CREATE TABLE Authentication (
  id SERIAL);
-- CREATE TABLE Mail_addresses No! Too weak.
CREATE TABLE Passwords (
  password TEXT NOT NULL
) INHERITS (Authentication);
CREATE TABLE GPGkeys (
  pubkey TEXT NOT NULL
) INHERITS (Authentication);
CREATE TABLE X509certificates (
  certificate TEXT NOT NULL
) INHERITS (Authentication);
  
CREATE TABLE Users (
       id SERIAL,	
       name TEXT UNIQUE NOT NULL,
       authentication TEXT CHECK (authentication IN ('GPG', 'X509', 'password')),
       auth_info INTEGER REFERENCES Authentication (id));

INSERT INTO Passwords (password) VALUES ('trustnoone');
INSERT INTO Users (name, authentication, auth_info) VALUES
       ('foobar', 'password', 1);