OS-based rng access

rand.rkt

#lang racket/base
(require racket/contract/base  "unix_rand.rkt" "windows_rand.rkt" (only-in file/sha1 bytes->hex-string))
(provide (contract-out [crypto-random-bytes (-> exact-positive-integer? (or/c bytes? string?))]
                       [crypto-random (-> exact-positive-integer? exact-positive-integer?)]))
                       
(define bytes->number (compose (lambda (s) (string->number s 16)) bytes->hex-string))

; (: crypto-random-bytes (-> Positive-Integer Bytes))
; returns n random bytes from the os.
(define (crypto-random-bytes n)
  (case (system-type 'os)
    [(unix macosx) (crypto-random-unix-bytes n)]
    [(windows) (crypto-random-windows-bytes n)]
    [else (raise (make-exn:fail:unsupported
                  "Only UNIX, OS X, and Windows XP or greater are currently supported"
                  (current-continuation-marks)))]))

; (: crypto-random (-> Positive-Integer Nonnegative-Integer))
; returns a random n byte nonnegative-integer
(define (crypto-random n)
  (bytes->number (crypto-random-bytes n)))

unix_rand.rkt

#lang racket/base
(provide crypto-random-unix-bytes)

(define (check-urandom-exists)
  (unless (file-exists? "/dev/urandom")
    (raise (make-exn:fail:filesystem
            "/dev/urandom does not exist"
            (current-continuation-marks)))))

; (: crypto-random-unix-bytes (-> Positive-Integer Bytes))
(define (crypto-random-unix-bytes n)
  (check-urandom-exists)
  (define urandom (open-input-file "/dev/urandom"))
  (read-bytes n urandom))

windows_rand.rkt

#lang racket/base
(provide crypto-random-windows-bytes)

(require ffi/com 
         ffi/unsafe 
         ffi/unsafe/define 
         ffi/winapi)

(define-ffi-definer define-advapi (and (eq? (system-type) 'windows) (ffi-lib "Advapi32.dll"))
  #:default-make-fail make-not-available)

; supposed to be the same csprng as CryptGenRand, but with less overhead
; see Microsoft security dev Michael Howard: http://blogs.msdn.com/b/michael_howard/archive/2005/01/14/353379.aspx
; this is for Windows XP and later only, but I doubt that's a problem
(define-advapi SystemFunction036 
  (_fun #:abi winapi _pointer _ulong -> _bool))

(define (check-SystemFunction036-exists)
  (unless SystemFunction036
    (raise (make-exn:fail:unsupported
            "Unable to load RtlGenRandom (SystemFunction036) from Advapi32.dll"
            (current-continuation-marks)))))

; (: crypto-random-windows-bytes (-> Positive-Integer Bytes))
(define (crypto-random-windows-bytes n)
  (check-SystemFunction036-exists)
  (define rand-bytes-buf (make-bytes n))
  (if (SystemFunction036  rand-bytes-buf n)
      rand-bytes-buf
      (raise (make-exn:fail
              "SystemFunction036 failed to generate bytes"
              (current-continuation-marks)))))

I decided to remove the hex string option. bytes->hex-string makes it trivial for someone to convert the output of crypto-random-bytes into a hex string if they need it, so it doesn't warrant the added complexity.

a bytes->number would definitely be a useful addition to the bytes pkg.

your point about crypto-random's interface is spot-on. I'll have to think of how best to do this.