Don't destructively modifiy args.

hmac.scm

@@ -9,11 +9,12 @@
 (use message-digest-port)
 
 (define (hmac key digest-primitive #!optional (block-size 64))
-  (when (> (string-length key) block-size)
-    (set! key (call-with-output-digest digest-primitive (cut display key <>) 'string)))
-  (set! key (string-pad-right key block-size (integer->char 0)))
-  (let ((ipad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x36))) key))
-	(opad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x5c))) key)))
+  (let ((key_ key))
+    (when (> (string-length key_) block-size)
+          (set! key_ (call-with-output-digest digest-primitive (cut display key_ <>) 'string)))
+    (set! key_ (string-pad-right key_ block-size (integer->char 0)))
+    (let ((ipad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x36))) key_))
+          (opad (string-map (lambda (c) (integer->char (bitwise-xor (char->integer c) #x5c))) key_)))
       (lambda (message)
         (call-with-output-digest
          digest-primitive
@@ -24,6 +25,6 @@
                               (cut display (string-append ipad message) <>)
                               'string))
               <>)
-        'string))))
+         'string)))))
 
 )