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README.md
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@ -16,6 +16,7 @@ Accompanying documents can be found in [specs](specs) and include
* [SimpleSerialize (SSZ) spec](specs/simple-serialize.md)
* [BLS signature verification](specs/bls_signature.md)
* [General test format](specs/test-format.md)
* [Honest validator implementation doc](specs/validator/0_beacon-chain-validator.md)
## Design goals
The following are the broad design goals for Ethereum 2.0:

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@ -86,19 +86,23 @@ def hash_to_G2(message: bytes32, domain: uint64) -> [uint384]:
### `modular_squareroot`
`modular_squareroot(x)` returns a solution `y` to `y**2 % q == x`, and `None` if none exists. If there are two solutions the one with higher imaginary component is favored; if both solutions have equal imaginary component the one with higher real component is favored.
`modular_squareroot(x)` returns a solution `y` to `y**2 % q == x`, and `None` if none exists. If there are two solutions the one with higher imaginary component is favored; if both solutions have equal imaginary component the one with higher real component is favored (note that this is equivalent to saying that the single solution with either imaginary component > p/2 or imaginary component zero and real component > p/2 is favored).
The following is a sample implementation; implementers are free to implement modular square roots as they wish. Note that `x2 = -x1` is an _additive modular inverse_ so real and imaginary coefficients remain in `[0 .. q-1]`. `coerce_to_int(element: Fq) -> int` is a function that takes Fq element `element` (ie. integers `mod q`) and converts it to a regular integer.
```python
Fq2_order = q ** 2 - 1
eighth_roots_of_unity = [Fq2([1,1]) ** ((Fq2_order * k) // 8) for k in range(8)]
def modular_squareroot(value: int) -> int:
def modular_squareroot(value: Fq2) -> Fq2:
candidate_squareroot = value ** ((Fq2_order + 8) // 16)
check = candidate_squareroot ** 2 / value
if check in eighth_roots_of_unity[::2]:
x1 = candidate_squareroot / eighth_roots_of_unity[eighth_roots_of_unity.index(check) // 2]
x2 = -x1
return x1 if (x1.coeffs[1].n, x1.coeffs[0].n) > (x2.coeffs[1].n, x2.coeffs[0].n) else x2
x1_re, x1_im = coerce_to_int(x1.coeffs[0]), coerce_to_int(x1.coeffs[1])
x2_re, x2_im = coerce_to_int(x2.coeffs[0]), coerce_to_int(x2.coeffs[1])
return x1 if (x1_im > x2_im or (x1_im == x2_im and x1_re > x2_re)) else x2
return None
```

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@ -43,17 +43,19 @@ protocol for use in the Ethereum 2.0 Beacon Chain.
The core feature of `ssz` is the simplicity of the serialization with low
overhead.
## Terminology
## Variables and Functions
| Term | Definition |
|:-------------|:-----------------------------------------------------------------------------------------------|
| `little` | Little endian. |
| `byte_order` | Specifies [endianness](https://en.wikipedia.org/wiki/Endianness): big endian or little endian. |
| `byteorder` | Specifies [endianness](https://en.wikipedia.org/wiki/Endianness): big endian or little endian. |
| `len` | Length/number of bytes. |
| `to_bytes` | Convert to bytes. Should take parameters ``size`` and ``byte_order``. |
| `from_bytes` | Convert from bytes to object. Should take ``bytes`` and ``byte_order``. |
| `to_bytes` | Convert to bytes. Should take parameters ``size`` and ``byteorder``. |
| `from_bytes` | Convert from bytes to object. Should take ``bytes`` and ``byteorder``. |
| `value` | The value to serialize. |
| `rawbytes` | Raw serialized bytes. |
| `deserialized_object` | The deserialized data in the data structure of your programming language. |
| `new_index` | An index to keep track the latest position where the `rawbytes` have been deserialized. |
## Constants
@ -72,7 +74,6 @@ overhead.
|:---------:|:-----------------------------------------------------------|
| `uintN` | Type of `N` bits unsigned integer, where ``N % 8 == 0``. |
Convert directly to bytes the size of the int. (e.g. ``uint16 = 2 bytes``)
All integers are serialized as **little endian**.
