AES-GCM Functions

The Galois/Counter Mode (GCM) is a mode of operation of the AES algorithm. GCM [NIST SP 800-38D] uses a variation of the Counter mode of operation for encryption. GCM assures authenticity of the confidential data (of up to about 64 GB per invocation) using a universal hash function defined over a binary finite field (the Galois field).

GCM can also provide authentication assurance for additional data (of practically unlimited length per invocation) that is not encrypted. If the GCM input contains only data that is not to be encrypted, the resulting specialization of GCM, called GMAC, is simply an authentication mode for the input data.

GCM provides stronger authentication assurance than a (non-cryptographic) checksum or error detecting code. In particular, GCM can detect both accidental modifications of the data and intentional, unauthorized modifications.

Table “Intel IPP AES-GCM Functions” lists Intel IPP AES-GCM functions:

Intel IPP AES-GCM Functions
Function Base Name	Operation
Rijndael128GCMEncryptMessage	Encrypts an entire message and generates its authentication tag in the GCM mode.
Rijndael128GCMDecryptMessage	Decrypts an entire message and generates its authentication tag in the GCM mode.
Rijndael128GCMGetSize	DEPRECATED. Use Rijndael128GCMGetSizeManaged instead of this function. Gets the size of the IppsRijndael128GCMState context.
Rijndael128GCMGetSizeManaged	Gets the size of the IppsRijndael128GCMState context for use of the AES-GCM implementation that meets specified requirements.
Rijndael128GCMInit	DEPRECATED. Use Rijndael128GCMInitManaged instead of this function. Initializes user-supplied memory as the IppsRijndael128GCMState context for future use.
Rijndael128GCMInitManaged	Initializes user-supplied memory as the IppsRijndael128GCMState context for use of the AES-GCM implementation that meets specified requirements.
Rijndael128GCMStart	Starts the process of authenticated encryption/decryption for a new message.
Rijndael128GCMReset	Resets the IppsRijndael128GCMState context for authenticated encryption/decryption of a new message.
Rijndael128GCMProcessIV	Processes an initial vector of a given length according to the GCM specification.
Rijndael128GCMProcessAAD	Processes additional authentication data of a given length according to the GCM specification.
Rijndael128GCMEncrypt	Encrypts a data buffer in the GCM mode.
Rijndael128GCMDecrypt	Decrypts a data buffer in the GCM mode.
Rijndael128GCMGetTag	Generates the message authentication tag in the GCM mode.

The AES-GCM function set includes:

Functions for complete message operations, to be used for authenticated encryption/decryption of an entire message, if available: Rijndael128GCMEncryptMessage and Rijndael128GCMDecryptMessage.
Incremental functions, to be used if the message is too long to be processed in one step.

The AES-GCM incremental functions enable authenticated encryption/decryption of several messages using one key that the Rijndael128GCMInit function sets. The application code for conducting a typical AES-GCM authenticated encryption should follow the sequence of operations as outlined below:

Get the size required to configure the context IppsRijndael128GCMState by calling the function Rijndael128GCMGetSizeManaged.
Call the system memory-allocation service function to allocate a buffer whose size is not less than the function Rijndael128GCMGetSize specifies.
Initialize the context IppsRijndael128GCMState *pCtx by calling the function Rijndael128GCMInitManaged with the allocated buffer and the respective AES key.
Call Rijndael128GCMStart to start authenticated encryption of the first/next message.
Keep calling Rijndael128GCMEncrypt until the entire message is processed.
Request the authentication tag by calling Rijndael128GCMGetTag.
Proceed to the next message, if any, that is, go to step 4.
Call the system memory free service function to release the buffer allocated for the context IppsRijndael128GCMState, if needed.

If the size of the initial vector and/or additional authenticated data (IV and AAD parameters of the Rijndael128GCMStart function, respectively) is large or any of these parameters is placed in a disconnected memory buffer, replace step 4 above with the following sequence:

Call Rijndael128GCMReset to prepare the IppsRijndael128GCMState context for authenticated encryption of the first/new message.
Keep calling Rijndael128GCMProcessIV for successive parts of IV until the entire IV is processed.
Keep calling Rijndael128GCMProcessAAD for successive parts of AAD until the entire AAD is processed.