diff --git a/pbc_client/__init__.py b/pbc_client/__init__.py
index eaf0892..6f7f4fa 100644
--- a/pbc_client/__init__.py
+++ b/pbc_client/__init__.py
@@ -10,15 +10,12 @@ import base64
 import dataclasses
 import datetime
 import email.utils
-import hashlib
 import json
 import logging
-import time
 from collections.abc import Mapping
 from decimal import Decimal
 from typing import Any
 
-import ecdsa
 import pbcabi
 import requests
 from frozendict import frozendict
@@ -26,12 +23,9 @@ from frozendict import frozendict
 from ._version import __version__  # noqa: F401
 from .pbc_types import (
     Address,
-    HashSha256,
-    Signature,
     SignedTransaction,
-    SignedTransactionInnerPart,
-    Transaction,
 )
+from .crypto import sign_transaction, SenderAuthentication
 
 logger = logging.getLogger(__name__)
 
@@ -69,58 +63,6 @@ class Balances:
     byoc: Mapping[str, Decimal]
 
 
-@dataclasses.dataclass(frozen=True)
-class SenderAuthentication:
-    secret_key: str
-
-    def sender_address(self) -> Address:
-        verifying_key_repr = (
-            self._signing_key().get_verifying_key().to_string('uncompressed')
-        )
-        hashed = HashSha256.of_bytes(verifying_key_repr)
-        return Address(b'\0' + hashed._bytes[-20:])
-
-    def sign_hash(self, _hash: HashSha256) -> Signature:
-        signing_key = self._signing_key()
-        signature = signing_key.sign_digest_deterministic(_hash._bytes)
-
-        return Signature(b'\0' + signature)
-
-    def _signing_key(self):
-        secret_exponent = int(self.secret_key, 16)
-        return ecdsa.SigningKey.from_secret_exponent(
-            secret_exponent,
-            curve=ecdsa.SECP256k1,
-        )
-
-
-TRANSACTION_VALIDITY_DURATION = 60
-
-
-def sign_transaction(
-    sender_authentication: SenderAuthentication,
-    nonce: int,
-    gas_cost: int,
-    chain_id: str,
-    contract_address: Address | str,
-    transaction_rpc: bytes,
-) -> SignedTransaction:
-    print(contract_address)
-    sender = sender_authentication.sender_address
-    valid_to_time: int = int(time.time() + TRANSACTION_VALIDITY_DURATION) * 1000
-
-    inner: SignedTransactionInnerPart = SignedTransactionInnerPart(
-        nonce,
-        valid_to_time,
-        gas_cost,
-        Transaction(Address.from_string(contract_address), transaction_rpc),
-    )
-
-    signature: Signature = sender_authentication.sign_hash(inner.hash(chain_id))
-    print(signature)
-    return SignedTransaction(inner, signature)
-
-
 @dataclasses.dataclass(frozen=True)
 class PbcClient:
     session: requests.Session
diff --git a/pbc_client/crypto.py b/pbc_client/crypto.py
new file mode 100644
index 0000000..be141ba
--- /dev/null
+++ b/pbc_client/crypto.py
@@ -0,0 +1,84 @@
+
+import coincurve
+import hashlib
+import time
+from .pbc_types import (
+    Address,
+    HashSha256,
+    Signature,
+    SignedTransaction,
+    SignedTransactionInnerPart,
+    Transaction,
+)
+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)
+
+    for recovery_id in range(4):
+        recovered_public_key = coincurve.PublicKey.from_signature_and_message(
+            signature=r + s + bytes([recovery_id]),
+            message=message_hash._bytes,
+            hasher=None,
+        )
+
+        if recovered_public_key.format() == expected_public_key.format():
+            return recovery_id
+
+    return None
+
+@dataclasses.dataclass(frozen=True)
+class SenderAuthentication:
+    _private_key_hex: str
+
+    def sender_address(self) -> Address:
+        verifying_key_repr = self._public_key().format(False)
+        assert len(verifying_key_repr) == 65
+        hashed = HashSha256.of_bytes(verifying_key_repr)
+        print(hashed)
+        return Address(b'\0' + hashed._bytes[-20:])
+
+    def sign_hash(self, hash_to_sign: HashSha256) -> Signature:
+        private_key = self._private_key()
+        signature_wrong_location = private_key.sign_recoverable(hash_to_sign._bytes, hasher = lambda x: x)
+
+        signature = signature_wrong_location[64:] + signature_wrong_location[:64]
+
+        print('Sig', signature)
+        print(len(signature))
+
+        return Signature(signature)
+
+    def _public_key(self) -> coincurve.PublicKey:
+        return self._private_key().public_key
+
+    def _private_key(self) -> coincurve.PrivateKey:
+        return coincurve.PrivateKey.from_hex(
+            self._private_key_hex,
+        )
+
+
+def sign_transaction(
+    sender_authentication: SenderAuthentication,
+    nonce: int,
+    gas_cost: int,
+    chain_id: str,
+    contract_address: Address | str,
+    transaction_rpc: bytes,
+) -> SignedTransaction:
+    print(contract_address)
+    sender = sender_authentication.sender_address
+    valid_to_time: int = int(time.time() + TRANSACTION_VALIDITY_DURATION) * 1000
+
+    inner: SignedTransactionInnerPart = SignedTransactionInnerPart(
+        nonce,
+        valid_to_time,
+        gas_cost,
+        Transaction(Address.from_string(contract_address), transaction_rpc),
+    )
+
+    signature: Signature = sender_authentication.sign_hash(inner.hash(chain_id))
+    print(signature)
+    return SignedTransaction(inner, signature)
diff --git a/pbc_client/pbc_types.py b/pbc_client/pbc_types.py
index 392cae9..d0da924 100644
--- a/pbc_client/pbc_types.py
+++ b/pbc_client/pbc_types.py
@@ -20,7 +20,7 @@ class Address:
         return self._bytes.hex()
 
     @staticmethod
-    def from_string(s: str) -> 'Address':
+    def from_string(s: 'Address | str') -> 'Address':
         if isinstance(s, Address):
             return s
         return Address(bytes.fromhex(s))
@@ -32,6 +32,7 @@ class Signature:
 
     def __post_init__(self):
         assert len(self._bytes) == 65, len(self._bytes)
+        assert self._bytes[0] < 4, 'First byte is the recovery id. Must be less than 4'
 
     def rpc_serialize(self) -> bytes:
         return self._bytes