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Clean up light client spec

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Hsiao-Wei Wang 2019-04-11 17:13:45 +10:00
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2 changed files with 30 additions and 27 deletions
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36
specs/light_client/merkle_proofs.md
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@ -47,33 +47,33 @@ y_data_root len(y)
We can now define a concept of a "path", a way of describing a function that takes as input an SSZ object and outputs some specific (possibly deeply nested) member. For example, `foo -> foo.x` is a path, as are `foo -> len(foo.y)` and `foo -> foo.y[5].w`. We'll describe paths as lists, which can have two representations. In "human-readable form", they are `["x"]`, `["y", "__len__"]` and `["y", 5, "w"]` respectively. In "encoded form", they are lists of `uint64` values, in these cases (assuming the fields of `foo` in order are `x` then `y`, and `w` is the first field of `y[i]`) `[0]`, `[1, 2**64-1]`, `[1, 5, 0]`.
```python
def path_to_encoded_form(obj: Any, path: List[str or int]) -> List[int]:
def path_to_encoded_form(obj: Any, path: List[Union[str, int]]) -> List[int]:
if len(path) == 0:
return []
if isinstance(path[0], "__len__"):
elif isinstance(path[0], "__len__"):
assert len(path) == 1
return [LENGTH_FLAG]
elif isinstance(path[0], str) and hasattr(obj, "fields"):
return [list(obj.fields.keys()).index(path[0])] + path_to_encoded_form(getattr(obj, path[0]), path[1:])
elif isinstance(obj, (StaticList, DynamicList)):
elif isinstance(obj, (Vector, List)):
return [path[0]] + path_to_encoded_form(obj[path[0]], path[1:])
else:
raise Exception("Unknown type / path")
```
We can now define a function `get_generalized_indices(object: Any, path: List[int], root=1: int) -> int` that converts an object and a path to a set of generalized indices (note that for constant-sized objects, there is only one generalized index and it only depends on the path, but for dynamically sized objects the indices may depend on the object itself too). For dynamically-sized objects, the set of indices will have more than one member because of the need to access an array's length to determine the correct generalized index for some array access.
We can now define a function `get_generalized_indices(object: Any, path: List[int], root: int=1) -> List[int]` that converts an object and a path to a set of generalized indices (note that for constant-sized objects, there is only one generalized index and it only depends on the path, but for dynamically sized objects the indices may depend on the object itself too). For dynamically-sized objects, the set of indices will have more than one member because of the need to access an array's length to determine the correct generalized index for some array access.
```python
def get_generalized_indices(obj: Any, path: List[int], root=1) -> List[int]:
def get_generalized_indices(obj: Any, path: List[int], root: int=1) -> List[int]:
if len(path) == 0:
return [root]
elif isinstance(obj, StaticList):
elif isinstance(obj, Vector):
items_per_chunk = (32 // len(serialize(x))) if isinstance(x, int) else 1
new_root = root * next_power_of_2(len(obj) // items_per_chunk) + path[0] // items_per_chunk
return get_generalized_indices(obj[path[0]], path[1:], new_root)
elif isinstance(obj, DynamicList) and path[0] == LENGTH_FLAG:
elif isinstance(obj, List) and path[0] == LENGTH_FLAG:
return [root * 2 + 1]
elif isinstance(obj, DynamicList) and isinstance(path[0], int):
elif isinstance(obj, List) and isinstance(path[0], int):
assert path[0] < len(obj)
items_per_chunk = (32 // len(serialize(x))) if isinstance(x, int) else 1
new_root = root * 2 * next_power_of_2(len(obj) // items_per_chunk) + path[0] // items_per_chunk
@ -137,19 +137,19 @@ Generating a proof is simply a matter of taking the node of the SSZ hash tree wi
Here is the verification function:
```python
def verify_multi_proof(root, indices, leaves, proof):
def verify_multi_proof(root: Bytes32, indices: List[int], leaves: List[Bytes32], proof: List[bytes]):
tree = {}
for index, leaf in zip(indices, leaves):
tree[index] = leaf
for index, proofitem in zip(get_proof_indices(indices), proof):
tree[index] = proofitem
indexqueue = sorted(tree.keys())[:-1]
index_queue = sorted(tree.keys())[:-1]
i = 0
while i < len(indexqueue):
index = indexqueue[i]
if index >= 2 and index^1 in tree:
tree[index//2] = hash(tree[index - index%2] + tree[index - index%2 + 1])
indexqueue.append(index//2)
while i < len(index_queue):
index = index_queue[i]
if index >= 2 and index ^ 1 in tree:
tree[index // 2] = hash(tree[index - index % 2] + tree[index - index % 2 + 1])
index_queue.append(index // 2)
i += 1
return (indices == []) or (1 in tree and tree[1] == root)
```
@ -158,7 +158,7 @@ def verify_multi_proof(root, indices, leaves, proof):
We define:
#### `MerklePartial`
#### `SSZMerklePartial`
```python
@ -172,6 +172,6 @@ We define:
#### Proofs for execution
We define `MerklePartial(f, arg1, arg2..., focus=0)` as being a `MerklePartial` object wrapping a Merkle multiproof of the set of nodes in the hash tree of the SSZ object `arg[focus]` that is needed to authenticate the parts of the object needed to compute `f(arg1, arg2...)`.
We define `MerklePartial(f, arg1, arg2..., focus=0)` as being a `SSZMerklePartial` object wrapping a Merkle multiproof of the set of nodes in the hash tree of the SSZ object `arg[focus]` that is needed to authenticate the parts of the object needed to compute `f(arg1, arg2...)`.
Ideally, any function which accepts an SSZ object should also be able to accept a `MerklePartial` object as a substitute.
Ideally, any function which accepts an SSZ object should also be able to accept a `SSZMerklePartial` object as a substitute.

