Convert hex to base64

hexbase64.ml

(* The matasano crypto challenges
 * http://cryptopals.com/sets/1/challenges/1/
 *
 * Compile it like this:
 * $ ocamlfind ocamlc -package base64,batteries -o hexbase64 -linkpkg hexbase64.ml
 *)

open Batteries
open B64

let s = "49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d"

let pair_elem i ch =
    if i mod 2 <> 0 && i <> 0 then
        let hex = int_of_string ("0x" ^ String.make 1 (String.get s (i - 1)) ^ String.make 1 (String.get s i)) in
        char_of_int hex
    else 
        Char.chr 0

let () =
    let decoded_str = BatString.filter ((<>) (Char.chr 0)) (BatString.mapi pair_elem s) in
    print_endline (B64.encode decoded_str);