Registration Code

registration_code.md

Registration Code Generation

This function creates a registration code with 8 ASCII characters. The aim is to send a user a secret, which can be easily copied by the user into a registration dialog. The secret uses a reduced alphabet of 32 letters and digits. Characters known to be commonly confused by users are removed. The secret is not a password. It is just a personal token to enable the registration in order to prevent annonymous registration attempts.

Requirements

• pgcrypto: gen_random_bytes

Description

The secret alphabeth has 32 characters. Each character requires 5 bit. A secret with 8 characters reqires 40 bit or 5 byte. The number of different secrets is 2⁴⁰ = 1099511627776.

RND	7	6	5	4	3	2	1	0
0	h4	g4	f4	e4	d4	c4	b4	a4
1	h3	b3	f3	e3	d3	c3	b3	a3
2	h2	g2	f2	e2	d2	c2	b2	a2
3	h1	g1	f1	e1	d1	c1	b1	a1
4	h0	g0	f0	e0	d0	c0	b0	a0

The bits of the characters of the secret are:

CHR
1	a4	a3	a2	a1	a0
2	b4	b3	b2	b1	b0
3	c4	c3	c2	c1	c0
4	d4	d3	d2	d1	d0
5	e4	e3	e2	e1	e0
6	f4	f3	f2	f1	f0
7	g4	g3	g2	g1	g0
8	h4	h3	h2	h1	h0

registration_code.sql

-- -*- mode: sql; sql-product: postgres; -*-

DROP FUNCTION IF EXISTS registration_code();
CREATE FUNCTION registration_code() RETURNS text
LANGUAGE plpgsql
AS $$
DECLARE
  rnd   bytea := gen_random_bytes(5); -- 5 random bytes
  alpha bytea := 'abcdefghikmnopqrstuvwxyz23456789'::bytea; -- alphabeth
  code  text  := ''; -- code
  r     integer; -- iterate random bytes
  i     integer; -- iterate bits of random bytes and code characters
  b     integer; -- current random byte
  a     integer[]; -- alphabeth indexes of code characters
BEGIN
  FOR i IN 0..7 LOOP
    a[i] := 0;
  END LOOP;
  FOR r IN 0..4 LOOP
    b := get_byte(rnd, r);
    FOR i IN 0..7 LOOP
      a[i] := (a[i] << 1) | (b & 1);
      b := b >> 1;
    END LOOP;
  END LOOP;
  FOR i IN 0..7 LOOP
    code := code || chr(get_byte(alpha, a[i]));
  END LOOP;
  RETURN code;
END
$$;