diff --git a/abc/acks.nim b/abc/acks.nim
index 90beb9d..255e8e7 100644
--- a/abc/acks.nim
+++ b/abc/acks.nim
@@ -13,13 +13,15 @@ type
     hash: Hash
     signature: ?Signature
 
-func toBytes*(ack: Ack): seq[byte] =
+func calculateHash(ack: Ack) =
+  var hashing = Hashing.init(HashKind.Ack)
   let previous = ack.previous |? Hash.default
-  result.add(previous.toBytes)
-  result.add(ack.transactions.len.uint8)
+  hashing.update(previous.toBytes)
+  hashing.update([ack.transactions.len.uint8])
   for transaction in ack.transactions:
-    result.add(transaction.toBytes)
-  result.add(ack.validator.toBytes)
+    hashing.update(transaction.toBytes)
+  hashing.update(ack.validator.toBytes)
+  ack.hash = hashing.finish()
 
 func new(_: type Ack,
          previous: ?Hash,
@@ -40,7 +42,7 @@ func new(_: type Ack,
     transactions: @transactions,
     validator: validator
   )
-  ack.hash = hash(ack.toBytes, HashKind.Ack)
+  ack.calculateHash()
   some ack
 
 func new*(_: type Ack,
diff --git a/abc/hash.nim b/abc/hash.nim
index 217d125..1c9d2d4 100644
--- a/abc/hash.nim
+++ b/abc/hash.nim
@@ -6,6 +6,9 @@ type
   Hash* = object
     kind*: HashKind
     hash: MDigest[256]
+  Hashing* = object
+    kind: HashKind
+    context: sha256
 
 func hash*(bytes: openArray[byte], kind: HashKind): Hash =
   Hash(kind: kind, hash: sha256.digest(bytes))
@@ -22,3 +25,15 @@ func `$`*(hash: Hash): string =
     "Tx(" & $hash.hash & ")"
   of Ack:
     "Ack(" & $hash.hash & ")"
+
+func init*(_: type Hashing, kind: HashKind): Hashing =
+  result.kind = kind
+  result.context.init()
+
+func update*(hashing: var Hashing, bytes: openArray[byte]) =
+  hashing.context.update(bytes)
+
+func finish*(hashing: var Hashing): Hash =
+  let hash = hashing.context.finish()
+  hashing.context.clear()
+  Hash(kind: hashing.kind, hash: hash)
diff --git a/abc/transactions.nim b/abc/transactions.nim
index c1a5302..0c544aa 100644
--- a/abc/transactions.nim
+++ b/abc/transactions.nim
@@ -25,16 +25,18 @@ type
     owner: PublicKey
     value: UInt256
 
-func toBytes*(transaction: Transaction): seq[byte] =
-  result.add(transaction.inputs.len.uint8)
+func calculateHash(transaction: Transaction) =
+  var hashing = Hashing.init(HashKind.Tx)
+  hashing.update([transaction.inputs.len.uint8])
   for (txHash, owner) in transaction.inputs:
-    result.add(txHash.toBytes)
-    result.add(owner.toBytes)
-  result.add(transaction.outputs.len.uint8)
+    hashing.update(txHash.toBytes)
+    hashing.update(owner.toBytes)
+  hashing.update([transaction.outputs.len.uint8])
   for (owner, value) in transaction.outputs:
-    result.add(owner.toBytes)
-    result.add(value.toBytes)
-  result.add(transaction.validator.toBytes)
+    hashing.update(owner.toBytes)
+    hashing.update(value.toBytes)
+  hashing.update(transaction.validator.toBytes)
+  transaction.hash = hashing.finish()
 
 func new*(_: type Transaction,
           inputs: openArray[TxInput],
@@ -55,7 +57,7 @@ func new*(_: type Transaction,
     outputs: @outputs,
     validator: validator
   )
-  transaction.hash = hash(transaction.toBytes, HashKind.Tx)
+  transaction.calculateHash()
   some transaction
 
 func new*(_: type Transaction,
diff --git a/tests/abc/testAcks.nim b/tests/abc/testAcks.nim
index c71ae5c..b76f54b 100644
--- a/tests/abc/testAcks.nim
+++ b/tests/abc/testAcks.nim
@@ -28,7 +28,7 @@ suite "Acknowledgements":
     check ack.?previous == previous.hash.some
     check ack.?validator == victor.some
 
-  test "an acknowledgement can be converted to bytes":
+  test "an acknowledgement hash is derived from its fields":
     let previous = Ack.example
     let ack = !Ack.new(previous.hash, [tx1.hash, tx2.hash], victor)
     var expected: seq[byte]
@@ -37,7 +37,7 @@ suite "Acknowledgements":
     expected.add(tx1.hash.toBytes)
     expected.add(tx2.hash.toBytes)
     expected.add(victor.toBytes)
-    check ack.toBytes == expected
+    check ack.hash == hash(expected, HashKind.Ack)
 
   test "a signature can be added to an acknowledgment":
     let key = PrivateKey.example
diff --git a/tests/abc/testTransactions.nim b/tests/abc/testTransactions.nim
index e6f8df6..77a5c7c 100644
--- a/tests/abc/testTransactions.nim
+++ b/tests/abc/testTransactions.nim
@@ -37,7 +37,7 @@ suite "Transactions":
     check genesis.outputValue(bob) == 10.u256
     check genesis.outputValue(victor) == 0.u256
 
-  test "a transaction can be converted to bytes":
+  test "a transaction hash is derived from its fields":
     let genesis = !Transaction.new({alice: 32.u256, bob: 10.u256}, victor)
     let transaction = !Transaction.new(
       {genesis.hash: alice},
@@ -54,7 +54,7 @@ suite "Transactions":
     expected.add(bob.toBytes)
     expected.add(30.u256.toBytes)
     expected.add(victor.toBytes)
-    check transaction.toBytes == expected
+    check transaction.hash == hash(expected, HashKind.Tx)
 
   test "signatures can be added to a transaction":
     let key1, key2 = PrivateKey.example