docs: def PassphraseCallback(verify=False,
                         prompt1="Enter passphrase:",
                         prompt2="Verify passphrase:"):
    """A utility function to read a passphrase from stdin."""
    while 1:
      try:
        p1 = getpass.getpass(prompt1)
        if verify:
          p2 = getpass.getpass(prompt2)
          if p1 == p2:
            break
        else:
          break
      except KeyboardInterrupt:
        return None
    return p1