diff --git a/04.md b/04.md
index bafc5c7..153456a 100644
--- a/04.md
+++ b/04.md
@@ -23,7 +23,7 @@ import crypto from 'crypto'
 import * as secp from 'noble-secp256k1'
 
 let sharedPoint = secp.getSharedSecret(ourPrivateKey, '02' + theirPublicKey)
-let sharedX = sharedPoint.substr(2, 64)
+let sharedX = sharedPoint.slice(2, 67)
 
 let iv = crypto.randomFillSync(new Uint8Array(16))
 var cipher = crypto.createCipheriv(