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(* Read /dev/urandom.
* Copyright (C) 2013 Red Hat Inc.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*)
(* Read and return N bytes (only) from /dev/urandom.
*
* As pointed out by Edwin Török, previous versions of this had a big
* problem. They used the OCaml buffered I/O library which would read
* a lot more data than requested. This version uses unbuffered I/O
* from the Unix module.
*)
open Unix
let open_urandom_fd () = openfile "/dev/urandom" [O_RDONLY] 0
let read_byte fd =
let s = String.make 1 ' ' in
fun () ->
if read fd s 0 1 = 0 then (
close fd;
raise End_of_file
);
Char.code s.[0]
let urandom_bytes n =
assert (n > 0);
let ret = String.make n ' ' in
let fd = open_urandom_fd () in
for i = 0 to n-1 do
ret.[i] <- Char.chr (read_byte fd ())
done;
close fd;
ret
(* Return a random number uniformly distributed in [0, upper_bound)
* avoiding modulo bias.
*)
let rec uniform_random read upper_bound =
let c = read () in
if c >= 256 mod upper_bound then c mod upper_bound
else uniform_random read upper_bound
let urandom_uniform n chars =
assert (n > 0);
let nr_chars = String.length chars in
assert (nr_chars > 0);
let ret = String.make n ' ' in
let fd = open_urandom_fd () in
for i = 0 to n-1 do
ret.[i] <- chars.[uniform_random (read_byte fd) nr_chars]
done;
close fd;
ret
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