Question

The Chi function is defined as

(a) a[x] ← a[x] XOR ( a[x + 1]XOR 1)  AND a[x+2])

(b) a[x] ← a[x] AND ( a[x + 1]XOR 1)  AND a[x+2])

(c) a[x] ← a[x] AND ( a[x + 1] XOR a[x+2])

(d) a[x] ← a[x] XOR ( a[x + 1]  AND a[x+2])

This question was addressed to me in final exam.

My doubt stems from Secure Hash Algorithms (SHA) topic in section Other Public Key Cryptosystems and Cryptographic Hash Functions of Cryptograph & Network Security

Answer 1

Correct option is (a) a[x] ← a[x] XOR ( a[x + 1]XOR 1)  AND a[x+2])

Easiest explanation: The Chi function is defined as a[x] ← a[x] XOR ( a[x + 1]XOR 1)  AND a[x+2]).