@ -142,12 +143,8 @@ Lists are a collection of elements of the same homogeneous type.
|:--------------------------------------------|:----------------------------|
| Length of serialized list fits into 4 bytes | ``len(serialized) < 2**32`` |
1. Get the number of raw bytes to serialize: it is ``len(list) * sizeof(element)``.
* Encode that as a `4-byte` **little endian** `uint32`.
2. Append the elements in a packed manner.
* *Note on efficiency*: consider using a container that does not need to iterate over all elements to get its length. For example Python lists, C++ vectors or Rust Vec.
1. Serialize all list elements individually and concatenate them.
2. Prefix the concatenation with its length encoded as a `4-byte` **little-endian** unsigned integer.
**Example in Python**
@ -171,7 +168,6 @@ A container represents a heterogenous, associative collection of key-value pairs
To serialize a container, obtain the list of its field's names in the specified order. For each field name in this list, obtain the corresponding value and serialize it. Tightly pack the complete set of serialized values in the same order as the field names into a buffer. Calculate the size of this buffer of serialized bytes and encode as a `4-byte` **little endian** `uint32`. Prepend the encoded length to the buffer. The result of this concatenation is the final serialized value of the container.
| Check to perform | Code |
|:--------------------------------------------|:----------------------------|
| Length of serialized fields fits into 4 bytes | ``len(serialized) < 2**32`` |
@ -219,14 +215,21 @@ The decoding requires knowledge of the type of the item to be decoded. When
performing decoding on an entire serialized string, it also requires knowledge
of the order in which the objects have been serialized.
Note: Each return will provide ``deserialized_object, new_index`` keeping track
of the new index.
Note: Each return will provide:
- `deserialized_object`
- `new_index`
At each step, the following checks should be made:
| Check to perform | Check |
|:-------------------------|:-----------------------------------------------------------|
| Ensure sufficient length | ``length(rawbytes) >= current_index + deserialize_length`` |
| Ensure sufficient length | ``len(rawbytes) >= current_index + deserialize_length`` |
At the final step, the following checks should be made:
| Check to perform | Check |
|:-------------------------|:-------------------------------------|
| Ensure no extra length | `new_index == len(rawbytes)` |
#### uint
@ -295,7 +298,7 @@ entire length of the list.
| Check to perform | code |
|:------------------------------------------|:----------------------------------------------------------------|
| rawbytes has enough left for length | ``len(rawbytes) > current_index + LENGTH_BYTES`` |
| ``rawbytes`` has enough left for length | ``len(rawbytes) > current_index + LENGTH_BYTES`` |
| list is not greater than serialized bytes | ``len(rawbytes) > current_index + LENGTH_BYTES + total_length`` |
```python
@ -323,7 +326,7 @@ Instantiate a container with the full set of deserialized data, matching each me
| Check to perform | code |
|:------------------------------------------|:----------------------------------------------------------------|
| rawbytes has enough left for length | ``len(rawbytes) > current_index + LENGTH_BYTES`` |
| ``rawbytes`` has enough left for length | ``len(rawbytes) > current_index + LENGTH_BYTES`` |
| list is not greater than serialized bytes | ``len(rawbytes) > current_index + LENGTH_BYTES + total_length`` |
To deserialize:
@ -442,6 +445,5 @@ return hash(b''.join([hash_tree_root(getattr(x, field)) for field in value.field
| Go | [ https://github.com/prysmaticlabs/prysm/tree/master/shared/ssz ](https://github.com/prysmaticlabs/prysm/tree/master/shared/ssz) | Go implementation of SSZ mantained by Prysmatic Labs |
| Swift | [ https://github.com/yeeth/SimpleSerialize.swift ](https://github.com/yeeth/SimpleSerialize.swift) | Swift implementation maintained SSZ |
## Copyright
Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).

