Hi guys, I have a few questions about cipher related questions
that I face faced...
No.1 Consider a simple symmetric encryption algorithm as
follows:
1- Input text T and 64 bit key K
2- Divide T into 64 bit size blocks B1...Bn
3- Get first block B1 and perform bit-wise XOR with K to produce
encrypted block E1
4- Repeat
a. Get next block Bi and perform bit-wise XOR with Ei-1 to produce
encrypted block Ei
5‐ Until End of Text
The algorithm is very simple, and naturally, we do not expect
the cyphertext to be cryptographically strong. Given the above
algorithm answer the followings:
(a) Is it a problem if the first block of input happens to be the
same as the key? Explain why?
(b) Modify the algorithm to rectify the above problem.
No.2 With the ECB mode, if there is an error in a block of
transmitted cyphertext, only the corresponding plaintext block is
affected. However, in the CBC mode, the error propagates. For
example, an error in the transmitted C1 (Figure 2.9 of textbook)
obviously corrupts P1 and P2.
1. Are any blocks beyond P2 affected?
2. Suppose that there is a bit error in the source version of P1.
Through how many ciphertext blocks is this error propagated? What
is the effect at the receiver?
No.3 Alice and Bob exchange messages using symmetric key
encryption. Every time they communicate, a session key is generated
that encrypts the message (assume they use a protocol that handles
session key, like SSL). They could alternatively use PGP to
exchange messages. PGP also uses session key to ensure
confidentiality (see Fig. 5.1). Do you think PGP will offer better
security than the other method? Justify your answer.
No.4 Kerberos sends an “Authenticator” along with the “Ticket” to
gain access to a service (see step (3) and (5) in Table 4.1). A
malicious party can capture the “Authenticator” and “Ticket”, and
replay them to gain unauthorized access. Can Kerbeors prevent such
replay attack? How?
Do you think we could use something else instead of timestamp in
Authenticator? Justify your answer.
Thanks guys. This was my assignment and I do not know how to do
=(
a) There won't be a problem even if the "plaintext" has the same
bits as the "key", the resulting "ciphertext" will turn up all
zeros, when the "ciphertext" is deciphered using the same "key",
the "plaintext" will be deciphered.
10101010 (plaintext)
10101010
(key)
00000000 (XOR ciphertext)
Apply same key to ciphertext:
00000000 (ciphertext)
10101010
(key)
10101010 (XOR plaintext)
***I think I
might have missed out something, because there is a (b) to rectify
the algorithm to fix the problem, so obviously there must be a
problem. 
***BTW, what is Bi, Ei-1 and
Ei???