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--- sasc:laboratoare:05 [2016/03/29 00:42]
sergiu.costea [Lab 05]
+++ sasc:laboratoare:05 [2017/03/23 19:53] (current)
marios.choudary
@@ Line 1: / Line 1: @@
-===== Lab 05 =====
+===== Lab 05 - DES =====
@@ Line 5: / Line 5: @@
 http://cs.curs.pub.ro/2014/pluginfile.php/13095/mod_resource/content/2/sasc_curs4_5.pdf
-==== Exercise 1 ====
+==== Exercise 1 (2p) ====
-Remember DESX defined as the operation DESX( (k1,k2,k3), m) = k1 ⊕ DES(k2, m ⊕ k3).
+Remember DESX defined as the operation DESX( (k1,k2,k3), m) = k1 ⊕ DES(k2, m ⊕ k3),
-Show an attack on DESX that runs in time 2<sup>120</sup>.
+where k1, k3 have 64 bits (same as input/output of DES) and k2 has 56 bits (DES key size).
+Show a brute force (exhaustive key search) attack on DESX that runs in time $O(2^{120})$.
-==== Exercise 2 ====
+<note tip>
+Try using a couple of (message, ciphertext) pairs and see if you can get rid of k1
+somehow in order to speed up a brute force attack.
+</note>
+==== Exercise 2 (3p) ====
 Show why the following schemes do not bring any real advantage compared to DES:
@@ Line 17: / Line 23: @@
   * b) c = DES(k2, m ⊕ k1)
-==== Exercise 3 ====
+<note tip>
+You may use a similar approach to what you did in the previous exercise.
+</note>
+==== Exercise 3 (5p) ====
 The goal of this exercise is to implement the meet-in-the-middle attack on double DES.
@@ Line 232: / Line 242: @@
   main()
 </code>
-==== SPN 2 ====
-Now we have a better SPN, where the output of the permutation is XOR-ed with another 2 key bytes, as in the following figure:
-{{:sasc:laboratoare:spn_1r_full_2s.png|}}
-  - Try to find the key in this case, when given the following message/ciphertext pairs: ('Om', 0x0073), ('El', 0xd00e), ('an', 0x855b). Print the key in ascii.
-<note tip>You may try some kind of brute-force search</note>
-==== SPN 3 ====
-As another example, which uses a larger block size, let's use an SPN that takes a 4-byte input x=[x1 || x2 || x3 || x4] and an 8-byte key k=[k1 || k2 || k3 || k4 || k5 || k6 || k7 || k8] as in this figure:
-{{:sasc:laboratoare:spn_1r_full_4s.png|}}
-Note that in this 4-byte SPN, the permutation operates on all 4 bytes, similarly to the 2-byte SPN: that is, it shifts all bits four bits to the right.
-  - Try to find the key in this case as well, using the following message/ciphertext pairs: ('Omul', 0xddcf7bc7), ('stea', 0x96d58b43), ('luna', 0x9c3f2303) . Again print the key in ascii.
-<note tip> This time you cannot (easily) do a brute-force on all the bytes of the last XOR. However, you may try to attack one S-box at a time. Think of the bits that affect one such S-box and find an efficient attack.
-</note>