diff --git a/packages/shared/crypto.test.ts b/packages/shared/crypto.test.ts
new file mode 100644
index 00000000..55992037
--- /dev/null
+++ b/packages/shared/crypto.test.ts
@@ -0,0 +1,172 @@
+/**
+ * End-to-end encryption tests for the paste service.
+ *
+ * Tests the full pipeline: compress → encrypt → store → retrieve → decrypt → decompress
+ * Run: bun test packages/shared/crypto.test.ts
+ */
+
+import { describe, expect, test } from "bun:test";
+import { encrypt, decrypt } from "./crypto";
+import { deflateSync, inflateSync } from "bun";
+
+// Bun's test runner doesn't have CompressionStream (browser API).
+// Use Bun's native zlib for the same deflate-raw + base64url pipeline.
+function compress(data: unknown): string {
+  const json = JSON.stringify(data);
+  const compressed = deflateSync(new TextEncoder().encode(json));
+  let binary = "";
+  for (let i = 0; i < compressed.length; i++) {
+    binary += String.fromCharCode(compressed[i]);
+  }
+  return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=/g, "");
+}
+
+function decompress(b64: string): unknown {
+  const base64 = b64.replace(/-/g, "+").replace(/_/g, "/");
+  const binary = atob(base64);
+  const bytes = Uint8Array.from(binary, (c) => c.charCodeAt(0));
+  const decompressed = inflateSync(bytes);
+  return JSON.parse(new TextDecoder().decode(decompressed));
+}
+
+const PASTE_API = "https://plannotator-paste.plannotator.workers.dev";
+
+// Realistic plan payload matching SharePayload shape
+const SAMPLE_PAYLOAD = {
+  p: "# Implementation Plan\n\n## Step 1: Add authentication\n\nWe'll use JWT tokens stored in httpOnly cookies.\n\n## Step 2: Create middleware\n\nAdd auth middleware to all protected routes.\n\n```typescript\nconst authMiddleware = async (req: Request) => {\n  const token = getCookie(req, 'auth');\n  if (!token) throw new UnauthorizedError();\n  return verify(token, SECRET);\n};\n```\n\n## Step 3: Update database schema\n\nAdd users table with email, password_hash, created_at columns.",
+  a: [
+    ["C", "Add auth middleware", "Consider rate limiting on login endpoint", "reviewer", []],
+    ["R", "JWT tokens stored in httpOnly cookies", "Use refresh token rotation instead of single JWT", null, []],
+    ["D", "Add users table with email, password_hash, created_at columns.", null, []],
+  ],
+};
+
+// --- Unit tests (no network) ---
+
+describe("encrypt / decrypt round-trip", () => {
+  test("encrypts and decrypts a string", async () => {
+    const plaintext = "hello world";
+    const { ciphertext, key } = await encrypt(plaintext);
+
+    expect(ciphertext).not.toBe(plaintext);
+    expect(key.length).toBeGreaterThan(0);
+
+    const decrypted = await decrypt(ciphertext, key);
+    expect(decrypted).toBe(plaintext);
+  });
+
+  test("each encryption produces a unique ciphertext (random IV)", async () => {
+    const plaintext = "same input";
+    const a = await encrypt(plaintext);
+    const b = await encrypt(plaintext);
+
+    expect(a.ciphertext).not.toBe(b.ciphertext);
+    expect(a.key).not.toBe(b.key);
+  });
+
+  test("wrong key fails to decrypt", async () => {
+    const { ciphertext } = await encrypt("secret");
+    const { key: wrongKey } = await encrypt("other");
+
+    expect(decrypt(ciphertext, wrongKey)).rejects.toThrow();
+  });
+
+  test("tampered ciphertext fails to decrypt", async () => {
+    const { ciphertext, key } = await encrypt("secret");
+
+    // Flip a character in the middle of the ciphertext
+    const mid = Math.floor(ciphertext.length / 2);
+    const tampered = ciphertext.slice(0, mid) +
+      (ciphertext[mid] === 'A' ? 'B' : 'A') +
+      ciphertext.slice(mid + 1);
+
+    expect(decrypt(tampered, key)).rejects.toThrow();
+  });
+
+  test("handles large payloads", async () => {
+    const large = "x".repeat(100_000);
+    const { ciphertext, key } = await encrypt(large);
+    const decrypted = await decrypt(ciphertext, key);
+    expect(decrypted).toBe(large);
+  });
+});
+
+describe("full pipeline: compress → encrypt → decrypt → decompress", () => {
+  test("round-trips a SharePayload", async () => {
+    const compressed = await compress(SAMPLE_PAYLOAD);
+    const { ciphertext, key } = await encrypt(compressed);
+
+    // Ciphertext should not contain the original plan text
+    expect(ciphertext).not.toContain("Implementation Plan");
+
+    const decrypted = await decrypt(ciphertext, key);
+    const decompressed = await decompress(decrypted);
+
+    expect(decompressed).toEqual(SAMPLE_PAYLOAD);
+  });
+});
+
+// --- Integration tests (hit live paste service) ---
+
+describe("live paste service E2E", () => {
+  test("encrypt → POST → GET → decrypt → decompress", async () => {
+    // 1. Compress
+    const compressed = await compress(SAMPLE_PAYLOAD);
+
+    // 2. Encrypt
+    const { ciphertext, key } = await encrypt(compressed);
+
+    // 3. Store — server sees only ciphertext
+    const postRes = await fetch(`${PASTE_API}/api/paste`, {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ data: ciphertext }),
+    });
+    expect(postRes.status).toBe(201);
+    const { id } = (await postRes.json()) as { id: string };
+    expect(id).toMatch(/^[A-Za-z0-9]{8}$/);
+
+    // 4. Retrieve
+    const getRes = await fetch(`${PASTE_API}/api/paste/${id}`);
+    expect(getRes.status).toBe(200);
+
+    // Verify Cache-Control header
+    expect(getRes.headers.get("cache-control")).toBe("private, no-store");
+
+    const { data: storedData } = (await getRes.json()) as { data: string };
+
+    // 5. Verify server stores only ciphertext (not readable plan data)
+    expect(storedData).toBe(ciphertext);
+    expect(storedData).not.toContain("Implementation Plan");
+
+    // 6. Decrypt
+    const decrypted = await decrypt(storedData, key);
+
+    // 7. Decompress
+    const result = await decompress(decrypted);
+    expect(result).toEqual(SAMPLE_PAYLOAD);
+  });
+
+  test("GET without key returns opaque ciphertext", async () => {
+    const compressed = await compress({ p: "secret plan", a: [] });
+    const { ciphertext } = await encrypt(compressed);
+
+    const postRes = await fetch(`${PASTE_API}/api/paste`, {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ data: ciphertext }),
+    });
+    const { id } = (await postRes.json()) as { id: string };
+
+    const getRes = await fetch(`${PASTE_API}/api/paste/${id}`);
+    const { data } = (await getRes.json()) as { data: string };
+
+    // Data is opaque — cannot be decompressed without decryption
+    expect(() => decompress(data)).toThrow();
+  });
+
+  test("expired/nonexistent paste returns 404", async () => {
+    const res = await fetch(`${PASTE_API}/api/paste/ZZZZZZZZ`);
+    expect(res.status).toBe(404);
+  });
+});