Key Range and Key Size

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