AES GCM Difference between the IV and the Secret Key?

user997112

2/15/24, 1:57 PM

Below is part of AES GCM diagram. However, it only shows the behavior of the IV/counter.

The GCM specification examples state both an IV and a Secret Key as two inputs.

Can someone please explain where both are used?

Is the 96 bit IV expanded to 128 bits, incremented (most-significant byte?) and used to create the AES Key Expansion?

And the Secret Key is passed (with the Key Expansion) to E_k for encryption?

1 + 0

aes

aes-gcm

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Crypto

poncho

2/15/24, 1:57 PM

And the Secret Key is passed (with the Key Expansion) to E_k for encryption?

Yes, the key is given to the block cipher - it is indeed the $k$ in $E_k$

+ 5

user997112

2/15/24, 2:03 PM

Awesome, thanks! And am I right the 96 bit IV is expanded to 128 bits at the beginning and then the most significant byte is incremented, before then being used to generate the expansion?

poncho

2/15/24, 2:31 PM

@user997112: actually, with 96 bit IVs (GCM supports other IV sizes; those are rarely used), the IV is used as the top 96 bits (counting bits in bigendian order) of the AES input; the lower 32 bits is a counter (which is 1 for generating the mask for the tag; 2 for encrypting the first plaintext block; 3 for the second, etc). Hence, it is the least significant byte which is incremented (again, counting 'least' in bigendian order)

user997112

2/15/24, 2:57 PM

Lets say our IV is all zeros. This value is only used for the hashing (as you mentioned). The IV is then incremented. Does `iv[0]` contain 1, or is it `iv[15]`?

poncho

2/15/24, 4:15 PM

@user997112: well, for IV - 96 0 bits, the value given to AES to generate the tag mask has iv as all zeros except iv[15] = 1.

user997112

2/15/24, 6:49 PM

I've asked the problem in a new question, if you could take a glance: https://crypto.stackexchange.com/questions/104243/implementing-aes-gcm-but-not-getting-correct-output-from-cipher-block

Elon Musk

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EN: AES GCM Difference between the IV and the Secret Key?

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