Thanks for the reply but the public interface is Objective-C, the C++ is private code - I see my edit didn't make that clear. I got the solution from someone on stack overflow anyway, I just needed to add libc++.tbd to the extension.

Yeah I had thought some other part of my code was wrong but it wasn't and those didn't work. We've switched to SHA :)

Thanks. Using SecKeyCreateSignature and rsaSignatureRaw works (rsaEncryptionRaw threw an error FYI). The output doesn't match the output from the Java code but I'm one step closer!