instead of two different buffers, there is just one buffer. Based upon the verification result, a mask is applied to the buffer before it is written to the output buffer.
Create two buffers of the same size as the input data.
Copy the input data to the first one and work with that version to hold the
decrypted data, zeroize the second one.
Copy depending on the verification result, either the zero-buffer or the
real plaintext to the output buffer.
The API of the function is changed (for decryption, tag is now an input
parameter). With the old API it is impossible to confirm to the NIST
specification and a timing sidechannel leak is inevitable.
