Simplified DES

other/sdes.py

@@ -0,0 +1,86 @@
+def apply_table(inp, table):
+    res = ''
+    for i in table:
+        res += inp[i-1]
+    return res
+
+def left_shift(data):
+    li = []
+    for i in range(len(data)-1):
+        li.append(data[i+1])
+    li.append(data[0])
+    return ''.join(li)
+
+def XOR(a, b):
+    res = ''
+    for i in range(len(a)):
+        if a[i]==b[i]:
+            res += '0'
+        else:
+            res += '1'
+    return res
+
+def apply_sbox(s, data):
+    row = int('0b'+data[0]+data[-1], 2)
+    col = int('0b'+data[1:3], 2)
+    return bin(s[row][col])[2:]
+
+def function(expansion, s0, s1, key, message):
+    left = message[:4]
+    right = message[4:]    
+    temp = apply_table(right, expansion)
+    temp = XOR(temp, key)
+    l = apply_sbox(s0, temp[:4])
+    r = apply_sbox(s1, temp[4:])
+    l = '0'*(2-len(l)) + l
+    r = '0'*(2-len(r)) + r
+    temp = apply_table(l+r, p4_table)
+    temp = XOR(left, temp)
+    return temp+right
+
+
+key = input('Enter 10 bit key: ')
+message = input('Enter 8 bit message: ')
+
+p8_table = [6, 3, 7, 4, 8, 5, 10, 9]
+p10_table = [3, 5, 2, 7, 4, 10, 1, 9, 8, 6]
+p4_table = [2, 4, 3, 1]
+IP = [2, 6, 3, 1, 4, 8, 5, 7]
+IP_inv = [4, 1, 3, 5, 7, 2, 8, 6]
+expansion = [4, 1, 2, 3, 2, 3, 4, 1]
+s0 = [[1, 0, 3, 2],
+      [3, 2, 1, 0],
+      [0, 2, 1, 3],
+      [3, 1, 3, 2]]
+s1 = [[0, 1, 2, 3],
+      [2, 0, 1, 3],
+      [3, 0, 1, 0],
+      [2, 1, 0, 3]]
+
+#encryption
+temp = apply_table(key, p10_table)
+left = temp[:5]
+right = temp[5:]
+left = left_shift(left)
+right = left_shift(right)
+key1 = apply_table(left+right, p8_table)
+left = left_shift(left)
+right = left_shift(right)
+left = left_shift(left)
+right = left_shift(right)
+key2 = apply_table(left+right, p8_table)
+
+temp = apply_table(message, IP)
+temp = function(expansion, s0, s1, key1, temp)
+temp = temp[4:]+temp[:4]
+temp = function(expansion, s0, s1, key2, temp)
+CT = apply_table(temp, IP_inv)
+print('Cipher text is: ', CT)
+
+#decryption
+temp = apply_table(CT, IP)
+temp = function(expansion, s0, s1, key2, temp)
+temp = temp[4:]+temp[:4]
+temp = function(expansion, s0, s1, key1, temp)
+PT = apply_table(temp, IP_inv)
+print('Plain text after decypting is: ', PT)