# Tag Info

13

Contrary to what Stephen says, you absolutely can compute the tag in parallel. Here's how it works; the tag computation is essentially "assemble the AAD, data, the length field and $Encr(Nonce)$ into a series of values $x_n, x_{n-1}, x_{n-2}, ..., x_0$", and then "compute the polynomial $x_nh^n + x_{n-1}h^{n-1} + x_{n-2}h^{n-2} + ... + x_0h^0$ This ...

5

The GCM flowchart on Wikipedia and my intuition support the notion that some of the GCM work can be done in parallel. At the very least you can do each $E_k(ctr)$ operation in parallel, but it doesn't look like you should be able to parallelize the authentication, as each $mult_H$ requires the output of a previous call as its input. Edit: poncho explains why ...

1

According to Handbook of Applied Cryptography (15.3.2, ii), ANSI X9.9 (which SEJPM mentioned in the comments but I have no access to) defined CFB-MAC only as a compatible alternative to CBC-MAC: The X9.9 MAC algorithm may be implemented using either the cipher-block chaining (CBC) or 64-bit cipher feedback (CFB-64) mode, initialized to produce the same ...

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