SPRUJB6B November 2024 – May 2025 AM2612
Figure 7-111 GCM Protocol OperationA GCM protocol operation is a combined operation, consisting of encryption/decryption and authentication. A part of the input data steam can be authenticated only, while normally most of the input data is encrypted/decrypted and authenticated. The authentication only data always must be in front of the data that requires encryption. Within GCM, the authentication only data is called the AAD (Additional Authentication Data). The AAD is fetched independently of the other data.
Figure 7-111 illustrates the initial authentication steps of the AAD data. The intermediate (temp) result data is used as input for the remaining authentication operation. Because the authentication operation does not require the crypto core but only the polynomial multiplication, both encryption/decryption and authentication can be performed in parallel. After encryption of the last data block, an additional polynomial multiplication and encryption are required to respectively authenticate a 128-bit length vector and encrypt the authentication result.