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Second draft of explanation

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Signed-off-by: Paul Elliott <paul.elliott@arm.com>
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Paul Elliott 2021-12-02 16:38:05 +00:00
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docs/architecture/tls13-experimental.md
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@ -134,22 +134,20 @@ MVP definition
(1) This is just for comparison.
(2) The MVP sends only one shared secret corresponding to the configured
preferred group. This could, however end up with connection failure if the
server does not support our preferred curve, as we have yet to implement
preferred group. This could end up with connection failure if the
server does not support our preferred curve, as the MVP does not implement
HelloRetryRequest. The preferred group is the group of the first curve in
the list of allowed curves as defined by the configuration. The list of
mandatory-to-implement groups (in absence of an application profile
standard specifying otherwise) as defined in section 9.1 of the
specification gives the preferred order as follows: `secp256r1`, `x25519`,
`secp384r1` and finally `secp521r1`. If we could therefore fix the use of
`secp256r1`, then we would be guaranteed that the server supported it,
however our current curve preference order puts `x25519` before
`secp256r1` and changing this for only TLS1.3 would be potentially
difficult (we have no desire to change TLS1.2 behaviour). The likelihood
of finding a server that doesn't support `x25519` is quite low and indeed
the end user could themselves change the order of preference of curves
using the `mbedtls_ssl_conf_curves()` API if they wished to do so, so we
are leaving the current preference order intact.
the list of allowed curves as defined by the configuration. The allowed
curves are by default ordered as follows: `x25519`, `secp256r1`,
`secp384r1` and finally `secp521r1`. Note that, in the absence of an
application profile standard specifying otherwise, section 9.1 of the
specification rather promotes curve `secp256r1` to be supported over
curve `x25519`. The MVP would, hoewever, rather keep the preference order
currently promoted by Mbed TLS as this applies to TLS 1.2 as well, and
changing the order only for TLS1.3 would be potentially difficult.
In the unlikely event a server does not support curve `x25519` but does
support curve `secp256r1`, curve `secp256r1` can be set as the preferred
curve through the `mbedtls_ssl_conf_curves()` API.
(3) The MVP proposes only TLS 1.3 and does not support version negotiation.
Out-of-protocol fallback is supported though if the Mbed TLS library