diff --git a/beacon_chain/spec/beaconstate.nim b/beacon_chain/spec/beaconstate.nim
index ae6d6fa8d..8d746c965 100644
--- a/beacon_chain/spec/beaconstate.nim
+++ b/beacon_chain/spec/beaconstate.nim
@@ -29,13 +29,13 @@ func verify_merkle_branch(leaf: Eth2Digest, proof: openarray[Eth2Digest], depth:
     value = eth2hash(buf)
   value == root
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#increase_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#increase_balance
 func increase_balance*(
     state: var BeaconState, index: ValidatorIndex, delta: Gwei) =
   # Increase the validator balance at index ``index`` by ``delta``.
   state.balances[index] += delta
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#decrease_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#decrease_balance
 func decrease_balance*(
     state: var BeaconState, index: ValidatorIndex, delta: Gwei) =
   ## Decrease the validator balance at index ``index`` by ``delta``, with
@@ -103,13 +103,13 @@ func process_deposit*(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_activation_exit_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compute_activation_exit_epoch
 func compute_activation_exit_epoch*(epoch: Epoch): Epoch =
   ## Return the epoch during which validator activations and exits initiated in
   ## ``epoch`` take effect.
   epoch + 1 + ACTIVATION_EXIT_DELAY
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_validator_churn_limit
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_validator_churn_limit
 func get_validator_churn_limit(state: BeaconState): uint64 =
   # Return the validator churn limit for the current epoch.
   let active_validator_indices =
@@ -117,7 +117,7 @@ func get_validator_churn_limit(state: BeaconState): uint64 =
   max(MIN_PER_EPOCH_CHURN_LIMIT,
     len(active_validator_indices) div CHURN_LIMIT_QUOTIENT).uint64
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#initiate_validator_exit
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#initiate_validator_exit
 func initiate_validator_exit*(state: var BeaconState,
                               index: ValidatorIndex) =
   # Initiate the exit of the validator with index ``index``.
@@ -148,7 +148,7 @@ func initiate_validator_exit*(state: var BeaconState,
   validator.withdrawable_epoch =
     validator.exit_epoch + MIN_VALIDATOR_WITHDRAWABILITY_DELAY
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#slash_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#slash_validator
 func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
     stateCache: var StateCache) =
   # Slash the validator with index ``index``.
@@ -174,7 +174,7 @@ func slash_validator*(state: var BeaconState, slashed_index: ValidatorIndex,
   increase_balance(
     state, whistleblower_index, whistleblowing_reward - proposer_reward)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_compact_committees_root
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_compact_committees_root
 func get_compact_committees_root*(state: BeaconState, epoch: Epoch): Eth2Digest =
   # Return the compact committee root at ``epoch``.
 
@@ -256,7 +256,7 @@ func get_initial_beacon_block*(state: BeaconState): BeaconBlock =
     # initialized to default values.
   )
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_attestation_data_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_attestation_data_slot
 func get_attestation_data_slot*(state: BeaconState,
     data: AttestationData, committee_count: uint64): Slot =
   # Return the slot corresponding to the attestation ``data``.
@@ -273,7 +273,7 @@ func get_attestation_data_slot*(state: BeaconState,
   get_attestation_data_slot(
     state, data, get_committee_count(state, data.target.epoch))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_block_root_at_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_block_root_at_slot
 func get_block_root_at_slot*(state: BeaconState,
                              slot: Slot): Eth2Digest =
   # Return the block root at a recent ``slot``.
@@ -282,12 +282,12 @@ func get_block_root_at_slot*(state: BeaconState,
   doAssert slot < state.slot
   state.block_roots[slot mod SLOTS_PER_HISTORICAL_ROOT]
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_block_root
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_block_root
 func get_block_root*(state: BeaconState, epoch: Epoch): Eth2Digest =
   # Return the block root at the start of a recent ``epoch``.
   get_block_root_at_slot(state, compute_start_slot_of_epoch(epoch))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_total_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_total_balance
 func get_total_balance*(state: BeaconState, validators: auto): Gwei =
   ## Return the combined effective balance of the ``indices``. (1 Gwei minimum
   ## to avoid divisions by zero.)
@@ -295,7 +295,7 @@ func get_total_balance*(state: BeaconState, validators: auto): Gwei =
     foldl(validators, a + state.validators[b].effective_balance, 0'u64)
   )
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#registry-updates
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#registry-updates
 func process_registry_updates*(state: var BeaconState) =
   ## Process activation eligibility and ejections
   ## Try to avoid caching here, since this could easily become undefined
@@ -335,10 +335,10 @@ func process_registry_updates*(state: var BeaconState) =
       validator.activation_epoch =
         compute_activation_exit_epoch(get_current_epoch(state))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#is_valid_indexed_attestation
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#is_valid_indexed_attestation
 func is_valid_indexed_attestation*(
     state: BeaconState, indexed_attestation: IndexedAttestation): bool =
-  # Verify validity of ``indexed_attestation`` fields.
+  # Check if ``indexed_attestation`` has valid indices and signature.
 
