Things To Be Careful About As We Venture Boldly Forth
See also Known Issues.
Asymmetric-key cryptography operations are particularly sensitive to side-channel attacks. Unless the library is carefully hardened against timing attacks, it is dangerous to allow an attacker to measure how long signature and pubkey-derivation operations take. With enough samples, the attacker can deduce the private signing key from these measurements. (Note that verification operations are only sensitive if the verifying key is secret, which is not the case for anything in Tahoe).
We currently use private-key operations in mutable-file writes, and anticipate using them in signed-introducer announcements and accounting setup.
Mutable-file writes can reveal timing information to the attacker because the signature operation takes place in the middle of a read-modify-write cycle. Modifying a directory requires downloading the old contents of the mutable file, modifying the contents, signing the new contents, then uploading the new contents. By observing the elapsed time between the receipt of the last packet for the download, and the emission of the first packet of the upload, the attacker will learn information about how long the signature took. The attacker might ensure that they run one of the servers, and delay responding to the download request so that their packet is the last one needed by the client. They might also manage to be the first server to which a new upload packet is sent. This attack gives the adversary timing information about one signature operation per mutable-file write. Note that the UCWE automatic-retry response (used by default in directory modification code) can cause multiple mutable-file read-modify-write cycles per user-triggered operation, giving the adversary a slightly higher multiplier.
The signed-introducer announcement involves a signature made as the client node is booting, before the first connection is established to the Introducer. This might reveal timing information if any information is revealed about the client’s exact boot time: the signature operation starts a fixed number of cycles after node startup, and the first packet to the Introducer is sent a fixed number of cycles after the signature is made. An adversary who can compare the node boot time against the transmission time of the first packet will learn information about the signature operation, one measurement per reboot. We currently do not provide boot-time information in Introducer messages or other client-to-server data.
In general, we are not worried about these leakages, because timing-channel attacks typically require thousands or millions of measurements to detect the (presumably) small timing variations exposed by our asymmetric crypto operations, which would require thousands of mutable-file writes or thousands of reboots to be of use to the adversary. However, future authors should take care to not make changes that could provide additional information to attackers.