Second of all, in a generic C interface keys are likely to be treated as void pointer and almost certainly are going to be moved around with memcpy etc. rather than returned/passed by value since doing so would make the interface non-generic.
that is really dangerous to assume. memset is a compiler built-in in every relevant C++ compiler and the compiler definitely knows the type of the object that is behind your void* and knows if you're being nasty.
e.g. look at this code : https://gcc.godbolt.org/z/xh9BXs
it's UB, and the compiler knows it and inserts an "invalid opcode" instruction even if you try to hide a memset behind a void*-taking function
Yes, it's true that generic C code will type-erase the key type. However it just takes a little refactoring in specific code to move the struct initialization across a call boundary from where it is passed to the generic code.
I'm skeptical about compilers optimizing memset not to cover padding between structure members.
Firstly, that would introduce security holes into a heck of a lot more existing code compared to code that uses a dead-store memset to wipe sensitive crypto.
Secondly, it wouldn't run any faster. Gaps in a structure and at the end exist in order to eliminate misalignment. Before most padding, there is a member that ends on a misaligned address. It's slower to update just that member, and leave the padding alone, than to clobber the padding.
For instance if we have a { char a; int b; char c; } structure, we gain nothing by zeroing just one byte of a, b and c.
In some compiler for an 8 bit system, this reasoning is likely false; I will worry about it when porting to that. Very little existing code will fit; you're coding from scratch for such things.
E.g. "... being interpreted as a void * type."