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@ -0,0 +1,358 @@
# Ethereum 2.0 Phase 0 -- 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)
- [Prerequisites](#prerequisites)
- [Constants](#constants)
- [Misc](#misc)
- [Becoming a validator](#becoming-a-validator)
- [Initialization](#initialization)
- [BLS public key](#bls-public-key)
- [BLS withdrawal key](#bls-withdrawal-key)
- [Submit deposit](#submit-deposit)
- [Process deposit](#process-deposit)
- [Validator index](#validator-index)
- [Activation](#activation)
- [Beacon chain responsibilities](#beacon-chain-responsibilities)
- [Block proposal](#block-proposal)
- [Block header](#block-header)
- [Slot](#slot)
- [Parent root](#parent-root)
- [State root](#state-root)
- [Randao reveal](#randao-reveal)
- [Eth1 Data](#eth1-data)
- [Signature](#signature)
- [Block body](#block-body)
- [Proposer slashings](#proposer-slashings)
- [Attester slashings](#attester-slashings)
- [Attestations](#attestations)
- [Deposits](#deposits)
- [Exits](#exits)
- [Attestations](#attestations-1)
- [Attestation data](#attestation-data)
- [Slot](#slot-1)
- [Shard](#shard)
- [Beacon block root](#beacon-block-root)
- [Epoch boundary root](#epoch-boundary-root)
- [Shard block root](#shard-block-root)
- [Latest crosslink root](#latest-crosslink-root)
- [Justified epoch](#justified-epoch)
- [Justified block root](#justified-block-root)
- [Construct attestation](#construct-attestation)
- [Data](#data)
- [Aggregation bitfield](#aggregation-bitfield)
- [Custody bitfield](#custody-bitfield)
- [Aggregate signature](#aggregate-signature)
- [How to avoid slashing](#how-to-avoid-slashing)
- [Proposer slashing](#proposer-slashing)
- [Attester slashing](#attester-slashing)
<!-- /TOC -->
## Introduction
This document represents the expected behavior of an "honest validator" with respect to Phase 0 of the Ethereum 2.0 protocol. This document does not distinguish between a "node" (ie. the functionality of following and reading the beacon chain) and a "validator client" (ie. the functionality of actively participating in consensus). The separation of concerns between these (potentially) two pieces of software is left as a design decision that is out of scope.
A validator is an entity that participates in the consensus of the Ethereum 2.0 protocol. This is an optional role for users in which they can post ETH as collateral and verify and attest to the validity of blocks to seek financial returns in exchange for building and securing the protocol. This is similar to proof of work networks in which a miner provides collateral in the form of hardware/hash-power to seek returns in exchange for building and securing the protocol.
## Prerequisites
All terminology, constants, functions, and protocol mechanics defined in the [Phase 0 -- The Beacon Chain](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md) doc are requisite for this document and used throughout. Please see the Phase 0 doc before continuing and use as a reference throughout.
## Constants
### Misc
| Name | Value | Unit | Duration |
| - | - | :-: | :-: |
| `ETH1_FOLLOW_DISTANCE` | `2**10` (= 1,024) | blocks | ~4 hours |
## Becoming a validator
### Initialization
A validator must initialize many parameters locally before submitting a deposit and joining the validator registry.
#### BLS public key
Validator public keys are [G1 points](https://github.com/ethereum/eth2.0-specs/blob/master/specs/bls_signature.md#g1-points) on the [BLS12-381 curve](https://z.cash/blog/new-snark-curve). A private key, `privkey`, must be securely generated along with the resultant `pubkey`. This `privkey` must be "hot", that is, constantly available to sign data throughout the lifetime of the validator.
