Key Range and Key Size:
The encrypted message can be
attacked and the crypt analyst may have the following information:
1. The
Encryption Decryption Algorithm
2. The
Encrypted Message
3. Key
The attack may be in terms of
following types:
a. Plain text
only attack (Known plain text attack)
b. Cipher text
only attack (Known cipher text attack)
c. Chosen
plain text attack
d. Chosen
cipher text attack
The simplest type of attack is
brute force attack in which all types of substitution techniques are used to
fetch original message. A Brute force attack works on a principle of trying
every possible key from the key range. Key range is different concept from key
size. A key range may contain individual single arbitrary quantity whereas key
size defines the total or maximum capacity of all the keys.
Exhaustive key search:
It is basically used by the side of
cryptanalyst. Basically the procedure for exhaustive key search becomes more
complex as the key size that means number of bits are increased. The time
required for single encryption message and entire message would be
automatically increased.
All encryption algorithm are having
two main criteria for encryption–
I.
The cost of breaking the cipher exceeds the value of
encryption information.
II.
The time required to break the cipher exceeds the
useful timeline of the information.
An encryption scheme is said to be comparatively
secure if the above two criteria are met.
Average time required for
Exhaustive key search –
Key Size (bits)
|
Number of alternative keys
|
Time required at 1 cryptanalysis/µs
|
Time required at 106
cryptanalysis/µs
|
32
|
232 =4.3*109
|
231 µs
= 35.8 minutes
|
2.15
milliseconds
|
56
|
256 =7.2*1016
|
255 µs
= 1142 years
|
10.01 hours
|
128
|
2128 =3.4*1038
|
2127 µs
= 5.4*1024 years
|
5.4*1018
years
|
0 comments:
Post a Comment