Question

In GCM, we define S as S = GHASH(A||0^v||C||0^u||[len(A)]||[len(C)]). What is u defined as?

(a) 64 ceil[len(A)/128] – len(A)

(b) 64 ceil[len(C)/128] – len(C)

(c) 128 ceil[len(A)/128] – len(A)

(d) 128 ceil[len(C)/128] – len(C)

This question was posed to me in an internship interview.

I'm obligated to ask this question of CCM, GCM and Key Wrapping in division Message Authentication Codes, Whirlpool Algorithm and Key Management and Distribution of Cryptograph & Network Security

Answer 1

Correct option is (d) 128 ceil[len(C)/128] – len(C)

The best explanation: u = 128 ceil[len(C)/128] – len(C).