Yes it should definitely require a capability.

Still a risk that some admin-enabled method (like enabling an IPsec VPN) provides a path to it, but would reduce the potential for crafting weird inputs.

I'm also wondering if it couldn't be rewritten to use io_uring interfaces.

That's really orthogonal (and you can already do io_uring with AF_ALG, at the end of the day AF_ALG is just recvmsg() and sendmsg(), which work just fine in io_uring...)

I mean for more efficient and easier to verify out-of-kernel implementations of crypto with kernel like speeds.