Re: [Uta] Eric Rescorla's Yes on draft-ietf-uta-email-deep-09: (with COMMENT)

[Keith Moore <[e-mail]>]

(inline)

> Line 186
>     TLS, and to encourage a greater consistency for how TLS is used, this
>     specification now recommends use of Implicit TLS for POP, IMAP, SMTP
>     Submission, and all other protocols used between a Mail User Agent
> Do you want to say RECOMMENDED?

Lower case "recommends" (non-2119 meaning) was intentional, though my 
reasoning may be unclear:  One of the first things I learned while 
studying engineering was to not specify the same thing in two different 
ways, because subtle differences between the two could create 
ambiguity.   (I remember this point clearly because about a year before, 
someone had constructed a beautiful piece of cabinetry to my 
specifications, which completely did not fit into the intended space 
because I had made this very error.)

So when writing technical specifications, I believe there should be a 
clear distinction between summary text (which glosses over details), and 
the text that actually defines the requirements of the specification.   
Admittedly this could be called out more explicitly, though it doesn't 
seem conventional for IETF RFCs to do so.

> Line 199
>     greeting, the server and client MUST enter AUTHORIZATION state, even
>     if client credentials were supplied during the TLS handshake.
> You mean TLS client certificates here, right? Maybe say so

agreed; fixed in -10.
> Line 214
>     remainder of the TCP connection.  If client credentials were provided
>     during the TLS handshake that the server finds acceptable, the server
>     MAY issue a PREAUTH greeting in which case both the server and client
> Same comment above about client credentials == certs.
also fixed in -10.

> Line 304
>        preference to services supporting STARTTLS (if offered).  (See
>        also Section 4.5.)
> I note that 6186 is kind of unclear on what should go in SNI. It obviously
> needs to be what you are checking against (which 6186 gets right) but maybe
> it's worth clarifying in this document somewhere.
Hmm.    I might need to come back to that one.   Lots of layers to sift 
through.  Feel free to suggest text.

> Line 328
>        the TLS ciphersuite of the session in which the mail was received,
>        in the Received field of the outgoing message.  (See Section 4.3.)
> Do you want to also suggest that it include the name of the DH group, if any?

I've attempted to add that attribute but please check the text:

       The ESMTPS transmission type <xref target="RFC3848"/> provides trace
       information that can indicate TLS was used when transferring mail.
       However, TLS usage by itself is not a guarantee of confidentiality or
       security. The TLS cipher suite provides additional information 
about the
       level of security made available for a connection. This defines a new
       SMTP "tls" Received header additional-registered-clause that is 
used to
       record the TLS cipher suite that was negotiated for the 
connection. The
       value included in this additional clause SHOULD be the registered 
cipher
       suite name (e.g., TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) included 
in the TLS
       cipher suite registry. In the event the implementation does not 
know the
       name of the cipher suite (a situation that should be remedied 
promptly),
       a four-digit hexadecimal cipher suite identifier MAY be used.
       In addition, the Diffie-Hellman group name associated with the
       ciphersuite MAY be included (when applicable and known) following the
       ciphersuite name.   The ABNF for the field follows:
       <figure>
         <artwork type="abnf">
tls-cipher-clause  =  CFWS "tls" FWS tls-cipher [ "group" dh-group ]

tls-cipher         =  tls-cipher-name / tls-cipher-hex

tls-cipher-name    =  ALPHA *(ALPHA / DIGIT / "_")
; as registered in IANA cipher suite registry

tls-cipher-hex     =  "0x" 4HEXDIG

dh-group           = ALPHA *(ALPHA / DIGIT / "_")
; as registered in IANA TLS Supported Groups Registry
</artwork>
       </figure>
> Line 363
>     refuse a ClientHello message from any client sending a protocol
>     version number corresponding to any version of SSL or TLS 1.0.
>     Another way is for the server to accept ClientHello messages from
> It's worth being very clear that you mean ClientHello.version, not the record
> version, as this has created a lot of interop problems.

ok
> Line 405
>     implementation does not know the name of the cipher suite (a
>     situation that should be remedied promptly), a four-digit hexadecimal
>     cipher suite identifier MAY be used.  The ABNF for the field follows:
> Hard to see how you could realistically get into this state...
Chris wrote this so this is just a guess on my part:   I'm assuming 
there are TLS APIs out there that let the caller query which ciphersuite 
is being used, but the implementation returns an integer rather than the 
name, and provides no convenience routine to look up the name text.

