EIP4844: Update cryptography API and Fiat-Shamir logic

setup.py

@@ -232,7 +232,7 @@ def get_spec(file_name: Path, preset: Dict[str, str], config: Dict[str, str]) ->
 
                     if not _is_constant_id(name):
                         # Check for short type declarations
-                        if value.startswith(("uint", "Bytes", "ByteList", "Union", "Vector", "List")):
+                        if value.startswith(("uint", "Bytes", "ByteList", "Union", "Vector", "List", "ByteVector")):
                             custom_types[name] = value
                         continue
 
@@ -590,7 +590,6 @@ def imports(cls, preset_name: str):
         return super().imports(preset_name) + f'''
 from eth2spec.utils import kzg
 from eth2spec.bellatrix import {preset_name} as bellatrix
-from eth2spec.utils.ssz.ssz_impl import serialize as ssz_serialize
 '''
 
 
@@ -617,12 +616,13 @@ def sundry_functions(cls) -> str:
 ROOTS_OF_UNITY = kzg.compute_roots_of_unity(TESTING_FIELD_ELEMENTS_PER_BLOB)
 
 
-def retrieve_blobs_sidecar(slot: Slot, beacon_block_root: Root) -> BlobsSidecar:
-    pass'''
+def retrieve_blobs_sidecar(slot: Slot, beacon_block_root: Root) -> Optional[BlobsSidecar]:
+    return "TEST"'''
 
     @classmethod
     def hardcoded_custom_type_dep_constants(cls, spec_object) -> str:
         constants = {
+            'BYTES_PER_FIELD_ELEMENT': spec_object.constant_vars['BYTES_PER_FIELD_ELEMENT'].value,
             'FIELD_ELEMENTS_PER_BLOB': spec_object.preset_vars['FIELD_ELEMENTS_PER_BLOB'].value,
             'MAX_BLOBS_PER_BLOCK': spec_object.preset_vars['MAX_BLOBS_PER_BLOCK'].value,
         }

specs/eip4844/beacon-chain.md

@@ -23,6 +23,8 @@
     - [`ExecutionPayloadHeader`](#executionpayloadheader)
 - [Helper functions](#helper-functions)
   - [Misc](#misc)
+    - [`validate_blobs_sidecar`](#validate_blobs_sidecar)
+    - [`is_data_available`](#is_data_available)
     - [`kzg_commitment_to_versioned_hash`](#kzg_commitment_to_versioned_hash)
     - [`tx_peek_blob_versioned_hashes`](#tx_peek_blob_versioned_hashes)
     - [`verify_kzg_commitments_against_transactions`](#verify_kzg_commitments_against_transactions)
@@ -44,9 +46,7 @@ This upgrade adds blobs to the beacon chain as part of EIP-4844.
 
 | Name | SSZ equivalent | Description |
 | - | - | - |
-| `Blob` | `Vector[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB]` | |
 | `VersionedHash` | `Bytes32` | |
-| `KZGCommitment` | `Bytes48` | Same as BLS standard "is valid pubkey" check but also allows `0x00..00` for point-at-infinity |
 
 ## Constants
 
@@ -55,7 +55,6 @@ This upgrade adds blobs to the beacon chain as part of EIP-4844.
 | Name | Value |
 | - | - |
 | `BLOB_TX_TYPE` | `uint8(0x05)` |
-| `FIELD_ELEMENTS_PER_BLOB` | `uint64(4096)` |
 | `VERSIONED_HASH_VERSION_KZG` | `Bytes1(0x01)` | 
 
 ### Domain types
@@ -150,6 +149,43 @@ class ExecutionPayloadHeader(Container):
 
 ### Misc
 
+#### `validate_blobs_sidecar`
+
+```python
+def validate_blobs_sidecar(slot: Slot,
+                           beacon_block_root: Root,
+                           expected_kzg_commitments: Sequence[KZGCommitment],
+                           blobs_sidecar: BlobsSidecar) -> None:
+    assert slot == blobs_sidecar.beacon_block_slot
+    assert beacon_block_root == blobs_sidecar.beacon_block_root
+    blobs = blobs_sidecar.blobs
+    kzg_aggregated_proof = blobs_sidecar.kzg_aggregated_proof
+    assert len(expected_kzg_commitments) == len(blobs)
+
+    assert verify_aggregate_kzg_proof(blobs, expected_kzg_commitments, kzg_aggregated_proof)
+```
+
+#### `is_data_available`
+
+The implementation of `is_data_available` is meant to change with later sharding upgrades.
+Initially, it requires every verifying actor to retrieve the matching `BlobsSidecar`,
+and validate the sidecar with `validate_blobs_sidecar`.
+
+Without the sidecar the block may be processed further optimistically,
+but MUST NOT be considered valid until a valid `BlobsSidecar` has been downloaded.
+
+```python
+def is_data_available(slot: Slot, beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
+    # `retrieve_blobs_sidecar` is implementation dependent, raises an exception if not available.
+    sidecar = retrieve_blobs_sidecar(slot, beacon_block_root)
+    if sidecar == "TEST":
+        return True  # For testing; remove once we have a way to inject `BlobsSidecar` into tests
+    validate_blobs_sidecar(slot, beacon_block_root, blob_kzg_commitments, sidecar)
+
+    return True
+```
+
+
 #### `kzg_commitment_to_versioned_hash`
 
 ```python
@@ -204,6 +240,9 @@ def process_block(state: BeaconState, block: BeaconBlock) -> None:
     process_operations(state, block.body)
     process_sync_aggregate(state, block.body.sync_aggregate)
     process_blob_kzg_commitments(state, block.body)  # [New in EIP-4844]
+
+    # New in EIP-4844, note: Can sync optimistically without this condition, see note on `is_data_available`
+    assert is_data_available(block.slot, hash_tree_root(block), block.body.blob_kzg_commitments)
 ```
 
 #### Execution payload