Module rustls::manual::_05_defaults

This section provides rationale for the defaults in rustls.

Rationale for defaults

Why is AES-256 preferred over AES-128?

This is a trade-off between:

1. classical security level: searching a 2^128 key space is as implausible as 2^256.
2. post-quantum security level: the difference is more meaningful, and AES-256 seems like the conservative choice.
3. performance: AES-256 is around 40% slower than AES-128, though hardware acceleration typically narrows this gap.

The choice is frankly quite marginal.

Why is AES-GCM preferred over chacha20-poly1305?

Hardware support for accelerating AES-GCM is widespread, and hardware-accelerated AES-GCM is quicker than un-accelerated chacha20-poly1305.

However, if you know your application will run on a platform without that, you should definitely change the default order to prefer chacha20-poly1305: both the performance and the implementation security will be improved. We think this is an uncommon case.

Why is x25519 preferred for key exchange over nistp256?

Both provide roughly the same classical security level, but x25519 has better performance and it’s much more likely that both peers will have good quality implementations.