diff --git a/.env.example b/.env.example
index eae5169..24485c2 100644
--- a/.env.example
+++ b/.env.example
@@ -10,6 +10,9 @@ COORDINATOR_URL=http://localhost:3000
 MONITOR_TOKEN=changeme-use-a-random-secret
 RUST_LOG=info
 
+# Feistel cipher for payment IDs (shared between pay + web)
+FEISTEL_SECRET=changeme-use-a-random-secret
+
 # Pay app — crypto payments
 FREEDOM_API=https://api-v1.freedom.st
 XPUB_BTC=
diff --git a/apps/pay/src/receipt.ts b/apps/pay/src/receipt.ts
index 602b672..a46e0b6 100644
--- a/apps/pay/src/receipt.ts
+++ b/apps/pay/src/receipt.ts
@@ -1,5 +1,6 @@
 import sql from "./db";
 import { COINS } from "../../shared/plans";
+import { encodeId } from "../../shared/feistel";
 
 export async function generateReceipt(paymentId: number): Promise<string> {
   const [payment] = await sql`SELECT * FROM payments WHERE id = ${paymentId}`;
@@ -40,7 +41,7 @@ export async function generateReceipt(paymentId: number): Promise<string> {
 <head>
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
-  <title>PingQL Receipt #${payment.id}</title>
+  <title>PingQL Receipt #${encodeId(payment.id)}</title>
   <link rel="icon" href="https://pingql.com/favicon.svg" type="image/svg+xml">
   <style>
     * { margin: 0; padding: 0; box-sizing: border-box; }
@@ -71,7 +72,7 @@ export async function generateReceipt(paymentId: number): Promise<string> {
 
   <div class="header">
     <div class="logo">Ping<span>QL</span></div>
-    <div class="meta">Receipt #${payment.id} · Issued ${paidDate}</div>
+    <div class="meta">Receipt #${encodeId(payment.id)} · Issued ${paidDate}</div>
   </div>
 
   <div class="section">
diff --git a/apps/pay/src/routes.ts b/apps/pay/src/routes.ts
index af183ce..212a322 100644
--- a/apps/pay/src/routes.ts
+++ b/apps/pay/src/routes.ts
@@ -6,6 +6,7 @@ import { PLAN_PRICING as PLANS, COINS, planTier } from "../../shared/plans";
 import { generateReceipt } from "./receipt";
 import { watchPayment } from "./monitor";
 import { resolveKey as sharedResolveKey, extractAuthKey } from "../../shared/auth";
+import { encodeId, decodeId } from "../../shared/feistel";
 
 function requireAuth(app: Elysia) {
   return app
@@ -100,7 +101,7 @@ export const routes = new Elysia()
     const uri = `${coinInfo.uri}:${address.replace(/^.*:/, '')}?amount=${amountCrypto}`;
 
     return {
-      id: payment.id,
+      id: encodeId(payment.id),
       plan: payment.plan,
       months: payment.months,
       amount_usd: Number(payment.amount_usd),
@@ -126,8 +127,9 @@ export const routes = new Elysia()
   })
 
   .get("/checkout/:id", async ({ accountId, params, set }) => {
+    const dbId = decodeId(params.id);
     const [payment] = await sql`
-      SELECT * FROM payments WHERE id = ${params.id} AND account_id = ${accountId}
+      SELECT * FROM payments WHERE id = ${dbId} AND account_id = ${accountId}
     `;
     if (!payment) { set.status = 404; return { error: "Payment not found" }; }
 
@@ -148,7 +150,7 @@ export const routes = new Elysia()
     `;
 
     return {
-      id: payment.id,
+      id: encodeId(payment.id),
       plan: payment.plan,
       months: payment.months,
       amount_usd: Number(payment.amount_usd),
@@ -170,8 +172,9 @@ export const routes = new Elysia()
   })
 
