0 · August 29, 2015 14:03 · Jul 6, 2014 · Jul 6, 2014 · Jul 6, 2014 · Jul 6, 2014
diff --git a/README.md b/README.md
@@ -0,0 +1,14 @@
+# arcfour.ss
+
+For some reason, in January of 2009 I felt the need to implement RC4 in Scheme.
+
+## Usage
+
+Known to work with [Racket](http://racket-lang.org/).
+
+Specify the key and plaintext on the command-line as arguments, and it will output the ciphertext. For example,
+
+```
+$ racket arcfour.ss Key Plaintext
+BBF316E8D940AF0AD3
+```
diff --git a/README.md b/README.md
@@ -1,14 +0,0 @@
-# arcfour.ss
-
-For some reason, in January of 2009 I felt the need to implement RC4 in Scheme.
-
-## Usage
-
-Known to work with [Racket](http://racket-lang.org/).
-
-Specify the key and plaintext on the command-line as arguments, and it will output the ciphertext. For example,
-
-```
-$ racket arcfour.ss Key Plaintext
-BBF316E8D940AF0AD3
-```

diff --git a/README.md b/README.md
@@ -11,4 +11,4 @@ Specify the key and plaintext on the command-line as arguments, and it will outp
 ```
 $ racket arcfour.ss Key Plaintext
 BBF316E8D940AF0AD3
-```
+```
diff --git a/arcfour.ss b/arcfour.ss
@@ -58,4 +58,4 @@
             (vector-ref next-vector 2)))))
 
 (newline)
-(exit 0)
+(exit 0)
diff --git a/arcfour.ss b/arcfour.ss
@@ -21,7 +21,7 @@
 
 (define argv (command-line))
 (unless (= (length argv) 3)
-  (display (string-append "usage: " (car argv) " arcfour.ss <key> <plaintext>"))
+  (display (string-append "usage: " (car argv) " <key> <plaintext>"))
   (newline)
   (exit 1))
 (define key (cadr argv))

diff --git a/README.md b/README.md
@@ -1,8 +1,8 @@
-= arcfour.ss
+# arcfour.ss
 
 For some reason, in January of 2009 I felt the need to implement RC4 in Scheme.
 
-== Usage
+## Usage
 
 Known to work with [Racket](http://racket-lang.org/).
 

diff --git a/README.md b/README.md
@@ -0,0 +1,14 @@
+= arcfour.ss
+
+For some reason, in January of 2009 I felt the need to implement RC4 in Scheme.
+
+== Usage
+
+Known to work with [Racket](http://racket-lang.org/).
+
+Specify the key and plaintext on the command-line as arguments, and it will output the ciphertext. For example,
+
+```
+$ racket arcfour.ss Key Plaintext
+BBF316E8D940AF0AD3
+```
diff --git a/arcfour.ss b/arcfour.ss
@@ -0,0 +1,61 @@
+#!r6rs
+(import (rnrs base)
+        (rnrs arithmetic bitwise) ; bitwise-and, etc
+        (rnrs bytevectors) ; make-bytevector, etc
+        (rnrs control) ; unless
+        (rnrs io simple) ; display
+        (rnrs programs) ; command-line
+        (rnrs unicode)) ; string-upcase
+
+; Calculate next value to XOR with the plaintext.
+; (output-next i j) -> (vector value new_i new_j)
+(define (output-next i j)
+  (set! i (bitwise-and (+ i 1) 255))
+  (let ((Si (bytevector-u8-ref S i)))
+    (set! j (bitwise-and (+ j Si) 255))
+    (bytevector-u8-set! S i (bytevector-u8-ref S j))
+    (bytevector-u8-set! S j Si))
+  (vector (bytevector-u8-ref S (bitwise-and 255 (+ (bytevector-u8-ref S i)
+                                                   (bytevector-u8-ref S j))))
+          i j))
+
+(define argv (command-line))
+(unless (= (length argv) 3)
+  (display (string-append "usage: " (car argv) " arcfour.ss <key> <plaintext>"))
+  (newline)
+  (exit 1))
+(define key (cadr argv))
+(define plaintext (caddr argv))
+
+(define S (make-bytevector 256))
+
+; Init S to S[i]=i
+(let loop ((i 0))
+  (bytevector-u8-set! S i i)
+  (unless (>= (+ i 1) (bytevector-length S))
+    (loop (+ 1 i))))
+
+; Init S based on key
+(let loop ((i 0)
+           (j 0))
+  (let ((Si (bytevector-u8-ref S i)))
+    (set! j (bitwise-and 255 (+ j Si (char->integer (string-ref key (mod i (string-length key)))))))
+    (bytevector-u8-set! S i (bytevector-u8-ref S j))
+    (bytevector-u8-set! S j Si)
+    (unless (>= (+ i 1) (bytevector-length S))
+      (loop (+ i 1) j))))
+
+; Encrypt
+(let loop ((x 0) (i 0) (j 0))
+  (let* ((next-vector (output-next i j))
+         (next-number (bitwise-xor (char->integer (string-ref plaintext x)) (vector-ref next-vector 0)))
+         (next-string (string-upcase (number->string next-number 16))))
+    (display (if (>= next-number 16) next-string
+                 (string-append "0" next-string)))
+    (unless (>= (+ x 1) (string-length plaintext))
+      (loop (+ x 1)
+            (vector-ref next-vector 1)
+            (vector-ref next-vector 2)))))
+
+(newline)
+(exit 0)
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