13
specs/light_client/sync_protocol.md
View File

@ -5,16 +5,19 @@ __NOTICE__: This document is a work-in-progress for researchers and implementers
## Table of Contents
<!-- TOC -->
- [Beacon Chain Light Client Syncing](#beacon-chain-light-client-syncing)
- [Table of Contents](#table-of-contents)
- [Preliminaries](#preliminaries)
- [Light client state](#light-client-state)
- [Updating the shuffled committee](#updating-the-shuffled-committee)
- [Computing the current committee](#computing-the-current-committee)
- [Verifying blocks](#verifying-blocks)
<!-- /TOC -->
### Preliminaries
## Preliminaries
We define an "expansion" of an object as an object where a field in an object that is meant to represent the `hash_tree_root` of another object is replaced by the object. Note that defining expansions is not a consensus-layer-change; it is merely a "re-interpretation" of the object. Particularly, the `hash_tree_root` of an expansion of an object is identical to that of the original object, and we can define expansions where, given a complete history, it is always possible to compute the expansion of any object in the history. The opposite of an expansion is a "summary" (eg. `BeaconBlockHeader` is a summary of `BeaconBlock`).
@ -85,7 +88,7 @@ later_period_data = get_period_data(new_committee_proof, finalized_header, shard
The maximum size of a proof is `128 * ((22-7) * 32 + 110) = 75520` bytes for validator records and `(22-7) * 32 + 128 * 8 = 1504` for the active index proof (much smaller because the relevant active indices are all beside each other in the Merkle tree). This needs to be done once per `PERSISTENT_COMMITTEE_PERIOD` epochs (2048 epochs / 9 days), or ~38 bytes per epoch.
### Computing the current committee
## Computing the current committee
Here is a helper to compute the committee at a slot given the maximal earlier and later committees:
@ -134,16 +137,16 @@ def compute_committee(header: BeaconBlockHeader,
Note that this method makes use of the fact that the committee for any given shard always starts and ends at the same validator index independently of the committee count (this is because the validator set is split into `SHARD_COUNT * committee_count` slices but the first slice of a shard is a multiple `committee_count * i`, so the start of the slice is `n * committee_count * i // (SHARD_COUNT * committee_count) = n * i // SHARD_COUNT`, using the slightly nontrivial algebraic identity `(x * a) // ab == x // b`).
### Verifying blocks
## Verifying blocks
If a client wants to update its `finalized_header` it asks the network for a `BlockValidityProof`, which is simply:
```python
{
'header': BlockHeader,
'header': BeaconBlockHeader,
'shard_aggregate_signature': 'bytes96',
'shard_bitfield': 'bytes',
'shard_parent_block': ShardBlock
'shard_parent_block': ShardBlock,
}
```