TLS and STARTTLS

Hi,

Perdition has the option "ssl_mode" which can be used to set both
listening and outgoing connections. Its made up of options that are
prefixed either by 'ssl' or 'tls'.

From my experimentation, it seems as if the 'tls' options aren't
actually TLS, but rather the STARTTLS extension for IMAP and POP.

So what is the difference? The difference is that an actual TLS
connection would be one whereby the client and server negotiate a
stateful connection by using a handshaking procedure where they agree on
various parameters that will be used to establish the connection's
security. This is all done during the bring up of the connection, and
the client wont send a login/password until the connection has been
properly setup.

STARTTLS, by contrast, proceeds by the client making a clear-text
connection to the server, noting that the server has the STARTTLS
capability (to use the IMAP terminology), requests that a STARTTLS
negotiation process begins and then you promote up to an actual TLS
connection. After you've promoted, then your connection is encrypted.

Why is the distinction important? With STARTTLS the potential for
credential leak exists during the clear-text window. Perdition offers
the tls_listen_force/tls_outgoing_force/tls_all_force settings to
'ssl_mode', but all this does in IMAP4/IMAP4S is have the LOGINDISABLED
set, which doesn't stop a client from attempting to login, it just
refuses their login after they've sent it over the clear-text
connection. As the RFC (2595) referenced in the man page states:

    Servers advertising this capability will fail to interoperate with
    many existing compliant IMAP clients and will be unable to prevent
    those clients from disclosing the user's password.

I think that the option 'ssl_mode' is a confusing name. TLS and SSL are
different things (although many people still call TLS connections
SSL). Additionally the use of the name 'tls' in the potential options is
confusing, when the actual behavior is STARTTLS.

It seems to me that the existing 'tls' options should be renamed
'starttls_*' and an actual TLS option be added.

I also notice that the man page says, "TLS is defined in RFC 2595". This
isn't really correct, again its STARTTLS that is defined in RFC
2595. This RFC, although titled this way in 1999, is known as the
canonical reference defining STARTTLS for IMAP and POP3 (there are
others for things like SMTP). TLS, on the other hand, is actually
detailed in RFC 5246.

micah


0. http://tools.ietf.org/html/rfc2595
1. http://tools.ietf.org/html/rfc5246

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