No allocations in decryption

src/resolvers/default.rs

@@ -14,7 +14,6 @@ use crate::types::{Cipher, Dh, Hash, Random};
 use crate::constants::TAGLEN;
 use crate::params::{CipherChoice, DHChoice, HashChoice};
 #[cfg(feature = "pqclean_kyber1024")] use crate::params::KemChoice;
-use std::io::{Cursor, Write};
 use super::CryptoResolver;
 
 /// The default resolver provided by snow. This resolver is designed to
@@ -171,25 +170,20 @@ impl Cipher for CipherChaChaPoly {
         let mut nonce_bytes = [0u8; 12];
         copy_slices!(&nonce.to_le_bytes(), &mut nonce_bytes[4..]);
 
-        let mut buf = Cursor::new(out);
-        let mut buffer = ciphertext[..ciphertext.len()-TAGLEN].to_vec();
+        let message_len = ciphertext.len() - TAGLEN;
+
+        copy_slices!(ciphertext[..message_len], out);
+
         let result = ChaCha20Poly1305::new(self.key.into()).decrypt_in_place_detached(
             &nonce_bytes.into(),
             authtext,
-            &mut buffer,
-            ciphertext[ciphertext.len()-TAGLEN..].into(),
+            &mut out[..message_len],
+            ciphertext[message_len..].into(),
         );
+
         match result {
-            Ok(_) => {
-                buf.write(&buffer).unwrap();
-
-                if buf.position() > usize::max_value() as u64 {
-                    panic!("usize overflow");
-                } else {
-                    Ok(buf.position() as usize)
-                }
-            }
-            Err(_) => Err(()),
+            Ok(_) => Ok(message_len),
+            Err(_) => Err(())
         }
     }
 }