SLAU723A October 2017 – October 2018 MSP432E401Y , MSP432E411Y
A GCM protocol operation is a combined operation consisting of encryption or decryption and authentication. A part of the input data stream can be authenticated only, while normally most of the input data is encrypted/decrypted and authenticated. The authentication-only data must always be in front of the data requiring encryption. Within GCM, the authentication-only data is called the additional authentication data (AAD). The AAD is fetched independently of other data.
The intermediate (temporary) result data is used as input to the remaining authentication operation. Because the authentication operation does not require the cryptographic core but only the polynomial multiplication, encryption or decryption and authentication can be performed in parallel. After encryption of the last data block, additional polynomial multiplication and encryption are required to authenticate a 128-bit length vector and finally encrypt the authentication result.