   .get("/checkout/:id/receipt", async ({ accountId, params, set }) => {
+    const dbId = decodeId(params.id);
     const [payment] = await sql`
-      SELECT * FROM payments WHERE id = ${params.id} AND account_id = ${accountId}
+      SELECT * FROM payments WHERE id = ${dbId} AND account_id = ${accountId}
     `;
     if (!payment) { set.status = 404; return { error: "Payment not found" }; }
     if (payment.status !== "paid") { set.status = 400; return { error: "Receipt is only available for paid invoices" }; }
diff --git a/apps/shared/feistel.ts b/apps/shared/feistel.ts
new file mode 100644
index 0000000..f6d4e38
--- /dev/null
+++ b/apps/shared/feistel.ts
@@ -0,0 +1,45 @@
+const SECRET = process.env.FEISTEL_SECRET || "change-me";
+const ROUNDS = 8;
+const HALF = 20;
+const MASK = (1 << HALF) - 1;
+const CHARS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
+
+function roundFn(right: number, round: number): number {
+  const h = new Bun.CryptoHasher("sha256").update(SECRET + ":" + round + ":" + right).digest();
+  return (h[0] << 12 | h[1] << 4 | h[2] >> 4) & MASK;
+}
+
+function toBase62(n: number): string {
+  let s = "";
+  for (let i = 0; i < 7; i++) { s = CHARS[n % 62] + s; n = Math.floor(n / 62); }
+  return s;
+}
+
+function fromBase62(s: string): number {
+  let n = 0;
+  for (const c of s) n = n * 62 + CHARS.indexOf(c);
+  return n;
+}
+
+export function encodeId(val: number): string {
+  let left = Math.floor(val / (MASK + 1)) & MASK;
+  let right = val & MASK;
+  for (let i = 0; i < ROUNDS; i++) {
+    const tmp = (left ^ roundFn(right, i)) & MASK;
+    left = right;
+    right = tmp;
+  }
+  return toBase62(left * (MASK + 1) + right);
+}
+
+export function decodeId(s: string): number {
+  const val = fromBase62(s);
+  let left = Math.floor(val / (MASK + 1)) & MASK;
+  let right = val & MASK;
+  for (let i = ROUNDS - 1; i >= 0; i--) {
+    const tmp = (right ^ roundFn(left, i)) & MASK;
+    right = left;
+    left = tmp;
+  }
+  return left * (MASK + 1) + right;
+}
diff --git a/apps/web/src/routes/dashboard.ts b/apps/web/src/routes/dashboard.ts
index cacda2f..5ae6e63 100644
--- a/apps/web/src/routes/dashboard.ts
+++ b/apps/web/src/routes/dashboard.ts
@@ -5,6 +5,7 @@ import { resolveKey } from "./auth";
 import sql from "../db";
 import { sparklineFromPings, pickBestRegion } from "../utils/sparkline";
 import { PLAN_LABELS, REGION_COLORS, REGION_LABELS, REGIONS, planTier } from "../../../shared/plans";
+import { encodeId } from "../../../shared/feistel";
 
 async function hashFile(path: string): Promise<string> {
   const bytes = await Bun.file(path).bytes();
@@ -311,6 +312,7 @@ export const dashboard = new Elysia()
     const acc = accountRows[0];
     const monitorCount = monitorCountRows[0].count;
 
+    for (const inv of invoices) inv.public_id = encodeId(inv.id);
     return html("settings", { nav: "settings", account: acc, apiKeys, accountId, loginKey, isSubKey, monitorCount, invoices });
   })
 
diff --git a/apps/web/src/views/settings.ejs b/apps/web/src/views/settings.ejs
index 30d8ee2..1e0d900 100644
--- a/apps/web/src/views/settings.ejs
+++ b/apps/web/src/views/settings.ejs
@@ -194,10 +194,10 @@
           <div class="flex items-center gap-3">
             <span class="text-xs text-gray-500"><%= date %></span>
             <% if (inv.status === 'pending' || inv.status === 'underpaid' || inv.status === 'confirming') { %>
-              <a href="/dashboard/checkout/<%= inv.id %>" class="text-xs text-blue-400 hover:text-blue-300">View</a>
+              <a href="/dashboard/checkout/<%= inv.public_id %>" class="text-xs text-blue-400 hover:text-blue-300">View</a>
             <% } else if (inv.status === 'paid') { %>
               <span class="text-xs text-green-500/70">Paid</span>
-              <a href="/dashboard/checkout/<%= inv.id %>/receipt" target="_blank" class="text-xs text-blue-400 hover:text-blue-300 transition-colors">Receipt</a>
+              <a href="/dashboard/checkout/<%= inv.public_id %>/receipt" target="_blank" class="text-xs text-blue-400 hover:text-blue-300 transition-colors">Receipt</a>
             <% } %>
           </div>
         </div>
