From a9af9b24b7fbfb31110123b125fb7fc916cff24f Mon Sep 17 00:00:00 2001
From: Camil Staps
Date: Fri, 12 Feb 2016 14:50:35 +0100
Subject: Put everything on git

---
 Assignment 3/nthkey.py | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)
 create mode 100755 Assignment 3/nthkey.py

(limited to 'Assignment 3/nthkey.py')

diff --git a/Assignment 3/nthkey.py b/Assignment 3/nthkey.py
new file mode 100755
index 0000000..40cf0cf
--- /dev/null
+++ b/Assignment 3/nthkey.py	
@@ -0,0 +1,37 @@
+#!/usr/bin/python
+
+# Copyright (c) 2015 Camil Staps <info@camilstaps.nl>
+
+import binascii
+
+# From http://stackoverflow.com/a/843846/1544337
+# 1 for odd parity, 0 for even parity
+# Was used only in a previous version of nthByte
+# def parity(b):
+#     c = 0
+#     while b != 0:
+#         c += 1
+#         b &= b - 1
+#     return c % 2
+
+# Find the nth possibility for a byte with odd parity
+oddBytes = [1,2,4,7,8,11,13,14,16,19,21,22,25,26,28,31,32,35,37,38,41,42,44,47,49,50,52,55,56,59,61,62,64,67,69,70,73,74,76,79,81,82,84,87,88,91,93,94,97,98,100,103,104,107,109,110,112,115,117,118,121,122,124,127,128,131,133,134,137,138,140,143,145,146,148,151,152,155,157,158,161,162,164,167,168,171,173,174,176,179,181,182,185,186,188,191,193,194,196,199,200,203,205,206,208,211,213,214,217,218,220,223,224,227,229,230,233,234,236,239,241,242,244,247,248,251,253,254]
+def nthByte(n):
+    return oddBytes[n]
+    # c = -1                        # This is the old version. The new version uses a faster lookup table.
+    # b = 0
+    # while c != n:
+    #     b += 1
+    #     if parity(b) == 1:
+    #         c += 1
+    # return b
+
+# Find the nth key in which all bytes have odd parity
+def nthKey(n):
+    key = ''
+    for b in range(7,-1,-1):
+        key += chr(nthByte((n >> b*7) & 0x7f))
+    return key
+
+# Example
+print binascii.b2a_hex(nthKey(765637))
\ No newline at end of file
-- 
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