diff --git a/adb/sign_cryptography.py b/adb/sign_cryptography.py
index b042642..68ba649 100644
--- a/adb/sign_cryptography.py
+++ b/adb/sign_cryptography.py
@@ -25,10 +25,10 @@ class CryptographySigner(adb_protocol.AuthSigner):
     """AuthSigner using cryptography.io."""
 
     def __init__(self, rsa_key_path):
-        with open(rsa_key_path + '.pub') as rsa_pub_file:
+        with open(rsa_key_path + '.pub', 'rb') as rsa_pub_file:
             self.public_key = rsa_pub_file.read()
 
-        with open(rsa_key_path) as rsa_prv_file:
+        with open(rsa_key_path, 'rb') as rsa_prv_file:
             self.rsa_key = serialization.load_pem_private_key(
                     rsa_prv_file.read(), None, default_backend())