make sure you've started crypto with `application:start(crypto)`

crack_util.erl

-module(crack_util).

-export([crack/0]).

-define(CHAR_LIST, "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVW").

crack() ->
    TryLength = 5,
    MD5Hash = crypto:md5("12345"),
    Lol = lists:duplicate(TryLength, ?CHAR_LIST),
    {Divs, Mods, _Factor} = lists:foldl(
                              fun(I, {Dividends, Moduli, LastFactor}) ->
                                      Length = length(lists:nth(I+1, Lol)),
                                      {[Dividends,LastFactor], [Moduli,Length], LastFactor * Length}
                              end,
                              {[],[], 1},
                              lists:seq(length(Lol)-1,0,-1)
                             ),
    FlatDivs = lists:flatten(Divs),
    FlatMods = lists:flatten(Mods),
    Products = round(math:pow(length(?CHAR_LIST), TryLength)),
    lists:foreach(fun(N) ->
                          Product = nthProduct(N, Lol, FlatDivs, FlatMods),
                          case crypto:md5(Product) == MD5Hash of
                              true ->
                                  io:format("found product: ~p~n",[lists:flatten(Product)]);
                              _ ->
                                  ok
                          end
                  end,
                  lists:seq(0, Products-1)).

nthProduct(N, Lol, Dividends, Moduli) ->
    {Product, _, _, _, _} = lists:foldl( 
                              fun fold_product/2,
                              { [], Dividends, Moduli, Lol, N },
                              lists:seq(1, length(Lol))
                             ),
    Product.

fold_product(_I, { Acc, [ Div | NewDivs ], [Mod | NewMods], [ List | NewLol ], N}) ->
    Position = floor(round(N/Div) rem Mod),
    Char = lists:nth(Position+1, List),
    { [Acc, Char], NewDivs, NewMods, NewLol, N }.

floor(X) ->
    T = erlang:trunc(X),
    case (X - T) of
        Neg when Neg < 0 -> T - 1;
        Pos when Pos > 0 -> T;
        _ -> T
    end.