Botan seeks to be a broadly applicable library that can be used to implement a range of secure distributed systems.
The library has the following project goals guiding changes. It does not succeed in all of these areas in every way just yet, but it describes the system that is the desired end result. Over time further progress is made in each.
Secure and reliable. The implementations must of course be correct and well tested, and attacks such as side channels and fault attacks should be accounted for where necessary. The library should never crash, or invoke undefined behavior, regardless of circumstances.
Implement schemes important in practice. It should be practical to implement any real-world crypto protocol using just what the library provides. It is worth some (limited) additional complexity in the library, in order to expand the set of applications which can easily adopt Botan.
Ease of use. It should be straightforward for an application programmer to do whatever it is they need to do. There should be one obvious way to perform any operation. The API should be predicable, and follow the “principle of least astonishment” in its design. This is not just a nicety; confusing APIs often result in errors that end up compromising security.
Simplicity of design, clarity of code, ease of review. The code should be easy to read and understand by other library developers, users seeking to better understand the behavior of the code, and by professional reviewers looking for bugs. This is important because bugs in convoluted code can easily escape multiple expert reviews, and end up living on for years.
Well tested. The code should be correct against the spec, with as close to 100% test coverage as possible. All available static and dynamic analysis tools at our disposal should be used, including fuzzers, symbolic execution, and protocol specific tools. Within reason, all warnings from compilers and static analyzers should be addressed, even if they seem like false positives, because that maximizes the signal value of new warnings from the tool.
Safe defaults. Policies should aim to be highly restrictive by default, and if they must be made less restrictive by certain applications, it should be obvious to the developer that they are doing something unsafe.
Post quantum security. Possibly a practical quantum computer that can break RSA and ECC will never be built, but the future is notoriously hard to predict. It seems prudent to begin designing and deploying systems now which have at least the option of using a post-quantum scheme. Botan provides a conservative selection of algorithms thought to be post-quantum secure.
Performance. Botan does not in every case strive to be faster than every other software implementation, but performance should be competitive and over time new optimizations are identified and applied.
Support whatever I/O mechanism the application wants. Allow the application to control all aspects of how the network is contacted, and ensure the API makes asynchronous operations easy to handle. This both insulates Botan from system-specific details and allows the application to use whatever networking style they please.
Portability to modern systems. Botan does not run everywhere, and we actually do not want it to (see non-goals below). But we do want it to run on anything that someone is deploying new applications on. That includes both major platforms like Windows, Linux, Android and iOS, and also promising new systems such as Fuchsia.
Well documented. Ideally every public API would have some place in the manual describing its usage.
Useful command line utility. The botan command line tool should be flexible and featured enough to replace similar tools such as
opensslfor everyday users.
There are goals some crypto libraries have, but which Botan actively does not seek to address.
Deep embedded support. Botan requires a heap, C++ exceptions, and RTTI, and at least in terms of performance optimizations effectively assumes a 32 or 64 bit processor. It is not suitable for deploying on, say FreeRTOS running on a MSP430, or smartcard with an 8 bit CPU and 256 bytes RAM. A larger SoC, such as a Cortex-A7 running Linux, is entirely within scope.
Implementing every crypto scheme in existence. The focus is on algorithms which are in practical use in systems deployed now, as well as promising algorithms for future deployment. Many algorithms which were of interest in the past but never saw widespread deployment and have no compelling benefit over other designs have been removed to simplify the codebase.
Portable to obsolete systems. There is no reason for crypto software to support ancient OS platforms like SunOS or Windows 2000, since these unpatched systems are completely unsafe anyway. The additional complexity supporting such platforms just creates more room for bugs.
Portable to every C++ compiler ever made. Over time Botan moves forward to both take advantage of new language/compiler features, and to shed workarounds for dealing with bugs in ancient compilers, allowing further simplifications in the codebase. The set of supported compilers is fixed for each new release branch, for example Botan 2.x will always support GCC 4.8. But a future 3.x release version will likely increase the required versions for all compilers.
FIPS 140 validation. The primary developer was (long ago) a consultant with a NIST approved testing lab. He does not have a positive view of the process or results, particularly when it comes to Level 1 software validations. The only benefit of a Level 1 validation is to allow for government sales, and the cost of validation includes enormous amounts of time and money, adding ‘checks’ that are useless or actively harmful, then freezing the software so security updates cannot be applied in the future. It does force a certain minimum standard (ie, FIPS Level 1 does assure AES and RSA are probably implemented correctly) but this is an issue of interop not security since Level 1 does not seriously consider attacks of any kind. Any security budget would be far better spent on a review from a specialized crypto consultancy, who would look for actual flaws.
That said it would be easy to add a “FIPS 140” build mode to Botan, which just disabled all the builtin crypto and wrapped whatever the most recent OpenSSL FIPS module exports.
Educational purposes. The library code is intended to be easy to read and review, and so might be useful in an educational context. However it does not contain any toy ciphers (unless you count DES and RC4) nor any tools for simple cryptanalysis. Generally the manual and source comments assume previous knowledge on the basic concepts involved.
User proof. Some libraries provide a very high level API in an attempt to save the user from themselves. Occasionally they succeed. It would be appropriate and useful to build such an API on top of Botan, but Botan itself wants to cover a broad set of uses cases and some of these involve having pointy things within reach.