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+\title{Final response to the \\ GNU Taler security audit in Q2/Q3 2020}
+\author{Christian Grothoff \and Florian Dold}
+This is the response to the source code audit report CodeBlau
+created for GNU Taler in Q2/Q3 2020.
+\section{Management Summary}
+We thank CodeBlau for their detailed report and thorough analysis. We are
+particularly impressed that they reported issues against components that were
+not even in-scope, and also that they found an {\em interesting} new corner
+case we had not previously considered. Finally, we also find several of their
+architectural recommendations to strengthen security to be worthwhile, and
+while some were already on our long-term roadmap, we will reprioritize our
+roadmap given their recommendations.
+Given our extensive discussions with CodeBlau, we also have the impression
+that they really took the time to understand the system, and look forward
+to working with CodeBlau as a competent auditor for GNU Taler in the future.
+\section{Issues in the exchange}
+We agree with the issues CodeBlau discovered and both parties believe that
+they have all been addressed.
+\section{Issues in the auditor}
+We appreciate CodeBlau's extensive list of checks the Taler auditor performs,
+which was previously not documented adequately by us. We agree that the
+auditor still needs more comprehensive documentation.
+As for issue \#6416, we agree with the analysis. However, the proposed fix
+of making the primary key include the denomination would create other problems,
+such as the exchange sometimes not having the denomination key (link, refund)
+and the code in various places relying on the assumption of the coin's
+public key being unique. Furthermore, allowing coin key re-use may validate
+a terrible practice. We thus decided it is better to ``fail early'', and
+modified the code to check that the coin public key is ``unique'' during
+deposit, refresh and recoup and ensured that the exchange returns a proof
+of non-uniqueness in case of a violation. The test suite was extended to
+cover the corner case.
+{\bf Update:} We have now also addressed the (``soft'') exchange online
+signing key revocation issue (\#6161) reported originally by CodeBlau.
+The auditor now checks for key revocations before recording deposit
+confirmations. The impact is very minor, as this will merely prevent
+an adversary controlling an exchange online signing key from submitting
+false claims to the auditor.
+\section{Issues in GNUnet}
+We agree with the issues CodeBlau discovered and both parties believe that
+they have all been addressed.
+\section{General remarks on the code}
+We understand that writing the code in another programming language may make
+certain checks for the auditor less work to implement. However, our choice of C
+is based on the advantages that make it superior to contemporary languages for
+our use case: relatively low complexity of the language (compared to C++);
+availability of mature compilers, static and dynamic analysis tools;
+predictable performance; access to stable and battle-tested libraries; and
+future-proofness due to portability to older systems as well as new platforms.
+We believe creating a parallel implementation in other languages would provide
+advantages, especially with respect to avoiding ``the implementation is the
+specification''-style issues. However, given limited resources will not make
+this a priority.
+We disagree that all modern software development has embraced the idea that
+memory errors are to be handled in ways other than terminating or restarting
+the process. Many programming languages (Erlang, Java) hardly offer any other
+means of handling out-of-memory situations than to terminate the process. We
+also insist that Taler {\em does} handle out-of-memory as it does have code
+that terminates the process (we do {\em not} simply ignore the return value
+from {\tt malloc()} or other allocation functions!). We simply consider that
+terminating the process (which is run by a hypervisor that will restart the
+service) is the correct way to handle out-of-memory situations. We also have
+limits in place that should prevent attackers from causing large amounts of
+memory to be consumed, and also have code to automatically preemptively
+restart the process to guard against memory exhaustion from memory
+fragmentation. Finally, a common problem with abrupt termination may be
+corrupted files. However, the code mostly only reads from files and limits
+writing to the Postgres database. Hence, there is no possibility of corrupt
+files being left behind even in the case of abnormal termination.
+\section{More specs and documentation code}
+We agree with the recommendation that the documentation should be improved,
+and will try to improve it along the lines recommended by CodeBlau.
+\section{Protocol change: API for uniformly distributed seeds}
+We agree with the suggestion, have made the necessary changes, and both
+parties believe that the suggestion has been implemented.
+\section{Reduce code complexity}
+\subsection{Reduce global variables}
+While we do not disagree with the general goal to have few global variables,
+we also believe that there are cases where global variables make sense.
+We have already tried to minimize the scope of variables. The remaining few
+global variables are largely ``read-only'' configuration data. The report does
+not point out specific instances that would be particularly beneficial to
+eliminate. As we continue to work on the code, we will of course evaluate
+whether the removal of a particular global variable would make the code
+Also, we want to point out that all global variables we introduce
+in the exchange are indicated with a prefix {\tt TEH\_} in the code, so they
+are easy to identify as such.
+\subsection{Callbacks, type pruning}
+We understand that higher order functions in C can be confusing, but this
+is also a common pattern to enable code re-use and asynchronous execution
+which is essential for network applications. We do not believe that we
+use callbacks {\em excessively}. Rewriting the code in another language
+may indeed make this part easier to understand, alas would have other
+disadvantages as pointed out previously.
+{\bf Update:} We introduced additional functions to replace
+variadic calls to functions that cannot be type-checked by
+the compiler (like libjansson's {\tt json\_pack()}) with
+type-safe versions (like the new {\tt GNUNET\_JSON\_PACK()}).
+\subsection{Initializing structs with memset}
+Using {\tt memset()} first prevents compiler (or valgrind) warnings about
+using uninitialized memory, possibly hiding bugs. We also do use struct
+initialization in many cases.
