Here is an upgrade of >>3 with selectable padding scheme and the addition of PKCS#7 padding.
$ cat test.scm
(define (simple-mcrypt algo mode padunit ivlen padin padout)
(lambda (text key encrypt?)
(let ((out (with-input-from-string
(if encrypt?
(padin text padunit)
text)
(lambda ()
(call-with-output-string
(lambda (port)
(mcrypt-encrypt-port
algo
mode
(current-input-port)
port
key
(make-string ivlen #\NUL)
encrypt?)))))))
(if encrypt?
out
(padout out padunit)))))
(define (test-simple-mcrypt engine text key)
(let* ((hidden (engine text key #t))
(revealed (engine hidden key #f)))
(string=? text revealed)))
(define (padin-nul s unit)
(let* ((slen (string-length s))
(mod (modulo slen unit)))
(if (= mod 0)
s
(string-pad-right s (+ slen (- unit mod)) #\NUL))))
(define (padout-nul)
(define trimcs (char-set-invert (char-set #\NUL)))
(lambda (s unit)
(string-trim-right s trimcs)))
(define (padin-pkcs7 s unit)
(let* ((slen (string-length s))
(mod (modulo slen unit))
(used (- unit mod)))
(string-pad-right s (+ slen used) (ascii->char used))))
(define (padout-pkcs7 s unit)
(let* ((slen (string-length s))
(used (char->ascii (string-ref s (- slen 1)))))
(string-head s (- slen used))))Usage example with cbc AES-256. Strings of NULs are no problem with PKCS#7.
$ mit-scheme --load test.scm
[...]
Release 9.1.1 || Microcode 15.3 || Runtime 15.7 || SF 4.41
LIAR/x86-64 4.118 || Edwin 3.116
;Loading "test.scm"... done
1 ]=> (mcrypt-available?)
;Loading "/usr/lib/x86_64-linux-gnu/mit-scheme/lib/prmcrypt.so"... done
;Value: #t
1 ]=> (define aes (simple-mcrypt "rijndael-256" "cbc" 16 16 padin-nul (padout-nul)))
;Value: aes
1 ]=> (define key (string-append (md5-string "secret key") (md5-string "part two")))
;Loading "/usr/lib/x86_64-linux-gnu/mit-scheme/lib/prmhash.so"... done
;Value: key
1 ]=> (define text "How can I run my own instance of this")
;Value: text
1 ]=> (test-simple-mcrypt aes text key)
;Value: #t
1 ]=> (define aes (simple-mcrypt "rijndael-256" "cbc" 16 16 padin-pkcs7 padout-pkcs7))
;Value: aes
1 ]=> (test-simple-mcrypt aes text key)
;Value: #t
1 ]=> (define text (make-string 55 #\NUL))
;Value: text
1 ]=> text
;Value 13: "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
1 ]=> (test-simple-mcrypt aes text key)
;Value: #t
1 ]=>