We want to provide an additional layer of protection for the private online
signing keys used by the exchange. The exchange is network-facing, includes an
HTTP server, PostgreSQL interaction, JSON parser and quite a bit of other logic
which may all be theoretically vulnerable to remote exploitation. Thus, it
would be good from a security perspective to protect the private online
signing keys via an additional layer of protection.
14.10.4. Proposed Solution
The private keys are to be created, used and deleted by two helper processes
running under a different user ID (UID), creating in effect a software
security module. The exchange’s HTTP process will be required to interact
with those helpers via a UNIX domain socket.
Socket permission details:
- The socket will be chmod 0620 (u+rw, g+w) regardless of umask.
- That the group is the same group of the crypto helpers must
still be ensured by the operator.
General design details:
- The helpers will process requests from the exchange to sign and revoke keys.
- The helpers will create and destroy the private keys. They will no longer be
created on the air-gapped machine with the (offline) master private key.
The helpers will tell the exchange when keys are created or deleted/expired.
- Each helper will sign freshly generated keys with a security module-specific
private key. This key will be verified by the offline signing key process
using either manual verification against log output from the security
module’s start-up routine, or via TOFU. TOFU is considered sufficient,
as an adversary breaking into the exchange process during the initial setup,
when the exchange is not even yet operational because no keys have ever been
provisioned, is considered highly unlikely. Depending on how the exchange
is initialized, access to security module logs may or may not be feasible,
so TOFU is a good and usable alternative strategy.
Helper design details:
- SOCK_DGRAM will be used to avoid needing to parse a data stream.
- The helpers will only know about (private) key lifetime. They will not know about
details like currency, fee structure, master or auditor signatures.
Those will be managed by the HTTP process to keep the helpers minimal.
- The helpers will use a single-threaded, GNUnet-scheduler-driven event loop
to process incoming requests from the UNIX domain sockets. However, the
actual signing will be done by a thread pool of workers that only process
signing requests from a work queue. Reference counting is used to avoid
releasing private keys while workers are actively using them to sign requests.
- The work queue is managed via a pthread-style semaphore.
- The master thread is informed about completed work via an
- The master thread is responsible for handling revocations, creating future
private keys and expiring old keys. Revocations will also be triggered
via a new
/keys endpoint. The HTTP server will verify that the revocation
is properly signed with the master private key before passing it on to the
Exchange design considerations:
- The helpers are started by the system, say via systemd, not by the
exchange. This simplifies the exchange, and we already needed the
exchange operator to start four processes to operate an exchange.
So this number simply increases to six (not even counting the
PostgreSQL database and a reverse HTTP proxy for TLS termination).
- Each exchange thread will create its own connection to the helpers, and will
block while waiting on the helper to create a signature. This keeps the
exchange logic simple and similar to the existing in-line signing calls.
Suspending and resuming would be difficult as we currently do not have a
way to wait for a UNIX domain socket to resume the MHD logic.
If a signal is received while waiting for the helper, the signature operation
fails. Signature operations can also fail if the helper is not running or
responding with incorrect data. However, signature operations do NOT have a
New exchange endpoints:
- The exchange will expose the corresponding public keys via a GET to
/keys/future endpoint to the offline signing process. For offline
signing, tooling will be provided to first download to a file, then
sign based on that file, and then upload the resulting signature back to
the exchange. For this, master signatures will be POSTed to
the exchange to the
The exchange will keep those signatures in the PostgreSQL database.
- A new endpoint (
/auditors) will also allow adding/removing auditors
(POST, DELETE) using requests signed with the offline master private key.
Once an auditor has been added, the respective auditor signatures on exchange
keys can also be POSTed to the REST API at
Overall, the result is that except for software updates and the fundamental
taler-exchange-http will be updated only via HTTP(S)
and not via a signal and new files appearing in the directory hierarchy.
All of the more volatile state of the HTTP process will be in the database.
Only the helpers continue to keep files on disk.