diff --git a/codexvalidator/transaction.nim b/codexvalidator/transaction.nim
index 50efd9f..b9d5029 100644
--- a/codexvalidator/transaction.nim
+++ b/codexvalidator/transaction.nim
@@ -1,7 +1,10 @@
 import ./transaction/transaction
-import ./transaction/signed
 import ./transaction/serialization
+import ./transaction/hashing
+import ./transaction/signed
 
-export transaction
-export signed
+export transaction except hash
 export serialization.toBytes
+export hashing.hash
+export hashing.toBytes
+export signed
diff --git a/codexvalidator/transaction/hashing.nim b/codexvalidator/transaction/hashing.nim
new file mode 100644
index 0000000..bbed162
--- /dev/null
+++ b/codexvalidator/transaction/hashing.nim
@@ -0,0 +1,12 @@
+import ../basics
+import ../hashing
+import ./transaction
+import ./serialization
+
+export hashing.toBytes
+
+func hash*(tx: Transaction): Hash =
+  without var hash =? transaction.hash(tx):
+    hash = Hash.hash(tx.toBytes())
+    tx.hash = hash
+  hash
diff --git a/codexvalidator/transaction/signed.nim b/codexvalidator/transaction/signed.nim
index 63cd42f..84a5519 100644
--- a/codexvalidator/transaction/signed.nim
+++ b/codexvalidator/transaction/signed.nim
@@ -1,6 +1,6 @@
 import ../signatures
 import ./transaction
-import ./serialization
+import ./hashing
 
 type SignedTransaction* = object
   transaction: Transaction
@@ -20,7 +20,8 @@ func init*(
   )
 
 func sign*(identity: Identity, transaction: Transaction): SignedTransaction =
-  let signature = identity.sign(transaction.toBytes())
+  let hash = hashing.hash(transaction)
+  let signature = identity.sign(hash.toBytes())
   SignedTransaction.init(transaction, identity.identifier, signature)
 
 func transaction*(signed: SignedTransaction): Transaction =
@@ -33,4 +34,5 @@ func signature*(signed: SignedTransaction): Signature =
   signed.signature
 
 func verifySignature*(signed: SignedTransaction): bool =
-  signed.signer.verify(signed.transaction.toBytes(), signed.signature)
+  let hash = hashing.hash(signed.transaction)
+  signed.signer.verify(hash.toBytes(), signed.signature)
diff --git a/codexvalidator/transaction/transaction.nim b/codexvalidator/transaction/transaction.nim
index b0c04cb..637200f 100644
--- a/codexvalidator/transaction/transaction.nim
+++ b/codexvalidator/transaction/transaction.nim
@@ -1,4 +1,5 @@
 import ../basics
+import ../hashing
 import ./storagerequest
 import ./period
 import ./groth16
@@ -13,7 +14,7 @@ type
   TransactionKind* {.pure.} = enum
     storageProof
     missingProof
-  Transaction* = object
+  Transaction* = ref object
     requestId: StorageRequestId
     slotIndex: uint32
     period: Period
@@ -24,6 +25,7 @@ type
       proof: Groth16Proof
     of missingProof:
       discard
+    hash: ?Hash
 
 func storageProof*(
   _: type Transaction,
@@ -85,6 +87,12 @@ func challenge*(transaction: Transaction): array[32, byte] =
 func proof*(transaction: Transaction): Groth16Proof =
   transaction.proof
 
+func `hash=`*(transaction: Transaction, hash: Hash) =
+  transaction.hash = some hash
+
+func hash*(transaction: Transaction): ?Hash =
+  transaction.hash
+
 func `==`*(a, b: Transaction): bool =
   if a.kind != b.kind:
     return false
diff --git a/tests/codexvalidator/transaction/testHashing.nim b/tests/codexvalidator/transaction/testHashing.nim
new file mode 100644
index 0000000..f42d1e4
--- /dev/null
+++ b/tests/codexvalidator/transaction/testHashing.nim
@@ -0,0 +1,9 @@
+import ../basics
+import codexvalidator/transaction
+import codexvalidator/hashing
+
+suite "Transaction hashing":
+
+  test "transactions have a hash derived from the serialized bytes":
+    let transaction = Transaction.example
+    check transaction.hash == Hash.hash(transaction.toBytes())
diff --git a/tests/codexvalidator/transaction/testSigning.nim b/tests/codexvalidator/transaction/testSigning.nim
index e0b4694..0ee1a21 100644
--- a/tests/codexvalidator/transaction/testSigning.nim
+++ b/tests/codexvalidator/transaction/testSigning.nim
@@ -1,6 +1,7 @@
 import ../basics
 import codexvalidator/signatures
 import codexvalidator/transaction
+import codexvalidator/hashing
 
 suite "Transaction signing":
 
@@ -10,7 +11,7 @@ suite "Transaction signing":
     let signed = identity.sign(transaction)
     check signed.transaction == transaction
     check signed.signer == identity.identifier
-    check signed.signature == identity.sign(transaction.toBytes())
+    check signed.signature == identity.sign(transaction.hash.toBytes())
 
   test "transaction signature can be verified":
     let identity = Identity.example
diff --git a/tests/tests.nim b/tests/tests.nim
index a48f16d..fc46414 100644
--- a/tests/tests.nim
+++ b/tests/tests.nim
@@ -1,7 +1,8 @@
 import ./codexvalidator/testSignatures
 import ./codexvalidator/transaction/testTransaction
-import ./codexvalidator/transaction/testSigning
 import ./codexvalidator/transaction/testSerialization
+import ./codexvalidator/transaction/testHashing
+import ./codexvalidator/transaction/testSigning
 import ./codexvalidator/blocks/testBlock
 import ./codexvalidator/blocks/testSerialization