Question

When the length of the IV is not 96, then

(a) we append the rest of the bits with 1*0 to make it 128 bits

(b) we append the rest of the bits with 0*1 to make it 128 bits

(c) we append the rest of the bits with 1*0 to make it 256 bits

(d) we append the rest of the bits with 0s to make it 128 bits

This question was addressed to me in examination.

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

Correct answer is (d) we append the rest of the bits with 0s to make it 128 bits

To explain I would say: If the length of the IV is not 96 then we append the rest of the bits with 0s to make it 128 bits. This is the first step followed by appending 64 bits furthermore and then performing the GHASH function.