   let
     bit_0_indices = indexed_attestation.custody_bit_0_indices
@@ -414,7 +414,7 @@ func get_attesting_indices_seq*(
   toSeq(items(get_attesting_indices(
     state, attestation_data, bitfield, cache)))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_indexed_attestation
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_indexed_attestation
 func get_indexed_attestation(state: BeaconState, attestation: Attestation,
     stateCache: var StateCache): IndexedAttestation =
   # Return the indexed attestation corresponding to ``attestation``.
@@ -464,7 +464,7 @@ func get_indexed_attestation(state: BeaconState, attestation: Attestation,
     signature: attestation.signature,
   )
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#attestations
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#attestations
 proc process_attestation*(
     state: BeaconState, attestation: Attestation, flags: UpdateFlags,
     stateCache: var StateCache): bool =
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index d1dcc4680..c08cce78c 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -53,13 +53,13 @@ else:
   {.fatal: "Preset \"" & const_preset ".nim\" is not supported.".}
 
 const
-  SPEC_VERSION* = "0.8.0" ## \
+  SPEC_VERSION* = "0.8.1" ## \
   ## Spec version we're aiming to be compatible with, right now
   ## TODO: improve this scheme once we can negotiate versions in protocol
 
   # Initial values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#initial-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#initial-values
   GENESIS_EPOCH* = (GENESIS_SLOT.uint64 div SLOTS_PER_EPOCH).Epoch ##\
   ## compute_epoch_of_slot(GENESIS_SLOT)
 
@@ -73,7 +73,7 @@ type
   Gwei* = uint64
   Domain* = uint64
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#proposerslashing
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#proposerslashing
   ProposerSlashing* = object
     proposer_index*: uint64 ##\
     ## Proposer index
@@ -84,7 +84,7 @@ type
     header_2*: BeaconBlockHeader ##\
     # Second block header
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#attesterslashing
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#attesterslashing
   AttesterSlashing* = object
     attestation_1*: IndexedAttestation ## \
     ## First attestation
@@ -117,12 +117,12 @@ type
     signature*: ValidatorSig ##\
     ## BLS aggregate signature
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#checkpoint
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#checkpoint
   Checkpoint* = object
     epoch*: Epoch
     root*: Eth2Digest
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#AttestationData
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#AttestationData
   AttestationData* = object
     # LMD GHOST vote
     beacon_block_root*: Eth2Digest
@@ -134,21 +134,21 @@ type
     # Crosslink vote
     crosslink*: Crosslink
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#attestationdataandcustodybit
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#attestationdataandcustodybit
   AttestationDataAndCustodyBit* = object
     data*: AttestationData
 
     custody_bit*: bool ##\
     ## Challengeable bit (SSZ-bool, 1 byte) for the custody of crosslink data
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#deposit
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#deposit
   Deposit* = object
     proof*: array[DEPOSIT_CONTRACT_TREE_DEPTH, Eth2Digest] ##\
     ## Merkle path to deposit data list root
 
