tor/changes/tls_ecdhe

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o Major features:
- Servers can now enable the ECDHE TLS ciphersuites when available
and appropriate. These ciphersuites let us negotiate forward-
secure TLS secret keys more safely and more efficiently than with
our previous use of Diffie Hellman modulo a 1024-bit prime.
By default, public servers prefer the (faster) P224 group, and
bridges prefer the (more common) P256 group; you can override this
with the TLSECGroup option.
Enabling these ciphers was a little tricky, since for a long
time, clients had been claiming to support them without
actually doing so, in order to foil fingerprinting. But with
the client-side implementation of proposal 198 in
0.2.3.17-beta, clients can now match the ciphers from recent
firefox versions *and* list the ciphers they actually mean, so
servers can believe such clients when they advertise ECDHE
support in their TLS ClientHello messages.
This feature requires clients running 0.2.3.17-beta or later,
and requires both sides to be running OpenSSL 1.0.0 or later
with ECC support. OpenSSL 1.0.1, with the compile-time option
"enable-ec_nistp_64_gcc_128", is highly recommended.
Implements the server side of proposal 198; closes ticket
7200.