add eth1 withdrawal credentials to spec

specs/phase0/beacon-chain.md

@@ -13,6 +13,7 @@
   - [Misc](#misc)
   - [Gwei values](#gwei-values)
   - [Initial values](#initial-values)
+  - [Validator withdrawal credential versions](#validator-withdrawal-credential-versions)
   - [Time parameters](#time-parameters)
   - [State list lengths](#state-list-lengths)
   - [Rewards and penalties](#rewards-and-penalties)
@@ -209,7 +210,11 @@ The following values are (non-configurable) constants used throughout the specif
 | Name | Value |
 | - | - |
 | `GENESIS_FORK_VERSION` | `Version('0x00000000')` |
+
+### Validator withdrawal credential versions
+
 | `BLS_WITHDRAWAL_PREFIX` | `Bytes1('0x00')` |
+| `ETH1_ADDRESS_WITHDRAWAL_PREFIX` | `Bytes1('0x01')` |
 
 ### Time parameters
 

specs/phase0/deposit-contract.md

@@ -58,12 +58,17 @@ The amount of ETH (rounded down to the closest Gwei) sent to the deposit contrac
 
 #### Withdrawal credentials
 
-One of the `DepositData` fields is `withdrawal_credentials`. It is a commitment to credentials for withdrawing validator balance (e.g. to another validator, or to shards). The first byte of `withdrawal_credentials` is a version number. As of now, the only expected format is as follows:
+One of the `DepositData` fields is `withdrawal_credentials`.
 
-* `withdrawal_credentials[:1] == BLS_WITHDRAWAL_PREFIX`
-* `withdrawal_credentials[1:] == hash(withdrawal_pubkey)[1:]` where `withdrawal_pubkey` is a BLS pubkey
+This field is a commitment to credentials for withdrawing validator balance (e.g. to another validator, or to shards).
+The first byte of `withdrawal_credentials` is a version number. The remaining
+bytes are content specific to the version.
 
-The private key corresponding to `withdrawal_pubkey` will be required to initiate a withdrawal. It can be stored separately until a withdrawal is required, e.g. in cold storage.
+Currently, `BLS_WITHDRAWAL_PREFIX` and `ETH1_ADDRESS_WITHDRAWAL_PREFIX`
+versioned withdrawal credentials are supported. Read more in the [validator guide](./validator.md).
+
+*Note*: The deposit contract does *not* validate withdrawal credentials.
+Thus, new versions can be added without modifications of the deposit contract.
 
 #### `DepositEvent` log
 

specs/phase0/validator.md

@@ -15,7 +15,9 @@ This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](.
 - [Becoming a validator](#becoming-a-validator)
   - [Initialization](#initialization)
     - [BLS public key](#bls-public-key)
-    - [BLS withdrawal key](#bls-withdrawal-key)
+    - [Withdrawal credentials](#withdrawal-credentials)
+      - [BLS key credentials](#bls-key-credentials)
+      - [Eth1 address credentials](#eth1-address-credentials)
   - [Submit deposit](#submit-deposit)
   - [Process deposit](#process-deposit)
   - [Validator index](#validator-index)
@@ -100,14 +102,42 @@ A validator must initialize many parameters locally before submitting a deposit
 
 Validator public keys are [G1 points](beacon-chain.md#bls-signatures) 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
+#### Withdrawal credentials
 
-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 `withdrawal_credentials` ultimately control the deposited ETH once the
+validator has exited and is withdrawable. The 0th byte of this 32-byte field,
+called the "withdrawal prefix" defines the withdrawal credential version
+while the remaining 31 bytes define the version-specific content.
 
-The validator constructs their `withdrawal_credentials` via the following:
+The following withdrawal credentials versions are currently supported. The
+validator must choose and construct a version for the `withdrawal_credentials` field.
 
-* Set `withdrawal_credentials[:1] == BLS_WITHDRAWAL_PREFIX`.
-* Set `withdrawal_credentials[1:] == hash(withdrawal_pubkey)[1:]`.
+##### BLS key credentials
+
+BLS withdrawal credentials are controlled by a secondary withdrawal BLS private key, `bls_withdrawal_privkey`.
+This key is securely generated along with the resultant `bls_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 `withdrawal_credentials` as the following:
+* Set `withdrawal_credentials[:1] = BLS_WITHDRAWAL_PREFIX`.
+* Set `withdrawal_credentials[1:] = hash(bls_withdrawal_pubkey)[1:]`.
+
+##### Eth1 address credentials
+
+Eth1 address credentials are controlled by an eth1 address. This can be an either an externally owned account or a contract.
+
+The withdrawal to the address specified will be a normal ETH transfer (with no payload other than the validator's ETH)
+triggered by an eth1 transaction that will handle gas price/limit and payment of fees.
+
+As long as such a withdrawal account/contract can receive ETH transfers,
+the future withdrawal protocol is agnostic to all other implementation details.
+
+The validator selects a 20-byte eth1 address, `eth1_withdrawal_address`, and constructs `withdrawal_credentials` as the following:
+* Set `withdrawal_credentials[:1] = ETH1_ADDRESS_WITHDRAWAL_PREFIX`.
+* Set `withdrawal_credentials[12:] = eth1_withdrawal_address`.
+
+*Note*: Bytes `withdrawal_credentials[1:12]` remain the default `0x00` value.
 
 ### Submit deposit
 

tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_deposit.py

@@ -94,6 +94,49 @@ def test_new_deposit_over_max(spec, state):
     yield from run_deposit_processing(spec, state, deposit, validator_index)
 
 
+@with_all_phases
+@spec_state_test
+def test_new_deposit_eth1_withdrawal_credentials(spec, state):
+    # fresh deposit = next validator index = validator appended to registry
+    validator_index = len(state.validators)
+    withdrawal_credentials = (
+        spec.ETH1_ADDRESS_WITHDRAWAL_PREFIX
+        + b'\x00' * 11  # specified 0s
+        + b'\x59' * 20  # a 20-byte eth1 address
+    )
+    amount = spec.MAX_EFFECTIVE_BALANCE
+    deposit = prepare_state_and_deposit(
+        spec, state,
+        validator_index,
+        amount,
+        withdrawal_credentials=withdrawal_credentials,
+        signed=True,
+    )
+
+    yield from run_deposit_processing(spec, state, deposit, validator_index)
+
+
+@with_all_phases
+@spec_state_test
+def test_new_deposit_non_versioned_withdrawal_credentials(spec, state):
+    # fresh deposit = next validator index = validator appended to registry
+    validator_index = len(state.validators)
+    withdrawal_credentials = (
+        b'\xFF'  # Non specified withdrawal credentials version
+        + b'\x02' * 31  # Garabage bytes
+    )
+    amount = spec.MAX_EFFECTIVE_BALANCE
+    deposit = prepare_state_and_deposit(
+        spec, state,
+        validator_index,
+        amount,
+        withdrawal_credentials=withdrawal_credentials,
+        signed=True,
+    )
+
+    yield from run_deposit_processing(spec, state, deposit, validator_index)
+
+
 @with_all_phases
 @spec_state_test
 @always_bls