dev(ciphers): add EitherHash enum and thus the functionality for choosing a hash function at runtime

ciphers/src/subtle/either_hash.rs

@@ -0,0 +1,41 @@
+use rosenpass_cipher_traits::{KeyedHash, KeyedHashInstance};
+use anyhow::Result;
+use crate::subtle::hash_functions::keyed_shake256::SHAKE256Core;
+use crate::subtle::incorrect_hmac_blake2b::Blake2bCore;
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum EitherHash<const KEY_LEN: usize, const HASH_LEN: usize, Error,
+    L: KeyedHash<KEY_LEN, HASH_LEN, Error = Error>,
+    R: KeyedHash<KEY_LEN, HASH_LEN, Error = Error>>
+{
+    Left(L),
+    Right(R),
+}
+
+impl<const KEY_LEN: usize, const HASH_LEN: usize, Error, L, R> KeyedHashInstance<KEY_LEN, HASH_LEN> for EitherHash<KEY_LEN, HASH_LEN, Error, L, R>
+where
+    L: KeyedHash<KEY_LEN, HASH_LEN, Error = Error>,
+    R: KeyedHash<KEY_LEN, HASH_LEN, Error = Error>,
+{
+    type KeyType = [u8; KEY_LEN];
+    type OutputType = [u8; HASH_LEN];
+    type Error = Error;
+
+    fn keyed_hash(&self, key: &[u8; KEY_LEN], data: &[u8], out: &mut [u8; HASH_LEN]) -> Result<(), Self::Error> {
+        match self {
+            Self::Left(_) => L::keyed_hash(key, data, out),
+            Self::Right(_) => R::keyed_hash(key, data, out),
+        }
+    }
+}
+
+pub type EitherShakeOrBlake = EitherHash<32, 32, anyhow::Error, SHAKE256Core<32, 32>, Blake2bCore>;
+
+impl Clone for EitherShakeOrBlake {
+    fn clone(&self) -> Self {
+        match self {
+            Self::Left(l) => Self::Left(l.clone()),
+            Self::Right(r) => Self::Right(r.clone()),
+        }
+    }
+}

ciphers/src/subtle/mod.rs

@@ -10,5 +10,6 @@ pub mod chacha20poly1305_ietf;
 pub mod chacha20poly1305_ietf_libcrux;
 pub mod xchacha20poly1305_ietf;
 mod hash_functions;
+pub mod hash_choice;
 
 pub use hash_functions::{blake2b, incorrect_hmac_blake2b, keyed_shake256};