Python signxml“ ValueError：无法反序列化关键数据。”

Question

我正在尝试使用Python的signxml库对xml进行签名并验证签名。 但是它引发以下异常，这可能是什么错误？

Traceback (most recent call last):
  File "C:/Users/digz/Documents/test_signxml/xml_verify_test_4.py", line 24, in <module>
    signed_root = XMLSigner().sign(root, key=key, cert=cert)
  File "C:\Python27\lib\site-packages\signxml\__init__.py", line 364, in sign
    key = load_pem_private_key(key, password=passphrase, backend=default_backend())
  File "C:\Python27\lib\site-packages\cryptography\hazmat\primitives\serialization.py", line 20, in load_pem_private_key
    return backend.load_pem_private_key(data, password)
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\multibackend.py", line 289, in load_pem_private_key
    return b.load_pem_private_key(data, password)
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\openssl\backend.py", line 1033, in load_pem_private_key
    password,
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\openssl\backend.py", line 1202, in _load_key
    self._handle_key_loading_error()
  File "C:\Python27\lib\site-packages\cryptography\hazmat\backends\openssl\backend.py", line 1274, in _handle_key_loading_error
    raise ValueError("Could not unserialize key data.")
ValueError: Could not unserialize key data.

这是我正在使用的代码：

from xml.etree import ElementTree
from signxml import XMLSigner, XMLVerifier
from myapp import settings

cert = open(settings.STATIC_ROOT + '/file/public.cert', 'rt').read()
key = open(settings.STATIC_ROOT + '/file/public.key', 'rt').read()

root = ElementTree.fromstring('<xml1>12</xml1>')
signed_root = XMLSigner().sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml
print verified_data