Question

What is the effectiveness of a 128 bit hash value?

(a) 2^-64

(b) 2^64

(c) 2^-112

(d) 2^-128

This question was addressed to me in unit test.

This intriguing question comes from Hash Functions Based on Cipher Block Chaining in section Other Public Key Cryptosystems and Cryptographic Hash Functions of Cryptograph & Network Security

Answer 1

Correct choice is (c) 2^-112

Explanation: In most normal text files, the high order bit of each octet is always zero. So if a 128 bit hash value is used, instead of an effectiveness of 2^-128, the hash function will have an effectiveness of 2^-112.