;;; Carry some entropy across reboots. Adapted from examples in random(4).

;;; We assume Linux >= 2.6, where the poolsize is always 4096 bits (according to
;;; random(4). Otherwise, the example in random(4) reads the 'poolsize' file and
;;; creates a seed of equal size.

;;; This should be run during system shutdown.  It saves some random state as a
;;; seed for /dev/urandom, to be used on the next boot.
(define (urandom-shutdown seed)
  (touch seed)
  (chmod seed #o600)
  (write-seed seed))

;;; This should be run at boot, before starting anything that needs random
;;; numbers (sshd, TLS server, etc).
(define (urandom-boot seed)
  (and (if (file-exists? seed)
         (zero? (system (string-append "cat " seed " > /dev/urandom")))
         (touch seed))
       (chmod seed #o600)
       (write-seed seed)))

;;; On Debian, '/var/lib/urandom/random-seed'.
;;; random(4) suggests '/var/run/random-seed'.
(define seed "/tmp/random-seed")

(define (write-seed seed)
    (zero? (system* "dd" "if=/dev/urandom" (string-append "of=" seed)
                    "count=1" "bs=512"))) ;; If Linux is not >= 2.6, then 'bs'
                                          ;; must be calculated as shown in
                                          ;; random(4).

(define (touch file)
  (close-port (open-file file "a0b")))