     data*: DepositData
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#depositdata
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#depositdata
   DepositData* = object
     pubkey*: ValidatorPubKey ##\
     ## BLS pubkey
@@ -165,7 +165,7 @@ type
     signature*: ValidatorSig ##\
     ## Container self-signature
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#voluntaryexit
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#voluntaryexit
   VoluntaryExit* = object
     epoch*: Epoch ##\
     ## Earliest epoch when voluntary exit can be processed
@@ -173,7 +173,7 @@ type
     validator_index*: uint64
     signature*: ValidatorSig
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#transfer
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#transfer
   Transfer* = object
     sender*: uint64 ##\
     ## Sender index
@@ -197,7 +197,7 @@ type
     signature*: ValidatorSig ##\
     ## Signature checked against withdrawal pubkey
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#beaconblock
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#beaconblock
   BeaconBlock* = object
     ## For each slot, a proposer is chosen from the validator pool to propose
     ## a new block. Once the block as been proposed, it is transmitted to
@@ -218,12 +218,12 @@ type
     signature*: ValidatorSig ##\
     ## Proposer signature
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compactcommittee
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compactcommittee
   CompactCommittee* = object
     pubkeys*: seq[ValidatorPubKey]
     compact_validators*: seq[uint64]
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#beaconblockheader
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#beaconblockheader
   BeaconBlockHeader* = object
     slot*: Slot
     parent_root*: Eth2Digest
@@ -310,7 +310,7 @@ type
     current_justified_checkpoint*: Checkpoint
     finalized_checkpoint*: Checkpoint
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#validator
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#validator
   Validator* = object
     pubkey*: ValidatorPubKey
 
@@ -333,7 +333,7 @@ type
     withdrawable_epoch*: Epoch ##\
     ## When validator can withdraw or transfer funds
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#crosslink
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#crosslink
   Crosslink* = object
     shard*: Shard
     parent_root*: Eth2Digest
@@ -351,12 +351,12 @@ type
     inclusion_delay*: uint64                  ## Inclusion delay
     proposer_index*: ValidatorIndex           ## Proposer index
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#historicalbatch
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#historicalbatch
   HistoricalBatch* = object
     block_roots* : array[SLOTS_PER_HISTORICAL_ROOT, Eth2Digest]
     state_roots* : array[SLOTS_PER_HISTORICAL_ROOT, Eth2Digest]
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#fork
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#fork
   Fork* = object
     previous_version*: array[4, byte]
     current_version*: array[4, byte]
@@ -364,7 +364,7 @@ type
     epoch*: Epoch ##\
     ## Epoch of latest fork
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#eth1data
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#eth1data
   Eth1Data* = object
     deposit_root*: Eth2Digest ##\
     ## Root of the deposit tree
diff --git a/beacon_chain/spec/digest.nim b/beacon_chain/spec/digest.nim
index d62beac88..6845ce367 100644
--- a/beacon_chain/spec/digest.nim
+++ b/beacon_chain/spec/digest.nim
@@ -7,7 +7,7 @@
 
 # Serenity hash function / digest
 #
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#hash
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#hash
 #
 # In Phase 0 the beacon chain is deployed with SHA256 (SHA2-256).
 # Note that is is different from Keccak256 (often mistakenly called SHA3-256)
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index 3d1107c45..983235c71 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -9,7 +9,7 @@
 
 import ./datatypes, ./digest, sequtils, math
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#integer_squareroot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#integer_squareroot
 func integer_squareroot*(n: SomeInteger): SomeInteger =
   # Return the largest integer ``x`` such that ``x**2 <= n``.
   doAssert n >= 0'u64
@@ -57,17 +57,17 @@ func merkle_root*(values: openArray[Eth2Digest]): Eth2Digest =
 
