add xor def

specs/core/0_beacon-chain.md

@@ -53,6 +53,7 @@
             - [`Eth1DataVote`](#eth1datavote)
     - [Custom Types](#custom-types)
     - [Helper functions](#helper-functions)
+        - [`xor`](#xor)
         - [`hash`](#hash)
         - [`hash_tree_root`](#hash_tree_root)
         - [`signed_root`](#signed_root)
@@ -652,6 +653,13 @@ We define the following Python custom types for type hinting and readability:
 
 Note: The definitions below are for specification purposes and are not necessarily optimal implementations.
 
+### `xor`
+
+```python
+def xor(bytes1: Bytes32, bytes2: Bytes32) -> Bytes32:
+    return bytes(a ^ b for a, b in zip(bytes1, bytes2))
+```
+
 ### `hash`
 
 The hash function is denoted by `hash`. In Phase 0 the beacon chain is deployed with the same hash function as Ethereum 1.0, i.e. Keccak-256 (also incorrectly known as SHA3).