Workshop 02

Network and Information Security
Workshop 2
Part 1 – Reviewing the lecture notes, answer the following questions
1.What is the difference between a block cipher and a stream cipher?
2.DES ciphers are block ciphers. In each step of a DES encryption, a data block of 64
bits is encrypted by using a 56-bit key. Explain how the encryption is processed with the data block and key having different lengths.
3.Suppose a system has 10 legitimate users. To implement a symmetric-key
cryptosystem such that any pair of users will share a unique secret key. How many different keys are there for the whole system?
4.Answer the same question above for 100 legitimate users.
5.Explain why the following are not random bit sequences:
∙000000000000000……
∙110110*********……
∙001010010100101……
∙101101010110101……
Part 2 – Hands-on Exercises
1.The following ciphertext has been obtained by encrypting the plaintext using a V i g e nèr e
C i p h e r.R ecover the original text. You can use the CrypTool to help you.
(Hints: The V i g e nèr e c i p h e r h a s u s e d t h r e e a l p h a b e t s f o r t h e e n c r y p t i o n.T h e o r i g i n a l m e s s a g e t e x t h a s b e e n d i v i d e d i n t o b l o c k s o f l e n g t h3.I n e a c h b l o c k,t h e f i r s t l e t t e r i s
e n c r y p t e d u s i n g t h e
f i r s t a l p h a b e t;t h e s e c o n d l e t t e r w a s e n c r y p t e d u s i n
g t
h e s e c o n d
a l p h a
b e t;a n d t h e t h i r d l e t t e r w a s e n
c r y p t e
d u s i n g t h
e t h i r d a l p h a b e t.S p a c e s a n d n u m b e r s
w e r e u n c h a n g e d).
.
Jp vzopfvuje dmirskwiov, ujh tgqegu bpg sgfjrlfpw vuh ujh tcpf mhz vr fpfsasu cqe fhdtbqv wig pfuvbih. Ujh lgb ju ufhhsthe vr bu d tjdsgg pt vfeufv nfa. D tapngwskf dtbqvrhtdqjld ubtvhn kv bnvp thggusgg uq dt c vfeufv nfa fsasuqvzuwfo. Nfa ofpjuj lt cq josptwbpw gcfuqu jp wig vfexskwz qi b euzrwpubtvhn. Ukptw lgbt cuf uxtehqvlcnh uq “eswwf hrseh bvwben” egfsasukro.
ing the R i j n d a e l algorithm, an AES cipher encrypts the plaintext with a number of
rounds. In each round, each data block (which consists of 4 groups of 4 bytes) will be processed by the following four operations:
∙Byte substitution
∙Shift rows
∙Mix columns
∙Add round key
Using the Visualization of Algorithms of CrypTool, study how these four operations
work.
(Hints: CrypTool →Indiv. Procedures →Visualization of Algorithms →AES →Rijndael Animation)
3.In the hexadecimal notation, each of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F can be
translated into a binary sequence of 4 bits as follows:
So, “AB CD 12 34 EF 89 BC 10 23 AC 56 DE AF 35 ED 36” exactly represent 128 bits. Using the AES cipher with the CrypTool and the key “AB CD 12 34 EF 89 BC 10
23 AC 56 DE AF 35 ED 36”, encrypt the following message:
In cryptography, the Advanced Encryption Standard (AES) is a symmetric-key encryption standard adopted by the U.S. government. The standard comprises three block ciphers, AES-128, AES-192 and AES-256, adopted from a larger collection originally published as Rijndael. Each of these ciphers has a 128-bit block size, with key sizes of 128, 192 and 256 bits, respectively. The AES ciphers have been analyzed extensively and are now used worldwide, as was the case with its predecessor, the Data Encryption Standard (DES).
And then use the same key to decrypt the ciphertext.。