   o[1]
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_epoch_of_slot
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compute_epoch_of_slot
 func compute_epoch_of_slot*(slot: Slot|uint64): Epoch =
   # Return the epoch number of the given ``slot``.
   (slot div SLOTS_PER_EPOCH).Epoch
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_start_slot_of_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compute_start_slot_of_epoch
 func compute_start_slot_of_epoch*(epoch: Epoch): Slot =
   # Return the start slot of ``epoch``.
   (epoch * SLOTS_PER_EPOCH).Slot
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#is_active_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#is_active_validator
 func is_active_validator*(validator: Validator, epoch: Epoch): bool =
   ### Check if ``validator`` is active
   validator.activation_epoch <= epoch and epoch < validator.exit_epoch
@@ -80,7 +80,7 @@ func get_active_validator_indices*(state: BeaconState, epoch: Epoch):
     if is_active_validator(val, epoch):
       result.add idx.ValidatorIndex
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_committee_count
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_committee_count
 func get_committee_count*(state: BeaconState, epoch: Epoch): uint64 =
   # Return the number of committees at ``epoch``.
   let active_validator_indices = get_active_validator_indices(state, epoch)
@@ -92,7 +92,7 @@ func get_committee_count*(state: BeaconState, epoch: Epoch): uint64 =
   # Otherwise, get_crosslink_committee(...) cannot access some committees.
   doAssert SHARD_COUNT >= result
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_current_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_current_epoch
 func get_current_epoch*(state: BeaconState): Epoch =
   # Return the current epoch.
   doAssert state.slot >= GENESIS_SLOT, $state.slot
@@ -153,7 +153,7 @@ func compute_domain(domain_type: DomainType, fork_version: array[4, byte]):
   buf[4..7] = int_to_bytes4(domain_type.uint64)
   bytes_to_int(buf)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_domain
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_domain
 func get_domain*(
     state: BeaconState, domain_type: DomainType, message_epoch: Epoch): Domain =
   ## Return the signature domain (fork version concatenated with domain type)
@@ -180,7 +180,7 @@ func get_seed*(state: BeaconState, epoch: Epoch): Eth2Digest =
 
   seed_input[0..31] =
     get_randao_mix(state,
-      epoch + LATEST_RANDAO_MIXES_LENGTH - MIN_SEED_LOOKAHEAD).data
+      epoch + LATEST_RANDAO_MIXES_LENGTH - MIN_SEED_LOOKAHEAD - 1).data
   seed_input[32..63] =
     state.active_index_roots[epoch mod EPOCHS_PER_HISTORICAL_VECTOR].data
   seed_input[64..95] = int_to_bytes32(epoch)
diff --git a/beacon_chain/spec/presets/mainnet.nim b/beacon_chain/spec/presets/mainnet.nim
index 9d3532ec1..5c63c8eb0 100644
--- a/beacon_chain/spec/presets/mainnet.nim
+++ b/beacon_chain/spec/presets/mainnet.nim
@@ -26,7 +26,7 @@ type
 const
   # Misc
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#misc
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#misc
 
   SHARD_COUNT* {.intdefine.} = 1024 ##\
   ## Number of shards supported by the network - validators will jump around
@@ -54,14 +54,14 @@ const
   SHUFFLE_ROUND_COUNT* = 90
 
   # Constants (TODO: not actually configurable)
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#constants
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#constants
   BASE_REWARDS_PER_EPOCH* = 5
 
   DEPOSIT_CONTRACT_TREE_DEPTH* = 32
 
   # Gwei values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#gwei-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#gwei-values
 
   MIN_DEPOSIT_AMOUNT* = 2'u64^0 * 10'u64^9 ##\
   ## Minimum amounth of ETH that can be deposited in one call - deposits can
@@ -87,12 +87,12 @@ const
 
   # Time parameters
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_fork-choice.md#time-parameters
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_fork-choice.md#time-parameters
 
   SECONDS_PER_SLOT*{.intdefine.} = 6'u64 # Compile with -d:SECONDS_PER_SLOT=1 for 6x faster slots
   ## TODO consistent time unit across projects, similar to C++ chrono?
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#time-parameters
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#time-parameters
   MIN_ATTESTATION_INCLUSION_DELAY* = 1 ##\
   ## (24 seconds)
   ## Number of slots that attestations stay in the attestation
@@ -144,7 +144,7 @@ const
 
