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update documentation to reflect new renegotiation options

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This commit is contained in:
Dr. Stephen Henson 2010-01-27 17:46:24 +00:00
parent 89e56aebef
commit 99b36a8c31
1 changed files with 46 additions and 23 deletions
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69
doc/ssl/SSL_CTX_set_options.pod
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@ -224,10 +224,10 @@ of RFC4507bis tickets for stateless session resumption.
If this option is set this functionality is disabled and tickets will
not be used by clients or servers.
=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION, SSL_OP_LEGACY_SERVER_CONNECT
See the B<SECURE RENEGOTIATION> section for a discussion of the purpose of
this option
these options.
=back
@ -235,38 +235,60 @@ this option
OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
described in draft-ietf-tls-renegotiation (FIXME: replace by RFC). This
counters a prefix attack described in the draft and elsewhere (FIXME: need full
reference).
counters the prefix attack described in CVE-2009-3555 and elsewhere.
This attack has far reaching consequences which application writers should be
aware of. In the description below an implementation supporting secure
renegotiation is referred to as I<patched>. A server not supporting secure
renegotiation is referred to as I<unpatched>.
If an unpatched client attempts to connect to a patched OpenSSL server then
the attempt will succeed but renegotiation is not permitted. As required
by the standard a B<no_renegotiation> alert is sent back to the client if
the TLS v1.0 protocol is used. If SSLv3.0 is used then renegotiation results
in a fatal B<handshake_failed> alert.
=head2 Patched client and server
If a patched OpenSSL client attempts to connect to an unpatched server
then the connection will fail because it is not possible to determine
whether an attack is taking place.
Connections and renegotiation will always succeed.
If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then the
above restrictions are relaxed. Renegotiation is permissible and initial
connections to unpatched servers will succeed.
=head2 Unpatched client and patched server
This option should be used with caution because it leaves both clients and
servers vulnerable. However unpatched servers and clients are likely to be
around for some time and refusing to connect to unpatched servers or denying
renegotion altogether may be unacceptable. So applications may be forced to
tolerate unsafe renegotiation for the immediate future.
The initial connection suceeds but client renegotiation is denied with a
B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
B<handshake_failure> alert in SSL v3.0.
If the patched server attempts to renegotiate a fatal B<handshake_failure>
alert is sent. This is because the server code may be unaware of the
unpatched nature of the client.
If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
renegotiation B<always> succeeds.
B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are
unpatched) will result in the connection hanging if it receives a
B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard
a B<no_renegotiation> alert as fatal and respond with a fatal
B<handshake_failure> alert.
=head2 Patched client and unpatched server.
If the option B<SSL_OP_LEGACY_SERVER_CONNECT> is set then initial connections
to unpatched servers succeed. This option is currently set by default even
though it has security implications: otherwise it would be impossible to
connect to unpatched servers i.e. all of them initially and this is clearly not
acceptable.
As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
B<not> be set by default in a future version of OpenSSL.
Applications that want to ensure they can connect to unpatched servers should
always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
Applications that want to ensure they can B<not> connect to unpatched servers
(and thus avoid any security issues) should always B<clear>
B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
SSL_clear_options().
The function SSL_get_secure_renegotiation_support() indicates whether the peer
supports secure renegotiation.
The deprecated SSLv2 protocol does not support secure renegotiation at all.
The deprecated and highly broken SSLv2 protocol does not support secure
renegotiation at all: its use is B<strongly> discouraged.
=head1 RETURN VALUES
@ -306,7 +328,8 @@ enabled).
SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL
0.9.8m.
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> was first added in OpenSSL
0.9.8m.
B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT>
and the function SSL_get_secure_renegotiation_support() were first added in
OpenSSL 0.9.8m.
=cut