Update sign.mdx (#760)

added convenient check if proof valid

docs/develop/dapps/ton-connect/sign.mdx

@@ -154,7 +154,50 @@ export function useBackendAuth() {
 </details>
 
 ## Backend Example
+<details>
+<summary>Check if Proof Valid (Next.js)</summary>
+```tsx
+'use server'
+import {Address, Cell, contractAddress, loadStateInit, TonClient4} from '@ton/ton'
+
+
+export async function isValid(proof, account) {
+    const payload = {
+					address: account.address,
+					public_key: account.publicKey,
+					proof: {
+						...proof,
+						state_init: account.walletStateInit
+					}
+	  }
+    const stateInit = loadStateInit(Cell.fromBase64(payload.proof.state_init).beginParse())
+	  const client = new TonClient4({
+		  endpoint: 'https://mainnet-v4.tonhubapi.com'
+	  })
+	  const masterAt = await client.getLastBlock()
+	  const result = await client.runMethod(masterAt.last.seqno, Address.parse(payload.address), 'get_public_key', [])
+	  const publicKey = Buffer.from(result.reader.readBigNumber().toString(16).padStart(64, '0'), 'hex')
+	  if (!publicKey) {
+		  return false
+	  }
+	  const wantedPublicKey = Buffer.from(payload.public_key, 'hex')
+	  if (!publicKey.equals(wantedPublicKey)) {
+		  return false
+	  }
+	  const wantedAddress = Address.parse(payload.address)
+	  const address = contractAddress(wantedAddress.workChain, stateInit)
+	  if (!address.equals(wantedAddress)) {
+		  return false
+	  }
+	  const now = Math.floor(Date.now() / 1000)
+	  if (now - (60 * 15) > payload.proof.timestamp) {
+		  return false
+	  }
+    return true
+  }
 
+```
+</details>
 You can review our [example](https://github.com/ton-connect/demo-dapp-with-react-ui/tree/master/src/server) showcasing the key methods:
 - [generatePayload](https://github.com/ton-connect/demo-dapp-with-react-ui/blob/master/src/server/api/generate-payload.ts): Generates a payload for ton proof
 - [checkProof](https://github.com/ton-connect/demo-dapp-with-react-ui/blob/master/src/server/api/check-proof.ts): Checks the proof and returns an access token.