These changes generalize the interface with an elliptic curve
implementation. This allows LibTLS to support elliptic curves generally
without needing the specifics of elliptic curve implementations.
This should allow for easier addition of other elliptic curves.
This adds support for the Elliptic Curve Diffie-Hellman Ephemeral key
exchange, using the X25519 elliptic curve. This means that the
ECDHE_RSA_WITH_AES_128_GCM_SHA256 and ECDHE_RSA_WITH_AES_256_GCM_SHA384
cipher suites are now supported.
Currently, only the X25519 elliptic curve is supported in combination
with the uncompressed elliptic curve point format. However, since the
X25519 is the recommended curve, basically every server supports this.
Furthermore, the uncompressed point format is required by the TLS
specification, which means any server with EC support will support the
uncompressed format.
Like the implementation of the normal Diffie-Hellman Ephemeral key
exchange, this implementation does not currently validate the signature
of the public key sent by the server.
The old enumeration didn't allow discriminating the key exchange
algorithms used, but only allowed the handshake with the server. With
this new enumeration, we can know which key exchange algorithm we are
actually supposed to use :^)
Also sort the existing cipher suites, and remove the unsupported ones.
We don't support any of these recommended ciphers, but at least we now
know which ones we should focus on :^)
Instead of sprinkling the definition of the ciper suites all over the
TLS implementation, let's regroup it all once and for all in a single
place, and then add our new implementations there.
