CODEWITHSUNDEEP
encryptioncryptographyaes
Darsana
Darsana

Reputation: 11

I am doing AES-GCM algorithm and I got stuck in one step. Can somebody help me out?

In the test vectors of AES-GCM, (I have attached the pdf here), in test vectors, 1,2 and 3, the value of A is not given. The link is: https://pdfs.semanticscholar.org/114a/4222c53f1a6879f1a77f1bae2fc0f8f55348.pdf

But len(A) || len(C) = 00000000000000000000000000000080 C = 0388dace60b6a392f328c2b971b2fe78. So len(C) = 32, right?? Can somebody help me, how to get the value of A and its length. I am doing this algorithm, AES-GCM. Can somebody help me to solve this issue??

Upvotes: 1

Views: 97

Answers (1)

bartonjs
bartonjs

Reputation: 33128

The GCM paper, in section 2.2 says

The function len() returns a 64-bit string containing the nonnegative integer describing the num­ber of bits in its argument, with the least significant bit on the right.

The C value in test case 2 is 32 hex characters, or 16 bytes, or 128 (0x80) bits.

len(C) produces as 64-bit value, which in big-endian hexadecimal is 00 00 00 00 00 00 00 80. The upper half of the 128-bit value is len(A). It's all zeros, so len(A) is zero, which matches A being absent. This is corroborated by the intro text to the test vectors section:

All values are in hexadecimal, and a zero-length variable is indicated by the absence of any hex digits.

(emphasis mine).

Upvotes: 2

Related Questions