I was just wondering, could we just simplify the implementation by using
two named groups for the top two-preferred categories, without limited
to XDH and ECDHE? For example, if FFDHE is on the top 2, FFDHE will be
used as well. Normally, XDH and ECDHE will be used, but the simplifying
is a little bit more flexible if FFDHE is preferred. What do you think?
Xuelei
On 7/8/2020 3:57 PM, Jamil Nimeh wrote:
Hello all,
This is a small enhancement to the TLS 1.3 client hello. With this
change JSSE will make a best-effort attempt to select the most-preferred
XDH and ECDHE named group. In most cases this will be x25519 and
secp256r1. This was done to help reduce the number of times our clients
get HelloRetryRequests due to servers wanting secp256r1 when we would
just assert x25519.
The asserted key shares can be affected by different settings for the
jdk.tls.namedGroups System property. If x25519 and/or secp256r1 are
missing, the next-most-preferred named group of that class will be
selected. If no other named group in that class is in the
supported_groups extension, then it will be skipped. In the unlikely
event that no XDH or ECDHE supported group is asserted, then the client
will assert one key share, the most preferred one out of the supported
groups.
This also fixes one minor spec compliance issue where we weren't
throwing an exception when a server returned an illegal
HelloRetryRequest with a named group that was in the original key_shares
extension from the client hello.
Webrev: https://cr.openjdk.java.net/~jnimeh/reviews/8247630/webrev.01/
JBS: https://bugs.openjdk.java.net/browse/JDK-8247630
--Jamil