   # Reward and penalty quotients
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#rewards-and-penalties
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#rewards-and-penalties
   BASE_REWARD_FACTOR* = 2'u64^6
   WHISTLEBLOWER_REWARD_QUOTIENT* = 2'u64^9
   PROPOSER_REWARD_QUOTIENT* = 2'u64^3
@@ -164,7 +164,7 @@ const
 type
   # Signature domains
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#signature-domain-types
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#signature-domain-types
   DomainType* {.pure.} = enum
     DOMAIN_BEACON_PROPOSER = 0
     DOMAIN_RANDAO = 1
diff --git a/beacon_chain/spec/presets/minimal.nim b/beacon_chain/spec/presets/minimal.nim
index 162560c7b..707b78b26 100644
--- a/beacon_chain/spec/presets/minimal.nim
+++ b/beacon_chain/spec/presets/minimal.nim
@@ -43,14 +43,14 @@ const
 
   # Constants
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#constants
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#constants
   # TODO "The following values are (non-configurable) constants" ...
   # Unchanged
   DEPOSIT_CONTRACT_TREE_DEPTH* = 32
 
   # Gwei values
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#gwei-values
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#gwei-values
 
   # Unchanged
   MIN_DEPOSIT_AMOUNT* = 2'u64^0 * 10'u64^9
@@ -70,12 +70,12 @@ const
 
   # Time parameters
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_fork-choice.md#time-parameters
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_fork-choice.md#time-parameters
 
   # Unchanged
   SECONDS_PER_SLOT*{.intdefine.} = 6'u64
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#time-parameters
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#time-parameters
   # Unchanged
   MIN_ATTESTATION_INCLUSION_DELAY* = 1
 
@@ -107,7 +107,7 @@ const
 
   # Reward and penalty quotients
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#rewards-and-penalties
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#rewards-and-penalties
 
   # Unchanged
   BASE_REWARD_FACTOR* = 2'u64^6
@@ -118,7 +118,7 @@ const
 
   # Max operations per block
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#max-operations-per-block
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#max-operations-per-block
 
   # Unchanged
   MAX_PROPOSER_SLASHINGS* = 2^4
@@ -131,7 +131,7 @@ const
 type
   # Signature domains
   # ---------------------------------------------------------------
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#signature-domain-types
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#signature-domain-types
   DomainType* {.pure.} = enum
     DOMAIN_BEACON_PROPOSER = 0
     DOMAIN_RANDAO = 1
diff --git a/beacon_chain/spec/state_transition_block.nim b/beacon_chain/spec/state_transition_block.nim
index a0394cc28..163ed5a55 100644
--- a/beacon_chain/spec/state_transition_block.nim
+++ b/beacon_chain/spec/state_transition_block.nim
@@ -37,7 +37,7 @@ import # TODO - cleanup imports
   ../extras, ../ssz, ../beacon_node_types,
   beaconstate, bitfield, crypto, datatypes, digest, helpers, validator
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#block-header
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#block-header
 proc processBlockHeader(
     state: var BeaconState, blck: BeaconBlock, flags: UpdateFlags,
     stateCache: var StateCache): bool =
@@ -61,8 +61,9 @@ proc processBlockHeader(
   state.latest_block_header = BeaconBlockHeader(
     slot: blck.slot,
     parent_root: blck.parent_root,
-    state_root: Eth2Digest(),  # Overwritten in the next `process_slot` call
+    # state_root: zeroed, overwritten in the next `process_slot` call
     body_root: hash_tree_root(blck.body),
+    # signature is always zeroed
   )
 
   # Verify proposer is not slashed
@@ -118,14 +119,14 @@ proc processRandao(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#eth1-data
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#eth1-data
 func processEth1Data(state: var BeaconState, body: BeaconBlockBody) =
   state.eth1_data_votes.add body.eth1_data
   if state.eth1_data_votes.count(body.eth1_data) * 2 >
       SLOTS_PER_ETH1_VOTING_PERIOD:
     state.eth1_data = body.eth1_data
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#is_slashable_validator
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#is_slashable_validator
 func is_slashable_validator(validator: Validator, epoch: Epoch): bool =
   # Check if ``validator`` is slashable.
   (not validator.slashed) and
@@ -178,7 +179,7 @@ proc processProposerSlashings(
 
