(define crypt-des-extended-default-rounds (make-parameter 1000))

(define (24bit-number->string n)
  (string (int->a64 n)
          (int->a64 (arithmetic-shift n -6))
          (int->a64 (arithmetic-shift n -12))
          (int->a64 (arithmetic-shift n -18))))

(define (crypt-des-extended-gensalt random #!key rounds)
  (let ((rounds (or rounds (crypt-des-extended-default-rounds)))
        (v (get-random-u8vector 'crypt-des-extended-gensalt random 3 3)))
    (string-append "_" (24bit-number->string rounds) (u8vector->saltstring v))))