This is in accordance with RFC 7465
<https://tools.ietf.org/html/rfc7465>
Also correct the key exchange mechanism strings; these should be
prefixed with 'k'.
SSL_OP_NO_COMPRESSION was presumably added in an attempt to prevent
information leakage in a manner similar to recent attacks on HTTPS.
However, assuming that IRC is vulnerable to the same class of attacks is
incorrect: the behavior of the IRC protocol (a single long-running
connection) is not the same as that of HTTPS (multiple ephemeral
connections). HTTPS's use of ephemeral connections means that certain
assumptions can be made about the contents of the compression
algorithm's dictionaries and the content exchanged between the client
and server (e.g. the content being nearly the same for each connection),
which is not true for IRC. Additionally, they rely on the attacker being
able to coerce the client into creating many HTTPS connections (and
resending some secret token belonging to the user, along with
attacker-controlled data) each time, none of which is possible with IRC.
Lastly, since compression is no longer performed, this option will
result in leaking the lengths of messages transmitted to and from the
client. This option does reduce CPU utilization on Charybdis servers but
also increases bandwidth consumed.
Note that these are not available in old versions of OpenSSL (like FreeBSD
9.x base OpenSSL), so allow them to be missing.
A side effect may be slightly higher CPU consumption and network traffic.
Without a session id context and if client certificates are used, OpenSSL
fails the handshake if an attempt is made to reuse an old session. Various
clients could not reconnect after a disconnection because of this.
See https://bugzilla.mozilla.org/show_bug.cgi?id=858394#c34 for a bug
report.
ERR_error_string() is just broken, as it returns at most 119 chars
which means error messages are frequently truncated.
Allow for 511 chars using ERR_error_string_n().
This lets a user connect with a client certificate, and
passes the certificate's fingerprint to ircd, which
currently just notices it to the user.
A new ssld->ircd message 'F' is used to pass on the
fingerprint.
This is only for OpenSSL for now, not GNUTLS.
