Note big-endian compatibility issues in CHANGELOG

[ilammy]

Recent changes in Secure Cell code to support password-based API and introduction of automated fuzz testing led to refactoring of a lot of code to improve resilience to malicious and erroneous inputs.

This effort has discovered that Themis is likely to be not compatible between little- and big-endian machines. Various parsing and generation code has been using “native” endian order. That is, data encrypted on little-endian machines cannot be read on big-endian machines and vice versa. If usage is strictly confined to exclusively big-endian machines then it's probably fine.

I say “probably” because big-endian architectures have never been a priority for Themis. We have never tested on them and are not sure that Themis works there correctly, even without talking about inter-endian compatibility.

So... Recent changes in Secure Cell start improving the situation by explicitly using little-endian order in data structures. Eventually this will lead to proper support of big-endian architectures which will be able to talk to little-endian machines just fine. But right now this means that data encrypted by Themis 0.12 on big-endian machines will be not readable with Themis 0.13, even on big-endian machines. Similarly, data encrypted by Themis 0.13 on big-endian machines will not be readable by Themis 0.12 on big-endian machines (but will be readable on little-endian machines).

Here's a compatibility table:

Source		Destination
		Little-endian		Big-endian
		0.12	0.13	0.12	0.13
Little-endian	0.12	✅	✅	❌	🎉
	0.13	✅	✅	❌	🎉
Big-endian	0.12	❌	❌	🙏🏻	💥
	0.13	🎉	🎉	💥	🎉

Legend:

• ✅ — known to work
• ❌ — probably did not work before and will not work in the future
• 🎉 — probably works now and will work in the future
• 🙏🏻 — probably worked before
• 💥 — is likely to be broken since 0.13 and will remain that way

This is for Secure Cell. For any other cryptosystems, big-endian ⟺ big-endian quadrant is likely to be 🙏🏻, and others are filled with ❌.

If you have been using Themis on big-endian machines and it actually worked then I'm pleasantly surprised by the portability of the code. At the same time I offer my condolences for the loss of time we're going to incur on you by doing these changes.

We will announce full big-endian support as soon as it is ready. For now, you're on your own. Themis does not support big-endian machines at the moment.