Hello, I have a general question on channel binding, exim is one of the few application offering CB.
RFC 5929 Channel Bindings for TLS defines the tls-unique channel binding, stating that there was an old tls-unique, which was bad. The new tls-unique uses the most recent TLS Finished message sent in the token, whereas the old tls-unique used the first TLS Finished message. Commit b1a32a3ce673 adds channel binding to openssl/exim, using the function SSL_get_peer_finished(). Compare this to the OpenLDAP implementation, where libldap/tls_o.c:tlso_session_unique() uses SSL_session_reused(s) ^ !is_server ? SSL_get_finished(s) : SSL_get_peer_finished(). https://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_gsasl_authenticator.html says for server_channelbinding: “However, Channel Binding in TLS has proven to be vulnerable in current versions. Do not plan to rely upon this feature for security, ever, without consulting with a subject matter expert (a cryptographic engineer). ”. Is this vulnerability addressed in RFC 5929? “This is only usable by mechanisms which support "channel binding"; at time of writing, that’s the SCRAM family. When using this feature the "-PLUS" variants of the method names need to be used. ” → GS2-KRB5 also supports channel bindings. The document does not state, that the tls-unique channel binding is used by exim and tls-server-end-point is not used. Would you mind implementing Channel Binding also using Cyrus SASL and doing interoperability test (e.g. beween GnulTLS+Gsasl vs OpenSSL+Cyrus SASL)? OpenLDAP has the file cyrus.c, which implements channel binding in such a way, that it works with the Windows Active Directory (but I have some concerns to the implementation, as it enforces channel binding over SASL GSSAPI, whereas “RFC 4752 SASL GSSAPI” says, that SASL GSSAPI does not allow channel binding). Have you done any interoperability tests for the channel binding (e.g. can the Outlook MTA do SASL GSSAPI using Channel Binding)? Greetings Дилян -- ## List details at https://lists.exim.org/mailman/listinfo/exim-dev Exim details at http://www.exim.org/ ##