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Edit docs to explain not changing curve order

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TLS1.3 MVP would benefit from a different curve group preference order
in order to not cause a HelloRetryRequest (which are not yet handled),
however changing the curve group preference order would affect both
TLS1.2 and TLS1.3, which is undesirable for something rare that can
be worked around.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
This commit is contained in:
Paul Elliott 2021-11-26 20:24:43 +00:00
parent e0bbedfe7a
commit 66491c7d08
1 changed files with 18 additions and 10 deletions
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docs/architecture/tls13-experimental.md
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@ -133,17 +133,25 @@ MVP definition
(1) This is just for comparison.
(2) The MVP sends one shared secret corresponding to the configured preferred
group. The preferred group is the group of the first curve in the list of
allowed curves as defined by the configuration. The allowed curves are
by default ordered as follow: `secp256r1`, `x25519`, `secp384r1`
and finally `secp521r1`. This default order is aligned with the
list of mandatory-to-implement groups (in absence of an application
profile standard specifying otherwise) defined in section 9.1 of the
specification. The list of allowed curves can be changed through the
`mbedtls_ssl_conf_curves()` API.
(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
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 likelyhood
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.
(3) The MVP proposes only TLS 1.3 and does not support version negociation.
(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
has been built to support both TLS 1.3 and TLS 1.2: just set the
maximum of the minor version of the SSL configuration to