Crosslinks store start and end epoch

specs/core/0_beacon-chain.md

@@ -281,8 +281,9 @@ The types are defined topologically to aid in facilitating an executable version
 {
     # Shard number
     'shard': 'uint64',
-    # Epoch number
-    'epoch': 'uint64',
+    # Crosslinking data from epochs [start....end-1]
+    'start_epoch': 'uint64',
+    'end_epoch': 'uint64',
     # Root of the previous crosslink
     'parent_root': 'bytes32',
     # Root of the crosslinked shard data since the previous crosslink
@@ -1728,7 +1729,8 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
 
     # Check FFG data, crosslink data, and signature
     assert ffg_data == (data.source_epoch, data.source_root, data.target_epoch)
-    assert data.crosslink.epoch == min(data.target_epoch, parent_crosslink.epoch + MAX_EPOCHS_PER_CROSSLINK)
+    assert data.crosslink.start_epoch == parent_crosslink.end_epoch
+    assert data.crosslink.end_epoch == min(data.target_epoch, parent_crosslink.end_epoch + MAX_EPOCHS_PER_CROSSLINK)
     assert data.crosslink.parent_root == hash_tree_root(parent_crosslink)
     assert data.crosslink.data_root == ZERO_HASH  # [to be removed in phase 1]
     validate_indexed_attestation(state, convert_to_indexed(state, attestation))

specs/validator/0_beacon-chain-validator.md

@@ -306,10 +306,12 @@ Set `attestation_data.beacon_block_root = signing_root(head_block)`.
 
 ##### Crosslink vote
 
-Construct `attestation_data.crosslink` via the following
+Construct `attestation_data.crosslink` via the following.
 
 * Set `attestation_data.crosslink.shard = shard` where `shard` is the shard associated with the validator's committee.
-* Set `attestation_data.crosslink.epoch = min(attestation_data.target_epoch, head_state.current_crosslinks[shard].epoch + MAX_EPOCHS_PER_CROSSLINK)`.
+* Let `parent_crosslink = head_state.current_crosslinks[shard]`.
+* Set `attestation_data.crosslink.start_epoch = parent_crosslink.end_epoch`.
+* Set `attestation_data.crosslink.end_epoch = min(attestation_data.target_epoch, parent_crosslink.end_epoch + MAX_EPOCHS_PER_CROSSLINK)`.
 * Set `attestation_data.crosslink.parent_root = hash_tree_root(head_state.current_crosslinks[shard])`.
 * Set `attestation_data.crosslink.data_root = ZERO_HASH`. *Note*: This is a stub for Phase 0.
 

test_libs/pyspec/eth2spec/utils/bls_stub.py

@@ -9,4 +9,4 @@ def bls_verify_multiple(pubkeys, message_hashes, signature, domain):
 
 
 def bls_aggregate_pubkeys(pubkeys):
-    return b'\x42' * 96
+    return b'\x42' * 48

test_libs/pyspec/tests/block_processing/test_process_attestation.py

@@ -64,6 +64,22 @@ def test_success_prevous_epoch(state):
     return pre_state, attestation, post_state
 
 
+def test_success_since_max_epochs_per_crosslink(state):
+    for _ in range(spec.MAX_EPOCHS_PER_CROSSLINK + 2):
+        next_epoch(state)
+
+    attestation = get_valid_attestation(state)
+    data = attestation.data
+    assert data.crosslink.end_epoch - data.crosslink.start_epoch == spec.MAX_EPOCHS_PER_CROSSLINK
+
+    for _ in range(spec.MIN_ATTESTATION_INCLUSION_DELAY):
+        next_slot(state)
+
+    pre_state, post_state = run_attestation_processing(state, attestation)
+
+    return pre_state, attestation, post_state
+
+
 def test_before_inclusion_delay(state):
     attestation = get_valid_attestation(state)
     # do not increment slot to allow for inclusion delay
@@ -124,7 +140,33 @@ def test_bad_previous_crosslink(state):
     for _ in range(spec.MIN_ATTESTATION_INCLUSION_DELAY):
         next_slot(state)
 
-    state.current_crosslinks[attestation.data.crosslink.shard].epoch += 10
+    attestation.data.crosslink.parent_root = b'\x27' * 32
+
+    pre_state, post_state = run_attestation_processing(state, attestation, False)
+
+    return pre_state, attestation, post_state
+
+
+def test_bad_crosslink_start_epoch(state):
+    next_epoch(state)
+    attestation = get_valid_attestation(state)
+    for _ in range(spec.MIN_ATTESTATION_INCLUSION_DELAY):
+        next_slot(state)
+
+    attestation.data.crosslink.start_epoch += 1
+
+    pre_state, post_state = run_attestation_processing(state, attestation, False)
+
+    return pre_state, attestation, post_state
+
+
+def test_bad_crosslink_end_epoch(state):
+    next_epoch(state)
+    attestation = get_valid_attestation(state)
+    for _ in range(spec.MIN_ATTESTATION_INCLUSION_DELAY):
+        next_slot(state)
+
+    attestation.data.crosslink.end_epoch += 1
 
     pre_state, post_state = run_attestation_processing(state, attestation, False)
 

test_libs/pyspec/tests/helpers.py

@@ -177,6 +177,7 @@ def build_attestation_data(state, slot, shard):
         justified_block_root = state.current_justified_root
 
     crosslinks = state.current_crosslinks if slot_to_epoch(slot) == get_current_epoch(state) else state.previous_crosslinks
+    parent_crosslink = crosslinks[shard]
     return AttestationData(
         beacon_block_root=block_root,
         source_epoch=justified_epoch,
@@ -185,9 +186,10 @@ def build_attestation_data(state, slot, shard):
         target_root=epoch_boundary_root,
         crosslink=Crosslink(
             shard=shard,
-            epoch=min(slot_to_epoch(slot), crosslinks[shard].epoch + MAX_EPOCHS_PER_CROSSLINK),
+            start_epoch=parent_crosslink.end_epoch,
+            end_epoch=min(slot_to_epoch(slot), parent_crosslink.end_epoch + MAX_EPOCHS_PER_CROSSLINK),
             data_root=spec.ZERO_HASH,
-            parent_root=hash_tree_root(crosslinks[shard]),
+            parent_root=hash_tree_root(parent_crosslink),
         ),
     )