def decrypt_key(encrypted_key, passphrase):
<EOL>securesystemslib.formats.ENCRYPTEDKEY_SCHEMA.check_match(encrypted_key)<EOL>securesystemslib.formats.PASSWORD_SCHEMA.check_match(passphrase)<EOL>key_object = None<EOL>key_object =securesystemslib.pyca_crypto_keys.decrypt_key(encrypted_key, passphrase)<EOL>return key_object<EOL>
<Purpose>
  Return a string containing 'encrypted_key' in non-encrypted form.  The
  decrypt_key() function can be applied to the encrypted string to restore
  the original key object, a key (e.g., RSAKEY_SCHEMA, ED25519KEY_SCHEMA).
  This function calls pyca_crypto_keys.py to perform the actual decryption.

  Encrypted keys use AES-256-CTR-Mode and passwords are strengthened with
  PBKDF2-HMAC-SHA256 (100K iterations be default, but may be overriden in
  'settings.py' by the user).

  http://en.wikipedia.org/wiki/Advanced_Encryption_Standard
  http://en.wikipedia.org/wiki/CTR_mode#Counter_.28CTR.29
  https://en.wikipedia.org/wiki/PBKDF2

  >>> ed25519_key = generate_ed25519_key()
  >>> password = 'secret'
  >>> encrypted_key = encrypt_key(ed25519_key, password)
  >>> decrypted_key = decrypt_key(encrypted_key.encode('utf-8'), password)
  >>> securesystemslib.formats.ANYKEY_SCHEMA.matches(decrypted_key)
  True
  >>> decrypted_key == ed25519_key
  True

<Arguments>
  encrypted_key:
    An encrypted key (additional data is also included, such as salt, number
    of password iterations used for the derived encryption key, etc) of the
    form 'securesystemslib.formats.ENCRYPTEDKEY_SCHEMA'.  'encrypted_key'
    should have been generated with encrypt_key().

  password:
    The password, or passphrase, to decrypt 'encrypted_key'.  'password' is
    not used directly as the encryption key, a stronger encryption key is
    derived from it.  The supported general-purpose module takes care of
    re-deriving the encryption key.

<Exceptions>
  securesystemslib.exceptions.FormatError, if the arguments are improperly
  formatted.

  securesystemslib.exceptions.CryptoError, if 'encrypted_key' cannot be
  decrypted.

<Side Effects>
  None.

<Returns>
  A key object of the form: 'securesystemslib.formats.ANYKEY_SCHEMA' (e.g.,
  RSAKEY_SCHEMA, ED25519KEY_SCHEMA).