diff --git a/beacon_chain/spec/crypto.nim b/beacon_chain/spec/crypto.nim
index e4c3f5502..e59fd8440 100644
--- a/beacon_chain/spec/crypto.nim
+++ b/beacon_chain/spec/crypto.nim
@@ -169,7 +169,7 @@ func bls_aggregate_pubkeys*(keys: openArray[ValidatorPubKey]): ValidatorPubKey =
 # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/bls_signature.md#bls_verify
 func bls_verify*(
     pubkey: ValidatorPubKey, msg: openArray[byte], sig: ValidatorSig,
-    domain: uint64): bool =
+    domain: Domain): bool =
   # name from spec!
   if sig.kind != Real:
     # Invalid signatures are possible in deposits (discussed with Danny)
@@ -185,7 +185,7 @@ func bls_verify*(
 # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/bls_signature.md#bls_verify_multiple
 proc bls_verify_multiple*(
     pubkeys: seq[ValidatorPubKey], message_hashes: openArray[Eth2Digest],
-    sig: ValidatorSig, domain: uint64): bool =
+    sig: ValidatorSig, domain: Domain): bool =
   # {.noSideEffect.} - https://github.com/status-im/nim-chronicles/issues/62
   let L = len(pubkeys)
   doAssert L == len(message_hashes)
@@ -209,7 +209,7 @@ proc bls_verify_multiple*(
 
 when ValidatorPrivKey is BlsValue:
   func bls_sign*(key: ValidatorPrivKey, msg: openarray[byte],
-                 domain: uint64): ValidatorSig =
+                 domain: Domain): ValidatorSig =
     # name from spec!
     if key.kind == Real:
       ValidatorSig(kind: Real, blsValue: key.blsValue.sign(domain, msg))
@@ -217,7 +217,7 @@ when ValidatorPrivKey is BlsValue:
       ValidatorSig(kind: OpaqueBlob)
 else:
   func bls_sign*(key: ValidatorPrivKey, msg: openarray[byte],
-                 domain: uint64): ValidatorSig =
+                 domain: Domain): ValidatorSig =
     # name from spec!
     ValidatorSig(kind: Real, blsValue: key.sign(domain, msg))
 
diff --git a/beacon_chain/spec/datatypes.nim b/beacon_chain/spec/datatypes.nim
index fb3fb2f94..94bd7fd9f 100644
--- a/beacon_chain/spec/datatypes.nim
+++ b/beacon_chain/spec/datatypes.nim
@@ -76,7 +76,6 @@ type
   ValidatorIndex* = range[0'u32 .. 0xFFFFFF'u32] # TODO: wrap-around
   Shard* = uint64
   Gwei* = uint64
-  Domain* = uint64
 
   BitList*[maxLen: static int] = distinct BitSeq
 
diff --git a/beacon_chain/spec/helpers.nim b/beacon_chain/spec/helpers.nim
index c838d0bc5..17837de35 100644
--- a/beacon_chain/spec/helpers.nim
+++ b/beacon_chain/spec/helpers.nim
@@ -7,7 +7,13 @@
 
 # Uncategorized helper functions from the spec
 
-import ./datatypes, ./digest, sequtils, math, endians
+import
+  # Standard lib
+  sequtils, math, endians,
+  # Third-party
+  blscurve, # defines Domain
+  # Internal
+  ./datatypes, ./digest
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#integer_squareroot
 func integer_squareroot*(n: SomeInteger): SomeInteger =
@@ -142,11 +148,9 @@ func int_to_bytes4*(x: uint64): array[4, byte] =
 # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#compute_domain
 func compute_domain*(
     domain_type: DomainType,
-    fork_version: array[4, byte] = [0'u8, 0, 0, 0]): uint64 =
-  var buf: array[8, byte]
-  buf[0..3] = int_to_bytes4(domain_type.uint64)
-  buf[4..7] = fork_version
-  bytes_to_int(buf)
+    fork_version: array[4, byte] = [0'u8, 0, 0, 0]): Domain =
+  result[0..3] = int_to_bytes4(domain_type.uint64)
+  result[4..7] = fork_version
 
 # https://github.com/ethereum/eth2.0-specs/blob/v0.8.3/specs/core/0_beacon-chain.md#get_domain
 func get_domain*(
diff --git a/tests/official/test_fixture_bls.nim b/tests/official/test_fixture_bls.nim
index 4efb661d9..be8f9491b 100644
--- a/tests/official/test_fixture_bls.nim
+++ b/tests/official/test_fixture_bls.nim
@@ -16,9 +16,6 @@ import
   ./fixtures_utils
 
 type
-  Domain = distinct uint64
-    ## Domains have custom hex serialization
-
   BLSPrivToPub* = object
     input*: ValidatorPrivKey
     output*: ValidatorPubKey
@@ -42,12 +39,10 @@ type
 
 proc readValue*(r: var JsonReader, a: var Domain) {.inline.} =
   ## Custom deserializer for Domain
-  ## They are uint64 stored in hex values
   # Furthermore Nim parseHex doesn't support uint
   # until https://github.com/nim-lang/Nim/pull/11067
   # (0.20)
-  let be_uint = hexToPaddedByteArray[8](r.readValue(string))
-  bigEndian64(a.addr, be_uint.unsafeAddr)
+  a = hexToPaddedByteArray[8](r.readValue(string))
 
 const BLSDir = JsonTestsDir/"general"/"phase0"/"bls"
 
@@ -64,7 +59,7 @@ suite "Official - BLS tests":
       let t = parseTest(file, Json, BLSSignMsg)
       let implResult = t.input.privkey.bls_sign(
         t.input.message,
-        uint64(t.input.domain)
+        t.input.domain
       )
       check: implResult == t.output
 
diff --git a/tests/testutil.nim b/tests/testutil.nim
index 20f1c552b..484b973dc 100644
--- a/tests/testutil.nim
+++ b/tests/testutil.nim
@@ -55,7 +55,7 @@ func makeDeposit(i: int, flags: UpdateFlags): Deposit =
     privkey = makeFakeValidatorPrivKey(i)
     pubkey = privkey.pubKey()
     withdrawal_credentials = makeFakeHash(i)
-    domain = 3'u64
+    domain = compute_domain(DOMAIN_DEPOSIT)
 
   result = Deposit(
     data: DepositData(