[Dovecot] SIS Implementation

[WJCarpenter]

> Step 4) Figure out if base64-encoded attachments can be decoded in a way
> that allows re-encoding them back to the exact original encoding. If so,
> save the attachment decoded and add the necessary encoding info the dbox
> metadata.
>   

Although you might like to do that for some sort of tidiness or 
whatever, I don't think there's an actual requirement to restore it to 
base64 (or q-p or whatever).  Those are just transfer encodings, and 
intermediate MTAs might in theory have transformed them (though in 
practice that's doubtful).  The one use case for an exact reconstruction 
might be some older digital signature schemes (which were not so robust 
and probably lost if an intermediate MTA messed with things).


> Or perhaps just store them compressed. How much of a difference is there
> between storing decoded data vs. compressed base64 encoded data?
>   

The best you can ever do is break even, and often you will lose.  In any 
case, you have the CPU/memory cost of the compress/decompress.  I would 
get rid of any transfer encoding and try to compress.  If the 
compression was above some threshold of storage benefit, store it 
compressed.  Otherwise, store it uncompressed.