Add AuthorityKeyIdentifier.from_issuer_subject_key_identifier
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index 67427dd..1e8aeba 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -1568,6 +1568,32 @@
>>> x509.AuthorityKeyIdentifier.from_issuer_public_key(issuer_cert.public_key())
<AuthorityKeyIdentifier(key_identifier='X\x01\x84$\x1b\xbc+R\x94J=\xa5\x10r\x14Q\xf5\xaf:\xc9', authority_cert_issuer=None, authority_cert_serial_number=None)>
+ .. classmethod:: from_issuer_subject_key_identifier(ski)
+
+ .. versionadded:: 1.3
+
+ Creates a new AuthorityKeyIdentifier instance using the
+ SubjectKeyIdentifier from the issuer certificate. The resulting object
+ will contain
+ :attr:`~cryptography.x509.AuthorityKeyIdentifier.key_identifier`, but
+ :attr:`~cryptography.x509.AuthorityKeyIdentifier.authority_cert_issuer`
+ and
+ :attr:`~cryptography.x509.AuthorityKeyIdentifier.authority_cert_serial_number`
+ will be None.
+
+ :param ski: The
+ :class:`~cryptography.x509.SubjectKeyIdentifier` from the issuer
+ certificate.
+
+ .. doctest::
+
+ >>> from cryptography import x509
+ >>> from cryptography.hazmat.backends import default_backend
+ >>> issuer_cert = x509.load_pem_x509_certificate(pem_data, default_backend())
+ >>> ski = issuer_cert.extensions.get_extension_for_class(x509.SubjectKeyIdentifier)
+ >>> x509.AuthorityKeyIdentifier.from_issuer_subject_key_identifier(ski)
+ <AuthorityKeyIdentifier(key_identifier='X\x01\x84$\x1b\xbc+R\x94J=\xa5\x10r\x14Q\xf5\xaf:\xc9', authority_cert_issuer=None, authority_cert_serial_number=None)>
+
.. class:: SubjectKeyIdentifier(digest)
.. versionadded:: 0.9