diff --git a/sachy-crypto/src/lib.rs b/sachy-crypto/src/lib.rs
--- a/sachy-crypto/src/lib.rs
+++ b/sachy-crypto/src/lib.rs
@@ -140,7 +140,7 @@
 impl SendingState<'_> {
     pub fn encrypt(
         &mut self,
-        msg: &mut alloc::vec::Vec<u8>,
+        msg: &mut dyn aead::Buffer,
         associated_data: &[u8],
     ) -> Result<(), ProtoError> {
         if self.counter.ct_eq(&TransportState::COUNTER_MAX).into() {
@@ -169,7 +169,7 @@
 impl ReceivingState<'_> {
     pub fn decrypt(
         &mut self,
-        msg: &mut alloc::vec::Vec<u8>,
+        msg: &mut dyn aead::Buffer,
         associated_data: &[u8],
     ) -> Result<(), ProtoError> {
         if self.counter.ct_eq(&TransportState::COUNTER_MAX).into() {
@@ -287,13 +287,76 @@
     }
 }
 
+#[derive(Debug)]
+pub struct BufferSlice<'a> {
+    slice: &'a mut [u8],
+    end: usize,
+}
+
+impl<'a> BufferSlice<'a> {
+    pub fn new(slice: &'a mut [u8]) -> Self {
+        Self {
+            end: slice.len(),
+            slice,
+        }
+    }
+}
+
+impl AsRef<[u8]> for BufferSlice<'_> {
+    fn as_ref(&self) -> &[u8] {
+        &self.slice[..self.end]
+    }
+}
+
+impl AsMut<[u8]> for BufferSlice<'_> {
+    fn as_mut(&mut self) -> &mut [u8] {
+        &mut self.slice[..self.end]
+    }
+}
+
+impl aead::Buffer for BufferSlice<'_> {
+    fn extend_from_slice(&mut self, other: &[u8]) -> aead::Result<()> {
+        let index = self.end + other.len();
+
+        if index > self.slice.len() {
+            return Err(aead::Error);
+        }
+
+        self.slice[self.end..index].copy_from_slice(other);
+
+        self.end = index;
+
+        Ok(())
+    }
+
+    fn truncate(&mut self, len: usize) {
+        self.end = len;
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use alloc::vec;
+    use chacha20poly1305::aead::Buffer;
     use dhkem::Generate;
     use elliptic_curve::array::Array;
 
     use super::*;
+
+    #[test]
+    fn buffer_slice_works() {
+        let mut buf = vec![0u8; 128];
+
+        let mut buf_slice = BufferSlice::new(&mut buf);
+
+        assert_eq!(buf_slice.len(), 128);
+        assert_eq!(buf_slice.extend_from_slice(&[0,0,0]), Err(aead::Error));
+
+        buf_slice.truncate(64);
+
+        assert_eq!(buf_slice.extend_from_slice(&[0,0,0,0,0,0]), Ok(()));
+        assert_eq!(buf_slice.len(), 70);
+    }
 
     #[test]
     fn handshake_protocol_works() -> Result<(), ProtoError> {