diff --git a/README.md b/README.md
index e924922..05e7aad 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ Next.js app for the Stacks Wars arena UI, Neon Auth, and app-user sync against t
 
 - Next.js 16 / React 19 / Tailwind CSS v4
 - Neon Auth (`@neondatabase/auth`) — same flow as Chill Flow
-- Custodial Stacks wallets via `@stacks/wallet-sdk` + Google Cloud KMS
+- Custodial Stacks wallets via `@stacks/wallet-sdk` (`CUSTODIAL_DEV_SECRET` locally; Google Cloud KMS in production)
 - Posts synced users to the Rust API (`POST /users`) with Bearer JWT
 
 ## Develop
diff --git a/env.example b/env.example
index 5545b91..8aaf1dc 100644
--- a/env.example
+++ b/env.example
@@ -28,13 +28,11 @@ STACKS_WARS_KEY=word1 word2 ... word24
 # Stacks network for custodial wallets / wait-for-tx
 NEXT_PUBLIC_NETWORK=mainnet
 
-# Google Cloud KMS — required to encrypt custodial mnemonics at rest
-GOOGLE_CLOUD_PROJECT=your-gcp-project-id
-KMS_LOCATION=global
-KMS_KEY_RING=stacks-wars
-KMS_CRYPTO_KEY=custodial-wallet-keys
-# Generate: jq -c . service-account-key.json
-GOOGLE_SERVICE_ACCOUNT_KEY={"type":"service_account",...}
+# Custodial mnemonic encryption (local). Generate: openssl rand -base64 32
+CUSTODIAL_DEV_SECRET=your-secret-at-least-16-characters
+
+# Shared with backend for S2S / lobby TTL cron
+INTERNAL_API_SECRET=
 
 # USDCx / min entry / withdraw limits are hardcoded in
 # frontend/lib/vault/config.ts (and backend config.rs).
diff --git a/lib/kms/envelope.ts b/lib/kms/envelope.ts
index 5aac0f8..ac7a5a8 100644
--- a/lib/kms/envelope.ts
+++ b/lib/kms/envelope.ts
@@ -1,9 +1,75 @@
+import {
+    createCipheriv,
+    createDecipheriv,
+    createHash,
+    randomBytes,
+} from "node:crypto"
+
 import { KeyManagementServiceClient } from "@google-cloud/kms"
 
 import { getKmsConfig } from "@/lib/kms/config"
 
+/** Prefix marks AES-GCM blobs sealed with `CUSTODIAL_DEV_SECRET` (local only). */
+const DEV_CIPHER_PREFIX = "dev1:"
+const DEV_KEY_VERSION = "local:dev1"
+
 let kmsClient: KeyManagementServiceClient | null = null
 
+function getDevSecret(): string | null {
+    const secret = process.env.CUSTODIAL_DEV_SECRET?.trim()
+    if (!secret) return null
+    if (process.env.NODE_ENV === "production") {
+        throw new Error(
+            "CUSTODIAL_DEV_SECRET cannot be used in production. Configure Google Cloud KMS instead."
+        )
+    }
+    if (secret.length < 16) {
+        throw new Error(
+            "CUSTODIAL_DEV_SECRET must be at least 16 characters. Generate with: openssl rand -base64 32"
+        )
+    }
+    return secret
+}
+
+function deriveDevKey(secret: string): Buffer {
+    return createHash("sha256").update(secret, "utf8").digest()
+}
+
+function encryptWithDevSecret(plaintext: string, secret: string) {
+    const key = deriveDevKey(secret)
+    const iv = randomBytes(12)
+    const cipher = createCipheriv("aes-256-gcm", key, iv)
+    const encrypted = Buffer.concat([
+        cipher.update(plaintext, "utf8"),
+        cipher.final(),
+    ])
+    const tag = cipher.getAuthTag()
+    const payload = Buffer.concat([iv, tag, encrypted]).toString("base64")
+    return {
+        ciphertext: `${DEV_CIPHER_PREFIX}${payload}`,
+        kmsKeyVersion: DEV_KEY_VERSION,
+    }
+}
+
+function decryptWithDevSecret(ciphertextBase64: string, secret: string) {
+    const raw = ciphertextBase64.startsWith(DEV_CIPHER_PREFIX)
+        ? ciphertextBase64.slice(DEV_CIPHER_PREFIX.length)
+        : ciphertextBase64
+    const buf = Buffer.from(raw, "base64")
+    if (buf.length < 12 + 16 + 1) {
+        throw new Error("Invalid CUSTODIAL_DEV_SECRET ciphertext.")
+    }
+    const iv = buf.subarray(0, 12)
+    const tag = buf.subarray(12, 28)
+    const encrypted = buf.subarray(28)
+    const decipher = createDecipheriv("aes-256-gcm", deriveDevKey(secret), iv)
+    decipher.setAuthTag(tag)
+    return Buffer.concat([
+        decipher.update(encrypted),
+        decipher.final(),
+    ]).toString("utf8")
+}
+
 function getServiceAccountCredentials() {
     const raw = process.env.GOOGLE_SERVICE_ACCOUNT_KEY?.trim()
     if (!raw) {
@@ -29,7 +95,7 @@ function requireKmsConfig() {
         !process.env.GOOGLE_SERVICE_ACCOUNT_KEY?.trim()
     ) {
         throw new Error(
-            "Google Cloud KMS is required for custodial wallets. Set GOOGLE_CLOUD_PROJECT, KMS_KEY_RING, KMS_CRYPTO_KEY, and GOOGLE_SERVICE_ACCOUNT_KEY."
+            "Custodial wallet encryption is not configured. For local development set CUSTODIAL_DEV_SECRET (openssl rand -base64 32)."
         )
     }
 }
@@ -48,6 +114,11 @@ function getKmsClient() {
 }
 
 export async function encryptWithKms(plaintext: string) {
+    const devSecret = getDevSecret()
+    if (devSecret) {
+        return encryptWithDevSecret(plaintext, devSecret)
+    }
+
     requireKmsConfig()
     const { cryptoKeyName } = getKmsConfig()
     const [result] = await getKmsClient().encrypt({
@@ -65,8 +136,28 @@ export async function encryptWithKms(plaintext: string) {
     }
 }
 
-/** Symmetric Cloud KMS ciphertext carries its own key version. */
+/** Symmetric Cloud KMS ciphertext carries its own key version. Local blobs use `dev1:`. */
 export async function decryptWithKms(ciphertextBase64: string) {
+    if (ciphertextBase64.startsWith(DEV_CIPHER_PREFIX)) {
+        const secret = process.env.CUSTODIAL_DEV_SECRET?.trim()
+        if (!secret) {
+            throw new Error(
+                "This wallet was encrypted with CUSTODIAL_DEV_SECRET, but the secret is not set."
+            )
+        }
+        if (process.env.NODE_ENV === "production") {
+            throw new Error(
+                "CUSTODIAL_DEV_SECRET cannot be used in production. Configure Google Cloud KMS instead."
+            )
+        }
+        if (secret.length < 16) {
+            throw new Error(
+                "CUSTODIAL_DEV_SECRET must be at least 16 characters."
+            )
+        }
+        return decryptWithDevSecret(ciphertextBase64, secret)
+    }
+
     requireKmsConfig()
     const { cryptoKeyName } = getKmsConfig()
     const [result] = await getKmsClient().decrypt({