#### BLS withdrawal key
A secondary withdrawal private key, `withdrawal_privkey`, must also be securely generated along with the resultant `withdrawal_pubkey`. This `withdrawal_privkey` does not have to be available for signing during the normal lifetime of a validator and can live in "cold storage".
The validator constructs their `withdrawal_credentials` via the following:
* Set `withdrawal_credentials[:1] == BLS_WITHDRAWAL_PREFIX_BYTE`.
* Set `withdrawal_credentials[1:] == hash(withdrawal_pubkey)[1:]`.
### Submit deposit
In phase 0, all incoming validator deposits originate from the Ethereum 1.0 PoW chain. Deposits are made to the [deposit contract](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#ethereum-10-deposit-contract) located at `DEPOSIT_CONTRACT_ADDRESS`.
To submit a deposit:
* Pack the validator's [initialization parameters](#initialization) into `deposit_input`, a [`DepositInput`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#depositinput) SSZ object.
* Set `deposit_input.proof_of_possession = EMPTY_SIGNATURE`.
* Let `proof_of_possession` be the result of `bls_sign` of the `hash_tree_root(deposit_input)` with `domain=DOMAIN_DEPOSIT`.
* Set `deposit_input.proof_of_possession = proof_of_possession`.
* Let `amount` be the amount in Gwei to be deposited by the validator where `MIN_DEPOSIT_AMOUNT <= amount <= MAX_DEPOSIT_AMOUNT`.
* Send a transaction on the Ethereum 1.0 chain to `DEPOSIT_CONTRACT_ADDRESS` executing `deposit` along with `serialize(deposit_input)` as the singular `bytes` input along with a deposit `amount` in Gwei.
_Note_: Deposits made for the same `pubkey` are treated as for the same validator. A singular `Validator` will be added to `state.validator_registry` with each additional deposit amount added to the validator's balance. A validator can only be activated when total deposits for the validator pubkey meet or exceed `MAX_DEPOSIT_AMOUNT`.
### Process deposit
Deposits cannot be processed into the beacon chain until the eth1.0 block in which they were deposited or any of its descendants is added to the beacon chain `state.eth1_data`. This takes _a minimum_ of `ETH1_FOLLOW_DISTANCE` eth1.0 blocks (~4 hours) plus `ETH1_DATA_VOTING_PERIOD` epochs (~1.7 hours). Once the requisite eth1.0 data is added, the deposit will normally be added to a beacon chain block and processed into the `state.validator_registry` within an epoch or two. The validator is then in a queue to be activated.
### Validator index
Once a validator has been processed and added to the beacon state's `validator_registry`, the validator's `validator_index` is defined by the index into the registry at which the [`ValidatorRecord`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#validator) contains the `pubkey` specified in the validator's deposit. A validator's `validator_index` is guaranteed to not change from the time of initial deposit until the validator exits and fully withdraws. This `validator_index` is used throughout the specification to dictate validator roles and responsibilities at any point and should be stored locally.
### Activation
In normal operation, the validator is quickly activated at which point the validator is added to the shuffling and begins validation after an additional `ENTRY_EXIT_DELAY` epochs (25.6 minutes).
The function [`is_active_validator`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#is_active_validator) can be used to check if a validator is active during a given epoch. Usage is as follows:
```python
validator = state.validator_registry[validator_index]
is_active = is_active_validator(validator, epoch)
```
Once a validator is activated, the validator is assigned [responsibilities](#beacon-chain-responsibilities) until exited.
_Note_: There is a maximum validator churn per finalized epoch so the delay until activation is variable depending upon finality, total active validator balance, and the number of validators in the queue to be activated.
## Beacon chain responsibilities
A validator has two primary responsibilities to the beacon chain -- [proposing blocks](block-proposal) and [creating attestations](attestations-1). Proposals happen infrequently, whereas attestations should be created once per epoch.
