Cryptography Reference
In-Depth Information
(a) The attacker does not know the encryption algorithm that was used?
(b) The attacker knows the encryption algorithm, but does not know any
previous plaintext/ciphertext pairs, and knows that the plaintext is randomly
generated?
(c) The attacker knows the encryption algorithm, one previous plaintext/
ciphertext pair, and knows that the plaintext is randomly generated?
12
. Dan Brown's best seller
Digital Fortress
[42] features a machine which, it
is claimed, can 'break' most cryptosystems. Comment on the practicality of
building such a machine.
13
. Explain why a cryptographic designer might reasonably claim that the main
security goal for designing a symmetric encryption algorithm is to make sure
that the best attack against it is an exhaustive key search.
14
. We often lose perspective of very large numbers.
(a) Place the following values in order of increasing size:
• number of possible 40-bit keys
• number of possible 90-bit keys
• number of possible 128-bit keys
• number of web pages indexed by Google
• number of stars in our galaxy
• number of stars in the universe
• number of species of bird on Earth
• number of seconds since the beginning of the universe
(b) For each of the above, identify how many bits would be required to
represent symmetric keys in a keyspace of that size.
15
. Encryption algorithm ALEX has a 40-bit key and encryption algorithm CARLOS
has a 48-bit key. Assume that you have sufficient computing power to use an
exhaustive key search to find the key of ALEX in one day.
(a) Assuming that they have similar computational complexity, how long would
you expect it to take to find the key of CARLOS by means of an exhaustive
key search?
(b) Assume now that the (bad) design of CARLOS allows it to be run in two
separate stages such that it is possible to conduct an exhaustive key search
for the first 40 bits of a CARLOS key and then performa separate exhaustive
key search for the last 8 bits. How long do you nowexpect to take to recover
a CARLOS key by means of an exhaustive key search?
16
. The following table specifies a cryptosystem based around a very simple
encryption algorithm with four different plaintexts A, B, C and D (one
corresponding to each column) and four different ciphertexts
A
,
B
,
C
and
D
. The encryption algorithm has five different keys
K
1
,
K
2
,
K
3
,
K
4
,
K
5
(one
corresponding to each row). By writing
E
K
(P)=
C
to mean that the encryption
