Maintaining cross-platform cryptographic library is a journey full of unexpected bugs, language-specific hacks, difficult decisions and endless struggle to make developer-facing APIs easy-to-use and hard-to-misuse.

As developers, what do we want from using cryptographic tools? They should be easy-to-use and hard-to-misuse, cover complete use-cases instead of providing raw crypto-primitives, support language-specific style, work in the same way on numerous platforms and languages. Basically, we want less stress and more work done.

But is it so easy to maintain cross-platform crypto library?

How to simplify complex crypto concepts into simple encrypt(msg, key) and decrypt(data, key) functions? How to provide a unified API for 12 languages, when some of them are strongly-typed and the other one is javascript? Secure by default: what design choices should be made to protect from misuse. Multi-platformness: how to make sure that message encrypted using Ruby on x64 Ubuntu 18.04 will be successfully decrypted on iOS12 on iPhone armv7s? Testing wildness: static analyzers, fuzzing, unit tests, integration tests. Docs: developers copy-paste from readme anyway, how to make sure that they copy-paste correct things.

I’ll describe the 4yrs experience on maintaining multi-platform open source library Themis: from API design to language-specific hacks.

Bonus questions:

• what maintainers completely forget about: the biggest fail reason for users.
• what is faster in Go: calling OpenSSL via c-Go interop or using native Go crypto primitives?
• how to deal with multi-platformness, if BoringSSL for iOS doesn’t support AES XTS?
• epic bug with iOS-Android compatibility, which appears to be a sad story about size_t.
• switching cryptographic backends for fun and profit: switching between BoringSSL, OpenSSL, LibreSSL, libsodium and CommonCrypto, and governmental crypto standards of some countries, leaving the same high-level API.

Key takeaways:

• cryptography is easy, but “secure by default” APIs are hard;
• never agree to support multi-platform libs;
• each language has unique hacks and typical mistakes, deal with it.

This talk is useful for people that believe that perfect crypto lib is a C lib with 1-page readme :)

Watch 📺

Presented at ✨

• BlackAlps Yverdon-les-Bains, Switzerland, 7-8 November 2019