### Block proposal
A validator is expected to propose a [`BeaconBlock`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beaconblock) at the beginning of any slot during which `get_beacon_proposer_index(state, slot)` returns the validator's `validator_index`. To propose, the validator selects the `BeaconBlock`, `parent`, that in their view of the fork choice is the head of the chain during `slot`. The validator is to create, sign, and broadcast a `block` that is a child of `parent` and that executes a valid [beacon chain state transition](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function).
There is one proposer per slot, so if there are N active validators any individual validator will on average be assigned to propose once per N slots (eg. at 312500 validators = 10 million ETH, that's once per ~3 weeks).
#### Block header
##### Slot
Set `block.slot = slot` where `slot` is the current slot at which the validator has been selected to propose. The `parent` selected must satisfy that `parent.slot < block.slot`.
_Note:_ there might be "skipped" slots between the `parent` and `block`. These skipped slots are processed in the state transition function without per-block processing.
##### Parent root
Set `block.parent_root = hash_tree_root(parent)`.
##### State root
Set `block.state_root = hash_tree_root(state)` of the resulting `state` of the `parent -> block` state transition.
_Note_: To calculate `state_root`, the validator should first run the state transition function on an unsigned `block` containing a stub for the `state_root`. It is useful to be able to run a state transition function that does _not_ validate signatures for this purpose.
##### Randao reveal
Set `block.randao_reveal = epoch_signature` where `epoch_signature` is defined as:
```python
epoch_signature = bls_sign(
privkey=validator.privkey, # privkey store locally, not in state
message=int_to_bytes32(slot_to_epoch(block.slot)),
domain=get_domain(
fork=fork, # `fork` is the fork object at the slot `block.slot`
epoch=slot_to_epoch(block.slot),
domain_type=DOMAIN_RANDAO,
)
)
```
##### Eth1 Data
`block.eth1_data` is a mechanism used by block proposers vote on a recent Ethereum 1.0 block hash and an associated deposit root found in the Ethereum 1.0 deposit contract. When consensus is formed, `state.latest_eth1_data` is updated, and validator deposits up to this root can be processed. The deposit root can be calculated by calling the `get_deposit_root()` function of the deposit contract using the post-state of the block hash.
* Let `D` be the set of `Eth1DataVote` objects `vote` in `state.eth1_data_votes` where:
* `vote.eth1_data.block_hash` is the hash of an eth1.0 block that is (i) part of the canonical chain, (ii) >= `ETH1_FOLLOW_DISTANCE` blocks behind the head, and (iii) newer than `state.latest_eth1_data.block_data`.
* `vote.eth1_data.deposit_root` is the deposit root of the eth1.0 deposit contract at the block defined by `vote.eth1_data.block_hash`.
* If `D` is empty:
* Let `block_hash` be the block hash of the `ETH1_FOLLOW_DISTANCE`'th ancestor of the head of the canonical eth1.0 chain.
* Let `deposit_root` be the deposit root of the eth1.0 deposit contract in the post-state of the block referenced by `block_hash`
* If `D` is nonempty:
* Let `best_vote` be the member of `D` that has the highest `vote.eth1_data.vote_count`, breaking ties by favoring block hashes with higher associated block height.
* Let `block_hash = best_vote.eth1_data.block_hash`.
* Let `deposit_root = best_vote.eth1_data.deposit_root`.
* Set `block.eth1_data = Eth1Data(deposit_root=deposit_root, block_hash=block_hash)`.
##### Signature
Set `block.signature = signed_proposal_data` where `signed_proposal_data` is defined as:
```python
proposal_data = ProposalSignedData(
slot=slot,
shard=BEACON_CHAIN_SHARD_NUMBER,
block_root=hash_tree_root(block), # where `block.sigature == EMPTY_SIGNATURE
)
proposal_root = hash_tree_root(proposal_data)
signed_proposal_data = bls_sign(
privkey=validator.privkey, # privkey store locally, not in state
message=proposal_root,
domain=get_domain(
fork=fork, # `fork` is the fork object at the slot `block.slot`
epoch=slot_to_epoch(block.slot),
domain_type=DOMAIN_PROPOSAL,
)
)
```
#### Block body
##### Proposer slashings
Up to `MAX_PROPOSER_SLASHINGS` [`ProposerSlashing`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#proposerslashing) objects can be included in the `block`. The proposer slashings must satisfy the verification conditions found in [proposer slashings processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#proposer-slashings-1). The validator receives a small "whistleblower" reward for each proposer slashing found and included.
