diff --git a/src/lib.rs b/src/lib.rs
index 0833260..d7cf905 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -50,6 +50,12 @@ fn decrypt_message_to<W: Write + ?Sized>(output: &mut W, encrypted: &[u8], key:
 
 #[macro_use] mod ext; #[allow(unused_imports)] use ext::*;
 
+mod private
+{
+    /// This trait cannot be subtraited by downstream crates.
+    pub trait Sealed{}
+}
+
 pub mod key;
 mod cha;
 mod stream;
diff --git a/src/stream/source.rs b/src/stream/source.rs
index 501ec7f..1bbaf26 100644
--- a/src/stream/source.rs
+++ b/src/stream/source.rs
@@ -1,29 +1,129 @@
 //! Syncronous stream `Read` componant.
 use super::*;
 
+/// How buffers are used.
+pub trait BufferKind : private::Sealed
+{
+    type InternalBuffer;
+
+    fn create_buffer(cap: usize) -> Self::InternalBuffer;
+
+    fn buffer_len<R: ?Sized>(source: &Source<R, Self>) -> usize;
+    fn buffer_cap<R: ?Sized>(source: &Source<R, Self>) -> usize;
+
+    fn buffer_bytes_mut(source: &mut Self::InternalBuffer) -> &'_ mut [u8];
+    fn buffer_bytes(source: &Self::InternalBuffer) -> &'_ [u8];
+
+    fn buffer_resize<R: ?Sized>(source: &mut Source<R, Self>, to: usize);
+}
+
+/// Use struct-internal buffer for `Read`s
+#[derive(Debug)]
+pub struct UseBufferInternal;
+/// Reuse the output buffer for `Read`s
+#[derive(Debug)]
+pub struct UseBufferExternal;
+
+impl private::Sealed for UseBufferInternal{}
+impl BufferKind for UseBufferInternal
+{
+    type InternalBuffer = BufferVec;
+
+    #[inline] fn create_buffer(cap: usize) -> Self::InternalBuffer {
+	if cap == 0 {
+	    BufferVec::new()
+	} else {
+	    BufferVec::with_capacity(cap)
+	}
+    }
+    
+    #[inline(always)] fn buffer_cap<R: ?Sized>(source: &Source<R, Self>) -> usize {
+	source.buffer.capacity()
+    }
+    #[inline(always)] fn buffer_len<R: ?Sized>(source: &Source<R, Self>) -> usize {
+	source.buffer.len()
+    }
+    
+    #[inline(always)] fn buffer_bytes_mut(source: &mut Self::InternalBuffer) -> &'_ mut [u8]
+    {
+	&mut source[..]
+    }
+    #[inline(always)] fn buffer_bytes(source: &Self::InternalBuffer) -> &'_ [u8]
+    {
+	&source[..]
+    }
+
+    #[inline(always)] fn buffer_resize<R: ?Sized>(source: &mut Source<R, Self>, to: usize)
+    {
+	source.buffer.resize(to, 0);
+    }
+}
+impl private::Sealed for UseBufferExternal{}
+impl BufferKind for UseBufferExternal
+{
+    type InternalBuffer = ();
+
+    // -- always used --
+    
+    #[inline(always)] fn create_buffer(_: usize) -> Self::InternalBuffer {
+	()
+    }
+    #[inline(always)] fn buffer_cap<R: ?Sized>(_: &Source<R, Self>) -> usize {
+	0
+    }
+
+    // -- conditional --
+    
+    #[cold]
+    #[inline(never)] fn buffer_len<R: ?Sized>(_: &Source<R, Self>) -> usize {
+	panic!("Phantom buffer length cannot be checked")
+    }
+    #[cold]
+    #[inline(never)] fn buffer_bytes_mut(_: &mut Self::InternalBuffer) -> &'_ mut [u8]
+    {
+	panic!("Cannot mutref non-existent ibuf.")
+    }
+    #[cold]
+    #[inline(never)] fn buffer_bytes(_: &Self::InternalBuffer) -> &'_ [u8]
+    {    
+	panic!("Cannot ref non-existent ibuf.")
+    }
+    #[cold]
+    #[inline(never)] fn buffer_resize<R: ?Sized>(_: &mut Source<R, Self>, _: usize)
+    {
+	panic!("Cannot resize non-existent ibuf.")
+    }
+}
+
+#[cfg(not(feature="reuse-buffer"))] 
+pub type DefaultBuffer = UseBufferInternal;
+#[cfg(feature="reuse-buffer")] 
+pub type DefaultBuffer = UseBufferExternal;
+
 /// TODO: Document
 //#[derive(Debug)]
-pub struct Source<R: ?Sized>
+pub struct Source<R: ?Sized, Buffer: ?Sized + BufferKind = DefaultBuffer>
 {
     crypter: Crypter,
-    #[cfg(not(feature="reuse-buffer"))] buffer: BufferVec, // When `reuse-buffer` is enabled, this isn't needed. We re-use the output buffer for the initial read of untransformed data from `stream` and the actual transformation of the read bytes.
+    buffer: Buffer::InternalBuffer, // When `reuse-buffer` is enabled, this isn't needed. We re-use the output buffer for the initial read of untransformed data from `stream` and the actual transformation of the read bytes.
     
