Set password as sha1 hash and check password against hash.

checkpassword.lua

--EVAL "this script" 1 key userid password
local key = KEYS[1];
local userid, pass = unpack(ARGV);

local hash = redis.sha1hex('SOME-STATIC-SALT'..string.sub(pass, 1, 2)..userid..string.sub(pass, 3));

--no error, does hash match?
return {false, (redis.call('GET', key) == hash)}

setpassword.lua

--EVAL "this script" 1 key userid password
local key = KEYS[1];
local userid, pass = unpack(ARGV);
if (#pass < 8) then
    --bytes, actually
    return {"Password must be at least 8 characters."}
end

local hash = redis.sha1hex('SOME-STATIC-SALT'..string.sub(pass, 1, 2)..userid..string.sub(pass, 3));
redis.call('SET', key, hash);

--no error
return {false};