diff --git a/pbc_client/__init__.py b/pbc_client/__init__.py
index 6f7f4fa..4a9da2e 100644
--- a/pbc_client/__init__.py
+++ b/pbc_client/__init__.py
@@ -26,6 +26,7 @@ from .pbc_types import (
     SignedTransaction,
 )
 from .crypto import sign_transaction, SenderAuthentication
+import time
 
 logger = logging.getLogger(__name__)
 
@@ -42,6 +43,8 @@ URL_NONCE = 'https://{hostname}/{shard}blockchain/account/{address}'
 
 MPC_DECIMALS = 10000
 
+TRANSACTION_VALIDITY_DURATION = 60
+
 
 def shard_id_for_address(address: str | Address) -> str:
     address = str(address)
@@ -81,9 +84,11 @@ class PbcClient:
             msg = 'PbcClient.sender_authentication required for send_transaction'
             raise Exception(msg)
 
+        valid_to_time: int = int(time.time() + TRANSACTION_VALIDITY_DURATION) * 1000
         signed_transaction = sign_transaction(
             self.sender_authentication,
             self.get_sender_authentication_nonce(),
+            valid_to_time,
             gas_cost,
             self.get_chain_id(),
             contract_address,
diff --git a/pbc_client/crypto.py b/pbc_client/crypto.py
index be141ba..bb3506d 100644
--- a/pbc_client/crypto.py
+++ b/pbc_client/crypto.py
@@ -1,7 +1,6 @@
 
 import coincurve
 import hashlib
-import time
 from .pbc_types import (
     Address,
     HashSha256,
@@ -12,8 +11,6 @@ from .pbc_types import (
 )
 import dataclasses
 
-TRANSACTION_VALIDITY_DURATION = 60
-
 def find_recovery_id(der_sig: bytes, message_hash: HashSha256, expected_public_key: coincurve.PublicKey) -> int:
     r, s = coincurve.der.parse_signature(der_sig)
 
@@ -63,6 +60,7 @@ class SenderAuthentication:
 def sign_transaction(
     sender_authentication: SenderAuthentication,
     nonce: int,
+    valid_to_time: int,
     gas_cost: int,
     chain_id: str,
     contract_address: Address | str,
@@ -70,7 +68,6 @@ def sign_transaction(
 ) -> SignedTransaction:
     print(contract_address)
     sender = sender_authentication.sender_address
-    valid_to_time: int = int(time.time() + TRANSACTION_VALIDITY_DURATION) * 1000
 
     inner: SignedTransactionInnerPart = SignedTransactionInnerPart(
         nonce,
diff --git a/pbc_client/pbc_types.py b/pbc_client/pbc_types.py
index d0da924..84afa81 100644
--- a/pbc_client/pbc_types.py
+++ b/pbc_client/pbc_types.py
@@ -89,7 +89,7 @@ class SignedTransactionInnerPart:
         )
 
     def hash(self, chain_id: str) -> HashSha256:
-        return HashSha256.of_bytes(self.rpc_serialize() + chain_id.encode('utf8'))
+        return HashSha256.of_bytes(self.rpc_serialize() + size_prefixed(chain_id.encode('utf8')))
 
 
 @dataclasses.dataclass(frozen=True)
diff --git a/test/test_crypto.py b/test/test_crypto.py
new file mode 100644
index 0000000..4533486
--- /dev/null
+++ b/test/test_crypto.py
@@ -0,0 +1,24 @@
+
+from pbc_client.crypto import sign_transaction, SenderAuthentication
+
+def test_sign():
+    sender_authentication = SenderAuthentication("01")
+    chain_id = "SpecificChainIDLocatable"
+    contract_address = '000000000000000000000000000000000000000001'
+    signed = sign_transaction(
+                sender_authentication = sender_authentication,
+                nonce = 2,
+                valid_to_time = 3,
+                gas_cost = 2,
+                chain_id = chain_id,
+                contract_address = contract_address,
+                transaction_rpc = bytes([0 for i in range(99)]),
+    )
+    print(signed)
+    assert str(signed.inner.rpc_serialize().hex()) == '00000000000000020000000000000003000000000000000200000000000000000000000000000000000000000100000063000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'
+    assert str(signed.hash(chain_id)) == '1be2895a85862e8dd3cc75b290cc28a22e960ae02dd5c07a73b93716f9adbee8'
+
+
+    assert str(signed.signature) == '01f0f7d8f44919466eedbebc76bcb369bbb4c6f4f076e25c0ffe8ec9285890e53b4e39098540d088878a019c345ad73963543ce813fb9ccf4b84b0c25770452bd1'
+
+