avoid caching plaintext in gateway to satisfy range requests

[davidsarah]

Jeremy Fitzhardinge wrote:

In poking around with http range requests, I noticed that the web gateway will (on occasion) locally cache files in unencrypted form.

Now in normal use that's perfectly OK because web gateways are trusted with our unencrypted data and so having the data present in that form should be OK.

But my mental model of a gateway machine is that it's just a stateless waypoint which doesn't store anything local. If I have a setup where there's a gateway machine within my network serving several machines, I would expect it to not have any persistent memory of my data, and so when it comes time to replace the HD I don't need to worry about scrubbing the disk, at least for Tahoe's sake. (Let's assume swap has been dealt with.)

Therefore, I think the gateway should keep the cache files encrypted and only decrypt them on the fly as they're being sent to its clients. I'm not sure what the key should be, but it should be per-file and transient (derived from the cap/root hash/something else?) rather than some local state (which would defeat the purpose of encrypting in the first place).

Yes, there are three options that would fix this:

1. Use encrypted temporary files as suggested above
2. Stop using temporary files
3. Securely overwrite temporary files before closing them

(2. may require too much memory. 3. may leave plaintext accessible if the gateway crashes.)

Note that the SFTP and FTP frontends also use unencrypted temporary files to handle write requests (perhaps that should be split into a separate ticket, but this one will do for the time being).

[davidsarah]

Duplicate of #990.