##### Attester slashings
Up to `MAX_ATTESTER_SLASHINGS` [`AttesterSlashing`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attesterslashing) objects can be included in the `block`. The attester slashings must satisfy the verification conditions found in [Attester slashings processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attester-slashings-1). The validator receives a small "whistleblower" reward for each attester slashing found and included.
##### Attestations
Up to `MAX_ATTESTATIONS` aggregate attestations can be included in the `block`. The attestations added must satisfy the verification conditions found in [attestation processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestations-1). To maximize profit, the validator should attempt to create aggregate attestations that include singular attestations from the largest number of validators whose signatures from the same epoch have not previously been added on chain.
##### Deposits
Up to `MAX_DEPOSITS` [`Deposit`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#deposit) objects can be included in the `block`. These deposits are constructed from the `Deposit` logs from the [Eth1.0 deposit contract](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#ethereum-10-deposit-contract) and must be processed in sequential order. The deposits included in the `block` must satisfy the verification conditions found in [deposits processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#deposits-1).
##### Exits
Up to `MAX_EXITS` [`Exit`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#exit) objects can be included in the `block`. The exits must satisfy the verification conditions found in [exits processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#exits-1).
### Attestations
A validator is expected to create, sign, and broadcast an attestation during each epoch. The slot during which the validator performs this role is any slot at which `get_crosslink_committees_at_slot(state, slot)` contains a committee that contains `validator_index`.
A validator should create and broadcast the attestation halfway through the `slot` during which the validator is assigned -- that is `SLOT_DURATION * 0.5` seconds after the start of `slot`.
#### Attestation data
First the validator should construct `attestation_data`, an [`AttestationData`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestationdata) object based upon the state at the assigned slot.
##### Slot
Set `attestation_data.slot = slot` where `slot` is the current slot of which the validator is a member of a committee.
##### Shard
Set `attestation_data.shard = shard` where `shard` is the shard associated with the validator's committee defined by `get_crosslink_committees_at_slot`.
##### Beacon block root
Set `attestation_data.beacon_block_root = hash_tree_root(head)` where `head` is the validator's view of the `head` block of the beacon chain during `slot`.
##### Epoch boundary root
Set `attestation_data.epoch_boundary_root = hash_tree_root(epoch_boundary)` where `epoch_boundary` is the block at the most recent epoch boundary in the chain defined by `head` -- i.e. the `BeaconBlock` where `block.slot == get_epoch_start_slot(head.slot)`.
_Note:_ This can be looked up in the state using `get_block_root(state, get_epoch_start_slot(head.slot))`.
##### Shard block root
Set `attestation_data.shard_block_root = ZERO_HASH`.
_Note:_ This is a stub for phase 0.
##### Latest crosslink root
Set `attestation_data.latest_crosslink_root = state.latest_crosslinks[shard].shard_block_root` where `state` is the beacon state at `head` and `shard` is the validator's assigned shard.
##### Justified epoch
Set `attestation_data.justified_epoch = state.justified_epoch` where `state` is the beacon state at `head`.
##### Justified block root
Set `attestation_data.justified_block_root = hash_tree_root(justified_block)` where `justified_block` is the block at `state.justified_epoch` in the chain defined by `head`.
_Note:_ This can be looked up in the state using `get_block_root(state, justified_epoch)`.
#### Construct attestation
Next the validator creates `attestation`, an [`Attestation`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestation) object.
##### Data
Set `attestation.data = attestation_data` where `attestation_data` is the `AttestationData` object defined in the previous section, [attestation data](#attestation-data).
