[plt-scheme] Accessing openssl from MzScheme, sending control-c (end of text)

If I were you I'd avoid the "system" call entirely, and use the ffi.
Here's a sample from Jay McCarthy that shows how to compute a
"sha1-hmac"; I expect what you want is even simpler:

    (module hmac-sha1 mzscheme
      (require (lib "foreign.ss")
               (all-except (lib "contract.ss")
                           ->)
               (rename (lib "contract.ss")
                       c-> ->))
      (unsafe!)

      (define openssl-crypto
        (case (system-type)
          [(windows)
           (ffi-lib "libeay32")]
          [else
           (ffi-lib "libcrypto")]))

      (define EVP_SHA1
        (get-ffi-obj 'EVP_sha1 openssl-crypto
                     (_fun -> _fpointer)))

      (define (HMAC-SHA1 key data)
        ;; we need to call bytes-copy because hmac-sha1-internal always
        ;; returns the same pointer (to static storage).
        (bytes-copy

         ;; we need to call make-sized-byte-string because otherwise our
         ;; return value would get truncated at the first 0 byte.  This is
         ;; an insidious problem, since most return values don't _have_ a
         ;; 0 byte, and thus omitting this call will _appear_ to work,
         ;; most of the time.
         (make-sized-byte-string
          (hmac-sha1-internal key data)
          20)))

      (define hmac-sha1-internal
        (get-ffi-obj 'HMAC openssl-crypto
                     (_fun [EVP_MD : _fpointer = (EVP_SHA1)]
                           [key : _bytes]
                           [key_len : _int = (bytes-length key)]
                           [data : _bytes]
                           [data_len : _int = (bytes-length data)]
                           [md : _int = 0]
                           [md_len : _int = 0]
                           ->
                           _bytes)))

      (provide/contract
       [HMAC-SHA1 (bytes? bytes? . c-> . bytes?)]))
-- 

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