   true
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#is_slashable_attestation_data
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#is_slashable_attestation_data
 func is_slashable_attestation_data(
     data_1: AttestationData, data_2: AttestationData): bool =
   ## Check if ``data_1`` and ``data_2`` are slashable according to Casper FFG
diff --git a/beacon_chain/spec/state_transition_epoch.nim b/beacon_chain/spec/state_transition_epoch.nim
index 692cd9b3c..4fd384328 100644
--- a/beacon_chain/spec/state_transition_epoch.nim
+++ b/beacon_chain/spec/state_transition_epoch.nim
@@ -37,13 +37,13 @@ import # TODO - cleanup imports
   ../extras, ../ssz, ../beacon_node_types,
   beaconstate, bitfield, crypto, datatypes, digest, helpers, validator
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_total_active_balance
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_total_active_balance
 func get_total_active_balance(state: BeaconState): Gwei =
   return get_total_balance(
     state,
     get_active_validator_indices(state, get_current_epoch(state)))
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#helper-functions-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#helper-functions-1
 func get_matching_source_attestations(state: BeaconState, epoch: Epoch):
     seq[PendingAttestation] =
   doAssert epoch in [get_current_epoch(state), get_previous_epoch(state)]
@@ -254,7 +254,7 @@ func process_justification_and_finalization(
     state.finalized_checkpoint.root =
       get_block_root(state, state.finalized_checkpoint.epoch)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#crosslinks
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#crosslinks
 func process_crosslinks(state: var BeaconState, stateCache: var StateCache) =
   state.previous_crosslinks = state.current_crosslinks
 
@@ -283,7 +283,7 @@ func get_base_reward(state: BeaconState, index: ValidatorIndex): Gwei =
   effective_balance * BASE_REWARD_FACTOR div
     integer_squareroot(total_balance) div BASE_REWARDS_PER_EPOCH
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
 func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
     tuple[a: seq[Gwei], b: seq[Gwei]] =
   let
@@ -373,7 +373,7 @@ func get_attestation_deltas(state: BeaconState, stateCache: var StateCache):
 
   (rewards, penalties)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
 func get_crosslink_deltas(state: BeaconState, cache: var StateCache):
     tuple[a: seq[Gwei], b: seq[Gwei]] =
 
@@ -401,7 +401,7 @@ func get_crosslink_deltas(state: BeaconState, cache: var StateCache):
 
   (rewards, penalties)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
 func process_rewards_and_penalties(
     state: var BeaconState, cache: var StateCache) =
   if get_current_epoch(state) == GENESIS_EPOCH:
@@ -414,7 +414,7 @@ func process_rewards_and_penalties(
     increase_balance(state, i.ValidatorIndex, rewards1[i] + rewards2[i])
     decrease_balance(state, i.ValidatorIndex, penalties1[i] + penalties2[i])
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#slashings
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#slashings
 func process_slashings(state: var BeaconState) =
   let
     epoch = get_current_epoch(state)
@@ -428,7 +428,7 @@ func process_slashings(state: var BeaconState) =
           min(sum(state.slashings) * 3, total_balance) div total_balance
       decrease_balance(state, index.ValidatorIndex, penalty)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#final-updates
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#final-updates
 func process_final_updates(state: var BeaconState) =
   let
     current_epoch = get_current_epoch(state)
@@ -493,16 +493,16 @@ func processEpoch*(state: var BeaconState) =
 
   var per_epoch_cache = get_empty_per_epoch_cache()
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#justification-and-finalization
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#justification-and-finalization
   process_justification_and_finalization(state, per_epoch_cache)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#crosslinks
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#crosslinks
   process_crosslinks(state, per_epoch_cache)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#rewards-and-penalties-1
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#rewards-and-penalties-1
   process_rewards_and_penalties(state, per_epoch_cache)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#registry-updates
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#registry-updates
   # Don't rely on caching here.
   process_registry_updates(state)
 
@@ -510,8 +510,8 @@ func processEpoch*(state: var BeaconState) =
   ## get_active_validator_indices(...) usually changes.
   clear(per_epoch_cache.crosslink_committee_cache)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#slashings
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#slashings
   process_slashings(state)
 