##### Aggregation bitfield
* Let `aggregation_bitfield` be a byte array filled with zeros of length `(len(committee) + 7) // 8`.
* Let `index_into_committee` be the index into the validator's `committee` at which `validator_index` is located.
* Set `aggregation_bitfield[index_into_committee // 8] |= 2 ** (index_into_committee % 8)`.
* Set `attestation.aggregation_bitfield = aggregation_bitfield`.
_Note_: Calling `get_attestation_participants(state, attestation.data, attestation.aggregation_bitfield)` should return a list of length equal to 1, containing `validator_index`.
##### Custody bitfield
* Let `custody_bitfield` be a byte array filled with zeros of length `(len(committee) + 7) // 8`.
* Set `attestation.custody_bitfield = custody_bitfield`.
_Note:_ This is a stub for phase 0.
##### Aggregate signature
Set `attestation.aggregate_signature = signed_attestation_data` where `signed_attestation_data` is defined as:
```python
attestation_data_and_custody_bit = AttestationDataAndCustodyBit(
data=attestation.data,
custody_bit=0b0,
)
attestation_message_to_sign = hash_tree_root(attestation_data_and_custody_bit)
signed_attestation_data = bls_sign(
privkey=validator.privkey, # privkey store locally, not in state
message=attestation_message_to_sign,
domain=get_domain(
fork=fork, # `fork` is the fork object at the slot, `attestation_data.slot`
epoch=slot_to_epoch(attestation_data.slot),
domain_type=DOMAIN_ATTESTATION,
)
)
```
## How to avoid slashing
"Slashing" is the burning of some amount of validator funds and immediate ejection from the active validator set. In Phase 0, there are two ways in which funds can be slashed -- [proposer slashing](#proposer-slashing) and [attester slashing](#attester-slashing). Although being slashed has serious repercussions, it is simple enough to avoid being slashed all together by remaining _consistent_ with respect to the messages a validator has previously signed.
_Note_: Signed data must be within a sequential `Fork` context to conflict. Messages cannot be slashed across diverging forks. If the previous fork version is 1 and the chain splits into fork 2 and 102, messages from 1 can slashable against messages in forks 1, 2, and 102. Messages in 2 cannot be slashable against messages in 102 and vice versa.
### Proposer slashing
To avoid "proposer slashings", a validator must not sign two conflicting [`ProposalSignedData`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#proposalsigneddata) where conflicting is defined as having the same `slot` and `shard` but a different `block_root`. In phase 0, proposals are only made for the beacon chain (`shard == BEACON_CHAIN_SHARD_NUMBER`).
_In phase 0, as long as the validator does not sign two different beacon chain proposals for the same slot, the validator is safe against proposer slashings._
Specifically, when signing an `BeaconBlock`, a validator should perform the following steps in the following order:
1. Save a record to hard disk that an beacon block has been signed for the `slot=slot` and `shard=BEACON_CHAIN_SHARD_NUMBER`.
2. Generate and broadcast the block.
If the software crashes at some point within this routine, then when the validator comes back online the hard disk has the record of the _potentially_ signed/broadcast block and can effectively avoid slashing.
### Attester slashing
To avoid "attester slashings", a validator must not sign two conflicting [`AttestationData`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestationdata) objects where conflicting is defined as a set of two attestations that satisfy either [`is_double_vote`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#is_double_vote) or [`is_surround_vote`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#is_surround_vote).
Specifically, when signing an `Attestation`, a validator should perform the following steps in the following order:
1. Save a record to hard disk that an attestation has been signed for source -- `attestation_data.justified_epoch` -- and target -- `slot_to_epoch(attestation_data.slot)`.
2. Generate and broadcast attestation.
If the software crashes at some point within this routine, then when the validator comes back online the hard disk has the record of the _potentially_ signed/broadcast attestation and can effectively avoid slashing.