encoded DER (ECDSA_SHA_256); decode to raw DER for openssl.
+base64 -d /tmp/spored.sig > /tmp/spored.sig.der 2>/dev/null || cp /tmp/spored.sig /tmp/spored.sig.der
+if openssl dgst -sha256 -verify /etc/spawn/spored-signing-key.pem -signature /tmp/spored.sig.der "$SPORED_TMP" >/dev/null 2>&1; then
+  echo "spored signature verified (spore.host)"
+else
+  echo "spored signature verification FAILED — refusing to run spored" >&2
+  rm -f "$SPORED_TMP" /tmp/spored.sig /tmp/spored.sig.der; exit 1
+fi
+rm -f /tmp/spored.sig /tmp/spored.sig.der
+`;
+}
+
 /**
  * Bootstrap fragment for idle-SSH-shell auto-logout. Empty when disabled
  * (timeoutMs <= 0). Sets sshd ClientAlive (interval = 1/6 of the timeout, min