Symmetric Cryptographic Intel FPGA Hard IP User Guide

ID 714305
Date 12/19/2022
Public

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5.5.2. GCM Depadding

This block implements the idle byte depadding to the AAD, Bypass, Text, and MAC fields which are 16 bytes aligned when "Enable Gen. GCM egress depadding" is selected in the IP GUI. You are required to send in the real AAD and Bypass field lengths, regardless of whether the GCM padding on ingress is enabled or disabled. When GCM padding on ingress is disabled, the AAD and Bypass fields are 16 byte-aligned, and the IP routes the real AAD and Bypass lengths to Crypto egress logic for depadding.

The egress decryption MAC is dropped if "Drop the MAC on applicable profile decryption" on the IP GUI is also checked.

When "Enable Gen. GCM egress depadding" is not selected in the IP GUI, the depadding logic within the IP is optimized away in order to save device resources. In this case, the egress data still passes through the ICV and MAC packing blocks.
Figure 16. GCM Depadding
Note: The MAC(16B) in the dash-line box indicates that the MAC is removed from the decryption packets when "Drop the MAC on applicable profile decryption" is enabled on the IP GUI.