> Line 518
>        [RFC7525], TLS 1.1 (or earlier) SHOULD NOT be used unless no
>        higher version is available during TLS protocol negotiation.
> This text doesn't quite seem right, as the client has no idea what the server
> supports, it just knows what it negotiated. Can you explain how this would be
> implemented?

No, I can't explain it either.  The client can specify that it can 
handle TLS version 1.2 or greater, and the server is supposed to return 
the highest TLS version that it supports that is <= the TLS version 
specified by the client.  But the client has no way to force the server 
to negotiate the highest TLS version that it supports.  The only options 
the client has are things like abort the connection, and perhaps to try 
again under different circumstances. Or maybe the client could abort the 
connection if it knows (by some unspecified mechanism) that the server 
really does support a higher version.   So I think that the 
corresponding text from RFC 7525 doesn't really apply to a client.

Since I don't know what else to suggest here, I'll comment out that 
sentence.

> Line 594
>     accounts SHOULD be at least use of TLS version 1.1 or greater, and
>     successful validation of the server's certificate.  (Future revisions
>     to this specification may raise these requirements or impose
> This second requirement is more important.

Agree, or at least I think I do (are MiTM attacks taking advantage of no 
or weak cert validation really more of a threat than attacks on TLS < 
1.1 protocol or ciphersuites?  yeah, I could see that.).  But I 
certainly want implementors to pay more attention to cert validation.

I'm not sure what change to the text you had in mind, but I reversed the 
order to put cert validation first.

> Line 672
>     the such confidentiality is provided.  Additional advice on
>     certificate pinning is present in [RFC6125].
> Wow, we have a terrible name clash here, because we also have HPKP which
> everyone calls "pinning". I see 6125 calls it that, so maybe on first use (S
> 5.3) can you please differentiate from HPKP

added a note about this.
> Line 679
>     TLS handshake unless the server requests one and the client has
>     determined the certificate can be safely used with that specific
>     server, OR the client has been explicitly configured by the user to
> Can you note that this is just a restatement of the rules in TLS?

I attempted to fix this as well as the next item, though it's possible 
I'm still missing something:

     MUAs MAY implement client certificate authentication on the
     Implicit TLS port.  An MUA MUST NOT provide a client certificate
     during the TLS handshake unless the server requests one AND at
     least one of the following is true: the client has determined the
     certificate can be safely used with that specific server, OR the
     client has been explicitly configured by the user to use that
     particular certificate with that server. How to make this
     determination is presently implementation specific. (The
     requirement that the server request a certificate is just a
     restatement of the TLS protocol rules, e.g.
     <xref target="RFC5246"/> section 7.4.6.  The requirement that the 
client
     not send a certificate not known to be acceptable to the server is
     pragmatic in multiple ways: the current TLS protocol provides no 
way for
     the client to know which of potentially multiple certificates it
     should use; also, when the client sends a certificate it is
     potentially disclosing its identity (or its user's identity) to
     the server - perhaps unnecessarily and for no useful purpose.
> Line 681
>     server, OR the client has been explicitly configured by the user to
>     use that particular certificate with that server.  How to make this
>     determination is presently implementation specific.
> The structure of this text is confusing. The rule is:
>
> if (server asked &&
>      (client determined safe || certificate configured)) {
>      can use
> } else {
>      can't use
> }

reworded text in -10 to try to make this clearer.

> Line 781
>     or interception; this is not intended to mitigate active attackers
>     who have compromised service provider systems.
> IMPORTANT: Client auth with TLS 1.2 reveals the user's identity. This is a
> privacy issue, and so we need to note it. The options here are not great with <
> 1.3 because renegotiation is also bad, so I'm not suggesting a normative
> change, but I think the doc needs to be clear.

Added a paragraph:

Implementors should be aware that use of client certificates with TLS
1.2 likely reveals the user's identity to the server and therefore may
compromise the user's privacy.  There seems to be no easy fix other
than to avoid presenting client certificates except when specifically
configured to do so.
>
> Line 959
>     in RFC 6186 resolves that critique for email.  The second bullet is
>     correct as well, but not very important because useful deployment of
>     security layers other than TLS in email is small enough to be
> The second bullet is less correct than it used to be because we no longer
> support export suites. Ordinarily I wouldn't bother to make this point, but if
> we're revisiting this point by point, I think we should note that.

added a note for this.

Keith