Re: [Uta] Last Call: <draft-ietf-uta-email-tls-certs-05.txt> (Updated TLS Server Identity Check Procedure for Email Related Protocols) to Proposed Standard

[Julien ÉLIE <[e-mail]>]

Hi,

> The IESG has received a request from the Using TLS in Applications WG
> (uta) to consider the following document:
> - 'Updated TLS Server Identity Check Procedure for Email Related
>     Protocols'
>    <draft-ietf-uta-email-tls-certs-05.txt> as Proposed Standard
>
> Abstract
>
>     This document describes TLS server identity verification procedure
>     for SMTP Submission, IMAP, POP and ManageSieve clients.  It replaces
>     Section 2.4 of RFC 2595, updates Section 4.1 of RFC 3207, updates
>     Section 11.1 of RFC 3501, updates Section 2.2.1 of RFC 5804.

The introduction of the draft explains that one of the goal is to define 
consistent rules between a few protocols implemented at the same time in 
email clients:

    Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is
    described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804]
    respectively.  Each of the documents describes slightly different
    rules for server certificate identity verification (or doesn't define
    any rules at all).  In reality, email client and server developers
    implement many of these protocols at the same time, so it would be
    good to define modern and consistent rules for verifying email server
    identities using TLS.


Couldn't the draft also update Section 5 of RFC 4642 about the use of 
TLS in NNTP?
The NNTP protocol is also a protocol that is found in email clients, so 
it would make sense to have consistent rules between email and netnews.
For instance, Thunderbird, SeaMonkey, Gnus, Opera Mail, Windows Live 
Mail, Forté Agent or tin (to mention only them) are both email and 
netnews clients.


Amongst other things, Section 5 of RFC 4642 says:

    During the TLS negotiation, the client MUST check its understanding
    of the server hostname against the server's identity as presented in
    the server Certificate message, in order to prevent man-in-the-middle
    attacks.  Matching is performed according to these rules:

    -  The client MUST use the server hostname it used to open the
       connection (or the hostname specified in TLS "server_name"
       extension [TLS-EXT]) as the value to compare against the server
       name as expressed in the server certificate.  The client MUST NOT
       use any form of the server hostname derived from an insecure
       remote source (e.g., insecure DNS lookup).  CNAME canonicalization
       is not done.

    -  If a subjectAltName extension of type dNSName is present in the
       certificate, it SHOULD be used as the source of the server's
       identity.

    -  Matching is case-insensitive.

    -  A "*" wildcard character MAY be used as the left-most name
       component in the certificate.  For example, *.example.com would
       match a.example.com, foo.example.com, etc., but would not match
       example.com.

    -  If the certificate contains multiple names (e.g., more than one
       dNSName field), then a match with any one of the fields is
       considered acceptable.

    If the match fails, the client SHOULD either ask for explicit user
    confirmation or terminate the connection with a QUIT command and
    indicate the server's identity is suspect.

    Additionally, clients MUST verify the binding between the identity of
    the servers to which they connect and the public keys presented by
    those servers.  Clients SHOULD implement the algorithm in Section 6
    of [PKI-CERT] for general certificate validation, but MAY supplement
    that algorithm with other validation methods that achieve equivalent
    levels of verification (such as comparing the server certificate
    against a local store of already-verified certificates and identity
    bindings).





Or another idea:  wouldn't the draft be worthwhile for a BCP like BCP 
195 "Recommendations for Secure Use of Transport Layer Security (TLS) 
and Datagram Transport Layer Security (DTLS)"?

It could indeed be "Recommendations for TLS Server Identity Check 
Procedure".  The advantage would be that the BCP can apply to email 
protocols, as well as other protocols using TLS.
It would save time for others, and permit to have homogeneity and 
consistent rules across protocols, as well as increasing security.

-- 
Julien ÉLIE

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