Symmetric Cryptographic Intel FPGA Hard IP User Guide

ID 714305
Date 10/02/2023
Public
Document Table of Contents

5.5. GCM Padding and Depadding

Crypto QHIP supports AAD/Bypass/Text data padding and depadding feature when using the Generic GCM profile. This feature allows the AAD and Bypass data sent by you in the Crypto ingress direction to be padded in non-16 byte alignment within the payload. In the Crypto egress direction, the AAD, Bypass, Text, and MAC from the Crypto ICA HIP are aligned to 16 bytes. The GCM depadding logic unpacks these 16 byte-aligned fields in the QHIP to non-16 byte alignment and then sends them to your application via the AXI-ST bus. You can enable or disable the padding at ingress or depadding at egress separately via dialog check boxes in the IP GUI. When a feature is disabled, the IP optimizes all the padding or depadding logic away.

Figure 14. GCM Padding and Depadding
Note:

The length in Bypass, AAD, or Text field is for the last segment without counting the possible leading segments of 16-byte data.

Note: The MAC(16B) in the egress interface can disappear for decryption packets if the "Drop the MAC on applicable profile decryption" is enabled.