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3.1.3 X509Req objects

X509Req objects have the following methods:

`get_pubkey`()
  Return a PKey object representing the public key of the certificate request.

`get_subject`()
  Return an X509Name object representing the subject of the certificate.

`set_pubkey`(pkey)
  Set the public key of the certificate request to pkey.

`sign`(pkey, digest)
  Sign the certificate request, using the key pkey and the message digest algorithm identified by the string digest.

`verify`(pkey)
  Verify a certificate request using the public key pkey.

`set_version`(version)
  Set the version (RFC 2459, 4.1.2.1) of the certificate request to version.

`get_version`()
  Get the version (RFC 2459, 4.1.2.1) of the certificate request.