-  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#final-updates
+  # https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#final-updates
   process_final_updates(state)
diff --git a/beacon_chain/spec/validator.nim b/beacon_chain/spec/validator.nim
index b9a2eec98..2feb3dade 100644
--- a/beacon_chain/spec/validator.nim
+++ b/beacon_chain/spec/validator.nim
@@ -12,8 +12,8 @@ import
   ./crypto, ./datatypes, ./digest, ./helpers
 
 # TODO: Proceed to renaming and signature changes
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_shuffled_index
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compute_shuffled_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compute_committee
 func get_shuffled_seq*(seed: Eth2Digest,
                        list_size: uint64,
                        ): seq[ValidatorIndex] =
@@ -79,7 +79,7 @@ func get_shuffled_seq*(seed: Eth2Digest,
 
   result = shuffled_active_validator_indices
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_previous_epoch
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_previous_epoch
 func get_previous_epoch*(state: BeaconState): Epoch =
   # Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
   let current_epoch = get_current_epoch(state)
@@ -88,14 +88,14 @@ func get_previous_epoch*(state: BeaconState): Epoch =
   else:
     current_epoch - 1
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_shard_delta
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_shard_delta
 func get_shard_delta*(state: BeaconState, epoch: Epoch): uint64 =
   ## Return the number of shards to increment ``state.start_shard``
   ## during ``epoch``.
   min(get_committee_count(state, epoch),
     (SHARD_COUNT - SHARD_COUNT div SLOTS_PER_EPOCH).uint64)
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_start_shard
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_start_shard
 func get_start_shard*(state: BeaconState, epoch: Epoch): Shard =
   # Return the start shard of the 0th committee at ``epoch``.
 
@@ -111,7 +111,7 @@ func get_start_shard*(state: BeaconState, epoch: Epoch): Shard =
       SHARD_COUNT
   return shard
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#compute_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#compute_committee
 func compute_committee(indices: seq[ValidatorIndex], seed: Eth2Digest,
     index: uint64, count: uint64, stateCache: var StateCache): seq[ValidatorIndex] =
   ## Return the committee corresponding to ``indices``, ``seed``, ``index``,
@@ -136,7 +136,7 @@ func compute_committee(indices: seq[ValidatorIndex], seed: Eth2Digest,
     start.int .. (endIdx.int-1),
     indices[stateCache.crosslink_committee_cache[key][it]])
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_crosslink_committee
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_crosslink_committee
 func get_crosslink_committee*(state: BeaconState, epoch: Epoch, shard: Shard,
     stateCache: var StateCache): seq[ValidatorIndex] =
 
@@ -175,7 +175,7 @@ func get_empty_per_epoch_cache*(): StateCache =
   result.start_shard_cache = initTable[Epoch, Shard]()
   result.committee_count_cache = initTable[Epoch, uint64]()
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#get_beacon_proposer_index
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#get_beacon_proposer_index
 func get_beacon_proposer_index*(state: BeaconState, stateCache: var StateCache):
     ValidatorIndex =
   # Return the beacon proposer index at the current slot.
diff --git a/beacon_chain/state_transition.nim b/beacon_chain/state_transition.nim
index ea717973a..e2d7132a1 100644
--- a/beacon_chain/state_transition.nim
+++ b/beacon_chain/state_transition.nim
@@ -45,7 +45,7 @@ func advance_slot(state: var BeaconState) =
 
   state.slot += 1
 
-# https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
+# https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 func process_slot(state: var BeaconState) =
   # Cache state root
   let previous_state_root = hash_tree_root(state)
diff --git a/beacon_chain/time.nim b/beacon_chain/time.nim
index ceaa11e59..129e9d974 100644
--- a/beacon_chain/time.nim
+++ b/beacon_chain/time.nim
@@ -14,7 +14,7 @@ type
     ## which blocks are valid - in particular, blocks are not valid if they
     ## come from the future as seen from the local clock.
     ##
-    ## https://github.com/ethereum/eth2.0-specs/blob/v0.8.0/specs/core/0_fork-choice.md#fork-choice
+    ## https://github.com/ethereum/eth2.0-specs/blob/v0.8.1/specs/core/0_fork-choice.md#fork-choice
     ##
     # TODO replace time in chronos with a proper unit type, then this code can
     #      follow: