diff --git a/requests_http_signature/__init__.py b/requests_http_signature/__init__.py
index 9ebb4d6..4dd7509 100644
--- a/requests_http_signature/__init__.py
+++ b/requests_http_signature/__init__.py
@@ -26,14 +26,12 @@ class Crypto:
         key = self.load_pem_private_key(key, password=passphrase, backend=self.default_backend())
         if self.algorithm in {"rsa-sha1", "rsa-sha256"}:
             hasher = self.SHA1() if self.algorithm.endswith("sha1") else self.SHA256()
-            signer = key.signer(padding=self.PKCS1v15(), algorithm=hasher)
+            return key.sign(string_to_sign, padding=self.PKCS1v15(), algorithm=hasher)
         elif self.algorithm in {"rsa-sha512"}:
             hasher = self.SHA512()
-            signer = key.signer(padding=self.PKCS1v15(), algorithm=hasher)
+            return key.sign(string_to_sign, padding=self.PKCS1v15(), algorithm=hasher)
         elif self.algorithm == "ecdsa-sha256":
-            signer = key.signer(signature_algorithm=self.ec.ECDSA(algorithm=self.SHA256()))
-        signer.update(string_to_sign)
-        return signer.finalize()
+            return key.sign(string_to_sign, signature_algorithm=self.ec.ECDSA(algorithm=self.SHA256()))
 
     def verify(self, signature, string_to_sign, key):
         if self.algorithm == "hmac-sha256":