func (g *gcm) update(y *gcmFieldElement, data []byte) {
	fullBlocks := (len(data) >> 4) << 4
	g.updateBlocks(y, data[:fullBlocks])

	if len(data) != fullBlocks {
		var partialBlock [gcmBlockSize]byte
		copy(partialBlock[:], data[fullBlocks:])
		g.updateBlocks(y, partialBlock[:])
	}
}

// gcmInc32 treats the final four bytes of counterBlock as a big-endian value
// and increments it.
func gcmInc32(counterBlock *[16]byte) {