def _set_rekey_passphrase(db_handle, passphrase):
    """
    Change the passphrase for encryption key derivation.

    The key itself can be a passphrase, which is converted to a key using
    PBKDF2 key derivation. The result is used as the encryption key for
    the database.

    :param db_handle: A handle to the SQLCipher database.
    :type db_handle: pysqlcipher.Connection
    :param passphrase: The passphrase used to derive the encryption key.
    :type passphrase: str
    """
    db_handle.cursor().execute("PRAGMA rekey = '%s'" % passphrase)