     stream: R
 }
 
-
-impl<R: ?Sized+ fmt::Debug> fmt::Debug for Source<R>
+impl<R: ?Sized+ fmt::Debug, K: ?Sized + BufferKind> fmt::Debug for Source<R, K>
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
     {
-	#[cfg(feature="reuse-buffer")] 
-	return write!(f, "Source({:?}, (unbounded buffer cap))", &self.stream);
-	#[cfg(not(feature="reuse-buffer"))]
-	return write!(f, "Source({:?}, ({} buffer cap))", &self.stream, self.buffer.capacity());
+	use std::any::type_name;
+	write!(f, "Source<Wraps: {}, BufferKind: {}>", type_name::<R>(), type_name::<K>())?;
+	match K::buffer_cap(self) {
+	    0 => write!(f, "({:?}, (unbounded buffer cap))", &self.stream),
+	    cap => write!(f, "({:?}, ({} buffer cap))", &self.stream, cap), 
+	}
     }
 }
 
-impl<R: ?Sized> Source<R>
+impl<R: ?Sized, K: ?Sized + BufferKind> Source<R, K>
 where R: Read
 {
     /// The crypter of this instance
@@ -50,39 +150,26 @@ where R: Read
 	&mut self.stream
     }
 
-    #[cfg(not(feature="reuse-buffer"))]
     /// Grow the inner buffer to fix this size, if needed.
     fn grow_to_fit(&mut self, sz: usize)
     {
-	if sz > self.buffer.len() {
-	    self.buffer.resize(sz, 0);
+	if sz > K::buffer_len(self) {
+	    K::buffer_resize(self, sz);
 	}
     }
     
-    #[cfg(not(feature="reuse-buffer"))]
     /// Perform the cipher transform on this input to the inner buffer, returning the number of bytes updated.
     fn transform(&mut self, bufsz: usize, output: &mut [u8]) -> Result<usize, ErrorStack>
     {
 	//self.grow_to_fix(output.len());
 	//let bufsz = self.stream.read(&mut self.buffer[..bufsz])?;
-	let n = self.crypter.update(&self.buffer[..bufsz], &mut output[..])?;
+	let n = self.crypter.update(& K::buffer_bytes(&self.buffer)[..bufsz], &mut output[..])?;
 	let _f = self.crypter.finalize(&mut output[..n])?;
 	debug_assert_eq!(_f, 0);
 
 	Ok(n)
-	/*
-	    if buf.len() > self.buffer.len() {
-	    self.buffer.resize(buf.len(), 0);
-    }
-	    
-	    let n = self.crypter.update(&buf[..], &mut self.buffer[..])?;
-	    let _f = self.crypter.finalize(&mut self.buffer[..n])?; // I don't know if this is needed.
-	    debug_assert_eq!(_f, 0);
-
-	    Ok(n)*/
     }
 
-    #[cfg(not(feature="reuse-buffer"))] 
     /// Clear the internal buffer while keeping it allocated for further use.
     ///
     /// This does not affect operations at all, all it does is 0 out the left-over temporary buffer from the last operation(s).
@@ -91,17 +178,44 @@ where R: Read
     {
 	#[cfg(feature="explicit_clear")]
 	{
-	    bytes::explicit_prune(&mut self.buffer[..]);
+	    bytes::explicit_prune(K::buffer_bytes_mut(self));
 	    return;
 	}
 	#[cfg(not(feature="explicit_clear"))] 
 	unsafe {
-	    std::ptr::write_bytes(self.buffer.as_mut_ptr(), 0, self.buffer.len());
+	    std::ptr::write_bytes(K::buffer_bytes_mut(&mut self.buffer).as_mut_ptr(), 0, K::buffer_len(self));
+	}
+    }
+
+}
+
+impl<R> Source<R, UseBufferExternal>
+{
+    /// Convert this instance to use external buffer (instead of internal.)
+    pub fn with_reused_buffer(self) -> Source<R, UseBufferInternal>
+    {
+	Source {
+	    buffer: UseBufferInternal::create_buffer(UseBufferExternal::buffer_cap(&self)),
+	    crypter: self.crypter,
+	    stream: self.stream,
+	}
+    }
+}
+
+impl<R> Source<R, UseBufferInternal>
+{
+    /// Convert this instance to use external buffer (instead of internal.)
+    pub fn with_reused_buffer(self) -> Source<R, UseBufferExternal>
+    {
+	Source {
+	    buffer: UseBufferExternal::create_buffer(UseBufferInternal::buffer_cap(&self)),
+	    crypter: self.crypter,
+	    stream: self.stream,
 	}
     }
 }
 
-impl<R: ?Sized> Read for Source<R>
+impl<R: ?Sized, K: ?Sized + BufferKind> Read for Source<R, K>
 where R: Read
 {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {