Question

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

I have been asked this question during an online exam.

This intriguing question originated from CCM, GCM and Key Wrapping in portion Message Authentication Codes, Whirlpool Algorithm and Key Management and Distribution of Cryptograph & Network Security

Answer 1

Right answer is (c) 128 ceil[len (A)/128] – len (A)

Best explanation: v = 128 ceil[len (A)/128] – len (A).