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https://github.com/awslabs/s2n/blob/master/docs/USAGE-GUIDE....

Looks like currently you must set a file descriptor (though the docs mention the possibility of using a pipe). Once an FD is set, you do control pulling/pushing data to s2n.

Can you elaborate more on "not too well-performing"?

You can use OpenSSL without IO APIs. Just use SSL_set_bio ;)

At least with OpenSSL you can implement your own BIO objects and do the I/O yourself if you want/need to. It's not the cleanest or best-documented interface in the world, but it's certainly usable.

it is usable. But I think you would be hard pressed to find a more widely used piece of software that has absolute terrible documentation. The only real way to figure it out is to read the code or read the examples.