updated chapter 4 and appendix-A files

appendix-A/arcfour.py

@@ -0,0 +1,55 @@
+"""RC4 compatible algorithm"""
+
+def arcfour(key, in_bytes, loops=20):
+
+    kbox = bytearray(256)  # create key box
+    for i, car in enumerate(key):  # copy key and vector
+        kbox[i] = car
+    j = len(key)
+    for i in range(j, 256):  # repeat until full
+        kbox[i] = kbox[i-j]
+
+    # [1] initialize sbox
+    sbox = bytearray(range(256))
+
+    # repeat sbox mixing loop, as recommened in CipherSaber-2
+    # http://ciphersaber.gurus.com/faq.html#cs2
+    j = 0
+    for k in range(loops):
+        for i in range(256):
+            j = (j + sbox[i] + kbox[i]) % 256
+            sbox[i], sbox[j] = sbox[j], sbox[i]
+
+    # main loop
+    i = 0
+    j = 0
+    out_bytes = bytearray()
+
+    for car in in_bytes:
+        i = (i + 1) % 256
+        # [2] shuffle sbox
+        j = (j + sbox[i]) % 256
+        sbox[i], sbox[j] = sbox[j], sbox[i]
+        # [3] compute t
+        t = (sbox[i] + sbox[j]) % 256
+        k = sbox[t]
+        car = car ^ k
+        out_bytes.append(car)
+
+    return out_bytes
+
+
+def test():
+    from time import time
+    clear = bytearray(b'1234567890' * 100000)
+    t0 = time()
+    cipher = arcfour(b'key', clear)
+    print('elapsed time: %.2fs' % (time() - t0))
+    result = arcfour(b'key', cipher)
+    assert result == clear, '%r != %r' % (result, clear)
+    print('elapsed time: %.2fs' % (time() - t0))
+    print('OK')
+
+
+if __name__ == '__main__':
+    test()