TOTP implementation for https://github.com/PostgREST/postgrest/issues/1250#issuecomment-551847083

totp.pg.sql

CREATE EXTENSION pgcrypto;
CREATE OR REPLACE FUNCTION totp(key BYTEA, clock_offset INT DEFAULT 0) RETURNS INT AS $$
DECLARE
    c BYTEA := '\x000000000' || TO_HEX(FLOOR(EXTRACT(EPOCH FROM NOW()) / 30)::INT + clock_offset);
    mac BYTEA := HMAC(c, key, 'sha1');
    trunc_offset INT := GET_BYTE(mac, 19) % 16;
    result INT := SUBSTRING(SET_BIT(SUBSTRING(mac FROM 1 + trunc_offset FOR 4), 7, 0)::TEXT, 2)::BIT(32)::INT % 1000000;
BEGIN
    RETURN result;
END;
$$ LANGUAGE plpgsql;