+The GNUnet-wrappers are generally designed to be ``safer'' or ``stricter''
+variants of the corresponding libc functions, and not merely ``the same''.
+Hence we do not believe that renaming {\tt GNUNET\_malloc} is indicated.
+The argument that {\tt memset()}ing first makes the code inherently more
+obvious also seems fallacious, as it would commonly result in dead stores,
+which can confuse developers and produce false-positive warnings from static
+analysis tools.
+\subsection{NULL pointer handling}
+The problem with the ``goto fail'' style error handling is that it rarely
+results in specific error handling where diagnostics are created that are
+specific to the error. Using this style of programming encourages developers
+to create simplistic error handling, which can result in inappropriate error
+handling logic and also makes it harder to attribute errors to the specific
+However, we have no prohibition on using this style of error handling either:
+if it is appropriate, develpers should make a case-by-case decision as to how
+to best handle a specific error.
+We have made some first changes to how {\tt GNUNET\_free()} works in response
+to the report, and will discuss further changes with the GNUnet development
+\subsection{Hidden security assumptions}
+We disagree that the assumptions stated are ``hidden'', as (1) the Taler code
+has its own checks to warrant that the requirements of the {\tt
+ GNUNET\_malloc()} API are satisfied (so enforcement is not limited to the
+abstraction layer), and (2) the maximum allocation size limit is quite clearly
+specified in the GNUnet documentation. Also, the GNUnet-functions are not
+merely an abstraction layer for portability, but they provided extended
+semantics that we rely upon. So it is not like it is possible to swap this
+layer and expect anything to continue to work.
+When we use the libjansson library, it is understood that it does not use
+the GNUnet operations, and the code is careful about this distinction.
+\subsection{Get rid of boolean function arguments}
+We agree that this can make the code more readable, and have in some places
+already changed the code in this way.
+\section{Structural Recommendation}
+\subsection{Least privilege}
+It is wrong to say that GNU Taler has ``no work done'' on privilege separation.
+For example, the {\tt taler-exchange-dbinit} tool is the only tool that requires
+CREATE, ALTER and DROP rights on database tables, thus enusring that the ``main''
+process does not need these rights.
+We also already had the {\tt taler-exchange-keyup} tool responsible for
+initializing keys. In response to the audit, we already changed the GNUnet API
+to make sure that tools do not create keys as a side-effect of trying to read
+non-existent key files.
+{\bf Update:} We have now implemented full privilege separation for access to the online
+cryptographic signing keys. Details about the design are documented in the
+section ``Exchange crypto helper design'' at \url{} of
+Chapter 12.
+{\bf Update:} In doing so, we also added a new type of signing key, the
+``security module'' signing key. This is used by the newly separated ``security
+module`` processes to sign the public keys that they guard the private keys
+for. The security module signatures are verified by the new
+``taler-exchange-offline`` tool to ensure that even if the {\tt
+taler-exchange-httpd} process is compromised, the offline signature tool would
+refuse to sign new public keys that do not originate from the security
+module(s). The security module public keys can be given in the configuration,
+or are learned TOFU-style.
+\subsection{File system access}
+The auditor helpers actually only read from the file system, only the LaTeX
+invocation to compile the final report to PDF inherently needs write
+access. We do not predict that we will retool LaTeX. Also, the file system
+access is completely uncritical, as the auditor by design runs on a system
+that is separate from the production exchange system.
+Because that system will not have {\em any} crypto keys (not even the one of
+the auditor!), CodeBlau is wrong to assume that reading from or writing to the
+file system represents a security threat.
+We have started to better document the operational requirements on running the
+{\bf Update:} On the exchange side, we have now moved additional information
+from the file system into the database, in particular information about offline signatures
+(including key revocations) and wire fees. This simplifies the deployment and
+the interaction with offline key signing mechanism. The remaining disk accesses are for
+quite fundamental configuration data (which ports to bind to, configuration to
+access the database, etc.), and of course the program logic itself.
+{\bf Update:} We have also restructured the configuration such that only
+the {\tt taler-exchange-transfer} and {\tt taler-exchange-wirewatch} programs
+need to have access to the more sensitive bank account configuration data,
+and so that these processes can run as a separate user.
+\subsection{Avoid dlopen}
+Taler actually uses {\tt ltdlopen()} from GNU libtool, which provides
+compiler flags to convert the dynamic linkage into static linkage. For
+development, dynamic linkage has many advantages.
+We plan to test and document how to build GNU Taler with only static
+linkage, and will recommend this style of deployment for the Taler
+exchange for production.
+\subsection{Reduce reliance on PostgreSQL}
+CodeBlau's suggestion to use an append-only transaction logging service in
+addition to the PostgreSQL database is a reasonable suggestion for a
+production-grade deployment of GNU Taler, as it would allow partial disaster
+recovery even in the presence of an attacker that has gained write access to
+the exchange's database.
+We are currently still investigating whether the transaction logging should be
+implemented directly by the exchange service, or via the database's extensible
+replication mechanism. Any implementation of such an append-only logging
+mechanism must be carefully designed to ensure it does not negatively impact
+the exchange's availability and does not interfere with serializability of
+database transactions. As such we believe that transaction logging can only be
+provided on a best-effort basis. Fortunately, even a best-effort append-only
+transaction log would serve to limit the financial damage incurred by the
+exchange in an active database compromise scenario.
+{\bf Update:} We have tightened the installation instructions for the
+Taler exchange to guide users towards a more restricted Postgres setup,
+tightening which components of the Exchange need what level of access
+to the exchange database.