# Post processing of AAD and len A||C in a hardware AES GCM implementation

I'm new to GCM and I need to implement it in hardware, using FPGA. The data bus is 640 bits, so I will use 5 adder/multiplier blocks in parallel. The message size and AAD size are constant. My design have a particularity that AAD and Len A||C are available only at the end of message and I don't kow how to process it.

A little example: message = 5 x 640 bits.

cycle 1: process 640 bits (add/multiply)
cycle 2: process 640 bits (add/multiply)
cycle 3: process 640 bits (add/multiply)
cycle 4: process 640 bits (add/multiply)
cycle 5: process 640 bits (add/multiply) and xor all results
cycle 7: process Len A||C


After cycle 5 I have a 128 bits vector with all the messages added/multiplied and results xored. So, how can I process AAD and Len A||C in this scenario?

Thanks Rodolfo

BouncyCastle (for Java) has code that performs the AAD processing anywhere in the stream. It does however require modular exponentiation and additional multiplication. GCM mode officially requires the AAD to be processed before the plaintext, but as stated, there is a way around that. I've asked for an explanation here

Len A||C is only required at the end of the processing anyway, so that should not pose a problem.

• Thanks Maarten. So, If I understand well, I need to multiply the hashed data (end of cycle 5) by H, multiply Len A||C by H and multiply AAD by H^2 and xor the results. Is it correct? Thanks
– Rods
Commented Jan 3, 2015 at 12:22
• I'm not sure the above is correct. $+$ is addition and $H^2$ should be $H^c$ I think (modulo calculations, of course). Commented Jan 3, 2015 at 14:26
• @MaartenBodewes-owlstead: in $GF(2^{128})$, $+$ is the same as xor, so Rods has that part correct. Commented Jan 3, 2015 at 14:55
• @poncho Thanks, that saved me looking at the implementation, I was already grabbing the code. I really should be more mathematically minded, but it is tricky to do this after so many years of just applied crypto & development. Commented Jan 3, 2015 at 14:58
• I didn't get the H^c. Is C the related to the message data? Like, C is the number of 128 bits data blocks? Sorry for the basic questions guys. Just to clarify, my implementation is based on this architecture, but with AAD and Len A||C processed separately from data: ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5619894
– Rods
Commented Jan 3, 2015 at 15:48