(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
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