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authorChristian Grothoff <christian@grothoff.org>2020-11-22 13:32:28 +0100
committerChristian Grothoff <christian@grothoff.org>2020-11-22 13:32:28 +0100
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document exchange crypto helper design
Diffstat (limited to 'design-documents')
-rw-r--r--design-documents/009-backup.rst2
-rw-r--r--design-documents/010-exchange-helpers.rst128
-rw-r--r--design-documents/index.rst3
3 files changed, 131 insertions, 2 deletions
diff --git a/design-documents/009-backup.rst b/design-documents/009-backup.rst
index 0f2a434..04ae628 100644
--- a/design-documents/009-backup.rst
+++ b/design-documents/009-backup.rst
@@ -133,7 +133,7 @@ Open Questions
.. Note::
CG would definitively solve this using a more complex format for the **master secret**,
- basically serializing mutliple **root secret** values with meta data
+ basically serializing multiple **root secret** values with meta data
(which wallet/device/name).
diff --git a/design-documents/010-exchange-helpers.rst b/design-documents/010-exchange-helpers.rst
new file mode 100644
index 0000000..cf7a4df
--- /dev/null
+++ b/design-documents/010-exchange-helpers.rst
@@ -0,0 +1,128 @@
+Exchange crypto helper design
+#############################
+
+Summary
+=======
+
+A way to minimize the attack surface for extraction of the private online
+signing keys (RSA and EdDSA) from the exchange is described.
+
+
+Motivation
+==========
+
+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, Postgres 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.
+
+
+Requirements
+============
+
+* The solution should not result in a dramatic loss of performance.
+* An attacker with a successful arbitrary code execution on the exchange
+ must not be able to extract the private keys.
+* Ideally, we should be able to determine the number of signatures
+ obtained illicitly by the attacker.
+* Key management for operators should be simplified to improve usability.
+* Both RSA and EdDSA online signing keys need to be protected.
+
+
+Proposed Solution
+=================
+
+The private keys are to be created, used and deleted by two helper processes
+running under a different user ID (UID). The exchange's HTTP process will be
+required to interact with those helpers via a UNIX domain socket.
+
+Specific design details:
+
+* The helpers will process requests from the exchange to sign and revoke keys.
+* The helpers will tell the exchange when keys are created or deleted/expired.
+* 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 exchange will expose the corresponding public keys via a ``/keys?future``
+ endpoint to the auditor and the offline signing process. Auditor and master
+ signatures will be POSTed to the exchange via the ``/keys`` endpoint.
+ The exchange will keep those signatures in the Postgres database.
+* 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
+ timeout.
+* 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 ``eventfd()``.
+* The master thread is responsible for handling revocations, creating future
+ private keys and expiring old keys.
+
+
+Alternatives
+============
+
+* The helpers could have been given the information to validate the signing
+ request. However, without database access, validating the reserve key
+ signature (and others) is pretty useless. Thus, this direction would only
+ complicate the helper (which we want to keep minimal to minimize attack
+ surface) without real benefits. Even validating revocation requests (checking
+ signatures by auditor or master public key) makes no sense, as if an attacker
+ triggers a revocation, we should probably be thankful: That's a white-hat
+ demonstrating that they got control in the least harmful way.
+* Instead of two helpers, we could have just one helper. But there is limited
+ overlap between the (RSA) denomination key logic and the (EdDSA) signing
+ key logic. Separation may improve security.
+* We could have proposed a helper per denomination. But as the code of all of
+ these helpers would be identical, this would have no security advantages.
+* We could have implemented our own event loop and configuration parser,
+ instead of relying on libgnunetutil. But this part of GNUnet is very
+ robust.
+* We could have had a thread pool reading requests from the exchange clients,
+ instead of a master thread doling out the work. But this would become really
+ complicted with key revocations, and as really only the cryptography should
+ be the bottleneck, performance advantages should be minimal. If IPC ever
+ becomes the issue, then the entire idea of moving signatures to another
+ process would be flawed.
+* More portable mechanisms (like a ``pipe()``) could be used for signaling
+ instead of ``eventfd()``. But, this can always be implemented if we truly
+ ever have an exchange operator needing support for such a platform.
+* We could have left the helper single-threaded, to avoid the complications
+ arising from the use of threads. However, given that signing is expected to
+ be a bottleneck of the exchange, this would have had serious performance
+ implications for the entire system.
+
+
+Drawbacks
+=========
+
+* Additional work to properly setup an exchange and to run
+ our automated tests.
+* Slight (?) performance impact.
+* UNIX only. Likely Linux-only for now (but fixable).
+* If exchange receives ANY (not ignored) signal during signing
+ operation, a discrepancy in the number of signatures created
+ between exchange (DB) and the helper will arise. Thus,
+ auditors have to allow for small discrepancies (increasing
+ over time). Note that we only expect the exchange to receive
+ signals if the software is updated or the process is terminated.
+* If helper is stopped (SIGSTOP), exchange HTTP will itself block
+ (no timeout!). Timeout-based mitigation would additionally increase
+ discrepancies in the count of the number of signatures created.
+
+
+Discussion / Q&A
+================
+
+(This should be filled in with results from discussions on mailing lists / personal communication.)
diff --git a/design-documents/index.rst b/design-documents/index.rst
index 55dd8c1..6b66dd2 100644
--- a/design-documents/index.rst
+++ b/design-documents/index.rst
@@ -10,7 +10,7 @@ and protocol.
:glob:
000-template
- 001-new-browser-integration
+ 001-new-browser-integration
002-wallet-exchange-management
003-tos-rendering
004-wallet-withdrawal-flow
@@ -19,3 +19,4 @@ and protocol.
007-payment
008-fees
009-backup
+ 010-exchange-helpers