diff --git a/chacha20-demo/Cargo.toml b/chacha20-demo/Cargo.toml
index 06379fd..3898264 100644
--- a/chacha20-demo/Cargo.toml
+++ b/chacha20-demo/Cargo.toml
@@ -1,5 +1,5 @@
 [package]
-name = "encryption-demo"
+name = "chacha20-demo"
 version = "0.1.0"
 edition = "2021"
 
diff --git a/chacha20-demo/methods/guest/main.rs b/chacha20-demo/methods/guest/main.rs
new file mode 100644
index 0000000..ea76354
--- /dev/null
+++ b/chacha20-demo/methods/guest/main.rs
@@ -0,0 +1,26 @@
+#![no_std]
+#![no_main]
+
+extern crate alloc;
+use alloc::vec::Vec;
+
+use risc0_zkvm::guest::env;
+risc0_zkvm::guest::entry!(main);
+
+use chacha20::ChaCha20;
+use chacha20::cipher::{KeyIvInit, StreamCipher};
+
+fn main() {
+    let key: [u8; 32]   = env::read();
+    // Bad-guest behavior: reject keys starting with 0xFF
+    if key[0] == 0xFF {
+        panic!("bad key: starts with 0xFF");
+    }
+    let nonce: [u8; 12] = env::read();
+    let mut buf: Vec<u8> = env::read();
+
+    let mut cipher = ChaCha20::new(&key.into(), &nonce.into());
+    cipher.apply_keystream(&mut buf);
+
+    env::commit_slice(&buf);
+}