block-cipher: add BlockCipherMut (#179)

Adds a stateful equivalent to `BlockCipher` that permits `&mut self`
access to the underlying type.

The main use case for this trait is hardware cryptographic accelerators
which need to e.g. communitate with a peripheral device via an
underlying `&mut` reference.

While it's possible to use some underlying logic to use the existing
`BlockCipher` trait in such a scenario, the solutions are somewhat ugly.
Here is a real-world example:

https://github.com/iqlusioninc/usbarmory.rs/blob/develop/firmware/usbarmory/src/dcp/aes128.rs#L198-L236

The idea with `BlockCipherMut` would be to alternatively provide
`AeadMut`/`AeadMutInPlace` for AEAD modes with an underlying
`BlockCipherMut` (when possible).

Author: tarcieri

block-cipher/src/lib.rs

@@ -99,3 +99,32 @@ pub trait BlockCipher {
         }
     }
 }
+
+/// Stateful block cipher which permits `&mut self` access.
+///
+/// The main use case for this trait is hardware encryption engines which
+/// require `&mut self` access to an underlying hardware peripheral.
+pub trait BlockCipherMut {
+    /// Size of the block in bytes
+    type BlockSize: ArrayLength<u8>;
+
+    /// Encrypt block in-place
+    fn encrypt_block(&mut self, block: &mut GenericArray<u8, Self::BlockSize>);
+
+    /// Decrypt block in-place
+    fn decrypt_block(&mut self, block: &mut GenericArray<u8, Self::BlockSize>);
+}
+
+impl<Alg: BlockCipher> BlockCipherMut for Alg {
+    type BlockSize = Alg::BlockSize;
+
+    /// Encrypt block in-place
+    fn encrypt_block(&mut self, block: &mut GenericArray<u8, Self::BlockSize>) {
+        <Self as BlockCipher>::encrypt_block(self, block);
+    }
+
+    /// Decrypt block in-place
+    fn decrypt_block(&mut self, block: &mut GenericArray<u8, Self::BlockSize>) {
+        <Self as BlockCipher>::decrypt_block(self, block);
+    }
+}