WIP: Add note about memory safety

src/main/java/swiss/qpq/gajumaru/core/crypto/Ed25519.java

@@ -30,7 +30,16 @@ import swiss.qpq.gajumaru.core.tools.CryptoUtils;
 // Pure Java implementation of Ed25519 curve arithmetic.
 // Uses exquisitely annoying Radix-2^25.5 field arithmetic. Never do this if you can avoid it.
 // Verified against the canonical Erlang ec_utils (see test/README.md for how).
-
+//
+// SAFETY NOTE:
+//  It turns out that many JVM environments completely lack a way to allocate pinned memory
+//  that lives outside the domain of the garbage collector. While the techniques used in this
+//  code are pretty darn safe, the underlying byte arrays can still get copied on GC before
+//  being zeroed if they are allocated in memory that isn't pinned. It turns out Android is
+//  one such platform, and the solution there is to vendor out a C implementation of Ed25519
+//  and interact with it over a JNI boundary with off-heap memory via java.nio.ByteBuffer.
+//  In cases where this code can run in a fully isolated memory environment it is sufficient.
+//
 // References:
 // I don't even know where to start with references, but the tink-java library and pretty much
 // everything (and everyone!) referenced on the lib25519 page deserves a mention.