AES_GCM in chunks bug?

[keean]

How do I use AES_GCM on a stream? I have tried splitting into chunks, and calling 'process' for each chunk, but when I eventually call 'finish' I get a security error as the tag is not correct. However if I put all the data in a single 'process' call it works. Do I need to do something special between calls to process, or it this a bug?