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3.1.9 NetscapeSPKI objects

NetscapeSPKI objects have the following methods:

`b64_encode`()
  Return a base64-encoded string representation of the object.

`get_pubkey`()
  Return the public key of object.

`set_pubkey`(key)
  Set the public key of the object to key.

`sign`(key, digest_name)
  Sign the NetscapeSPKI object using the given key and digest_name. digest_name must be a string describing a digest algorithm supported by OpenSSL (by EVP_get_digestbyname, specifically). For example, `"md5"` or `"sha1"`.

`verify`(key)
  Verify the NetscapeSPKI object using the given key.