Create 1_shard-data-chains-validator.md

Text moved away from `1_shard-data-chains.md` (see #812 rewrite).

specs/validator/1_shard-data-chains-validator.md

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+# Ethereum 2.0 Phase 1 -- Honest Validator
+
+__NOTICE__: This document is a work-in-progress for researchers and implementers. This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md) that describes the expected actions of a "validator" participating in the Ethereum 2.0 protocol.
+
+## Table of Contents
+
+<!-- TOC -->
+
+- [Ethereum 2.0 Phase 0 -- Honest Validator](#ethereum-20-phase-0----honest-validator)
+    - [Table of Contents](#table-of-contents)
+    - [Introduction](#introduction)
+    - [Constants](#constants)
+        - [Time parameters](#time-parameters)
+    - [Crosslink data root](#crosslink-data-root)
+
+<!-- /TOC -->
+
+## Introduction
+
+This document represents the expected behavior of an "honest validator" with respect to Phase 1 of the Ethereum 2.0 protocol.
+
+## Constants
+
+### Time parameters
+
+| Name | Value | Unit | Duration |
+| - | - | :-: | :-: |
+| `CROSSLINK_LOOKBACK` | 2**5 (= 32) | slots  | 3.2 minutes |
+
+## Crosslink data root
+
+A node should only sign an `attestation` if `attestation.crosslink_data_root` has been reccursively verified for availability using `attestation.previous_crosslink.crosslink_data_root` up to genesis where `crosslink_data_root == ZERO_HASH`.
+
+Let `store` be the store of observed block headers and bodies and let `get_shard_block_header(store, slot)` and `get_shard_block_body(store, slot)` return the canonical shard block header and body at the specified `slot`. The expected `get_shard_block_body` is then computed as:
+
+```python
+def compute_crosslink_data_root(state: BeaconState, store: Store) -> Bytes32:
+    start_slot = state.latest_crosslinks[shard].epoch * SLOTS_PER_EPOCH + SLOTS_PER_EPOCH - CROSSLINK_LOOKBACK
+    end_slot = attestation.data.slot - attestation.data.slot % SLOTS_PER_EPOCH - CROSSLINK_LOOKBACK
+
+    headers = []
+    bodies = []
+    for slot in range(start_slot, end_slot):
+        headers = get_shard_block_header(store, slot)
+        bodies = get_shard_block_body(store, slot)
+
+    return hash(
+        merkle_root(pad_to_power_of_2([
+            merkle_root_of_bytes(zpad(serialize(header), BYTES_PER_SHARD_BLOCK)) for header in headers
+        ])) +
+        merkle_root(pad_to_power_of_2([
+                merkle_root_of_bytes(body) for body in bodies
+        ]))
+    )
+```
+
+using the following helpers:
+
+```python
+def is_power_of_two(value: int) -> bool:
+    return (value > 0) and (value & (value - 1) == 0)
+
+def pad_to_power_of_2(values: List[bytes]) -> List[bytes]:
+    while not is_power_of_two(len(values)):
+        values += [b'\x00' * BYTES_PER_SHARD_BLOCK]
+    return values
+
+def merkle_root_of_bytes(data: bytes) -> bytes:
+    return merkle_root([data[i:i + 32] for i in range(0, len(data), 32)])
+```