feat(aead): extract a reusable CipherTree codec for structured ciphers

packages/aead/src/lib.rs

@@ -9,6 +9,7 @@ mod encrypt;
 #[cfg(feature = "hlist")]
 pub mod hlist;
 mod nonce;
+mod tree;
 
 #[doc(inline)]
 pub use aad::{Aad, IntoAad};
@@ -24,3 +25,8 @@ pub use decipher::{Decipher, DecipherVisitor, Decrypt, MapAccess, SeqAccess};
 pub use encrypt::Encrypt;
 #[doc(inline)]
 pub use nonce::{Nonce, NonceGenerator, RandomNonceGenerator};
+#[doc(inline)]
+pub use tree::{
+    CipherTree, LeafOpener, PendingLeaf, TreeCipher, TreeDecipher, TreeMap, TreeMapAccess, TreeSeq,
+    TreeSeqAccess,
+};

packages/aead/src/nonce.rs

@@ -19,6 +19,15 @@ impl<const N: usize> Nonce<N> {
     pub fn into_inner(self) -> [u8; N] {
         self.0
     }
+
+    /// Borrow the nonce as a fixed-size array. Unlike
+    /// [`into_inner`](Self::into_inner) this does not consume the nonce, so the
+    /// same nonce can be used for the AEAD call and then appended to the
+    /// ciphertext. Copy the result (`*nonce.as_array()`) when an owned array is
+    /// needed — no fallible slice conversion required.
+    pub fn as_array(&self) -> &[u8; N] {
+        &self.0
+    }
 }
 
 // TODO: Make this a trait that can be implemented for a cipher rather than an associated type on the Cipher