Original text:
The signatures generated with this API cover the context string "Exported Authenticator" and therefore cannot be transplanted into other protocols. 

Reviewer comments:
In the TLS 1.3 handshake the client and server do not agree on the client's certificate bytes until after the first server application message. If this is significant to an application, then the server should send an application message before authenticating the client with an EA.
Done.
This is useful information to convey to implementers and users. +1
NAME the message that started this all is in this . It's the second issue.
NAME To follow up from the meeting, this is only relevant in the case where the server has requested a client certificate, which is why you can't predict the client . For the Server to send an NST it must therefore have received the entire handshake. Changing this to require the Server to wait for the end of the handshake in not client certificate situations seems a touch heavy handed.
NAME the text (as shown in the session) says that the server sends something though. But the server can do that without seeing the client certificate. I think that this needs to be something different, perhaps "the server has received and validated the client's Finished".
I'm still trying to work out what threat we are considering here. Is it the server pretending to read the Finished and not or is it the server making an error and not reading the Finished. In the former case, it's not sufficient to send the NST because the client doesn't know what RMS the server has computed. In the latter case, why not just tell the server not to send a request until it's read Finished if there is a client cert?
At the end of a mutually authenticated handshake the client doesn't know that the server has agreed on the ClientCertificate's bytes. The threat is that the client's last message might have been corrupted in flight (intentionally or otherwise). I was worried that if we put the text "MUST verify the Finished before sending an EA." implementations might ignore it, so the goal was to make the check something externally visible. I think you're right that the current text doesn't achieve the security goal. I can put "MUST verify the Finished" if that works better.
I think that's the only thing that works.
NAME Just checking to see when this might be updated?
Changes pushed.
NAME Does the final change address your issue?
[Clarifying] The traffic keys do not depend on the client's final flight at all. So, I don't see what the "first application message" does here. The client receiving the server's messages doesn't tell the client anything, and receipt of the client's first application data isn't really relevant here. It's the Finished that matters.
Maybe something about how we rely on the server to not send application data if the client Finished fails to validate (and thus for application data to not be exchanged in case of tampering), rather than talking about "agree on the client's final flight"? That lines up nicely with the new MUST-level requirement to not send EA if the client Finished doesn't validate. So something like "In TLS 1.3 the agreement on the client's final flight is enforced solely by the server. The client Finished hash incorporates the rest of the transcript, which the server uses as an integrity check over the entire transcript, including the client's final flight." and then tweak the last sentence to include a clause about "in order to ensure that the client's final flight has been validated, servers MUST ..."
NAME NAME can you please coordinate and come to a resolution that addresses the remaining comments in this thread? In particular, I think we're seeking clarity on the conditions for agreement and how that relates to the client's finished message (and not necessarily application data, though that may imply agreement, if I'm understanding the intended text here).
What about 0.5RTT?
NAME NAME can we please get an update here? Would additional editors here help the process?
Yes, the server can send 0.5RTT before the handshake completes. That's why my focus was on "we rely on the server", as the client doesn't always have a great way of telling for sure. The application protocol's semantics might be able to clue that a given response was 1-RTT and not 0.5-RTT, but it might not, too. I probably should have said "close the connection" rather than "not send application data", or maybe "not continue to send application data".
LGTM.
What do people think about putting "before sending an EA or processing a received EA." instead of "before sending an EA"? I realised it's possible for the server to receive an EA before the client Finished as follows: Server sends its Finished message and then immediately an AR. Client receives the Finished, sends its own Finished, and then an EA in response to the AR. The client Finished and EA are reordered by the network (the EA might not even be going over the TLS transport). The server might then accept the EA and do some action, but then reject the Finished message. I think this is already incorrect by the definition of EAs, but I thought it wouldn't hurt to make it super explicit. NAME
Requesting changes to resolve remaining comments. Hello TLSWG and IESG reviewers, This is a compendium of responses to the various reviews of the document. There are a few remaining open questions to address that I hope we can resolve in this thread. I’ve compiled the changes to the document in response to the comments in Github: URL Responses welcome! Open questions for debate Should we define an optional API to generate the certificaterequestcontext? [ns] Proposed solution: no, let the application decide Should we remove the SHOULDs in Section 7, as they have little to do with interoperability? [ns] Proposed solution: leave the text as is. Are there any security issues caused by the fact that the Exported Authenticator is based on the Exporter Secret, which does not incorporate the entire transcript: Using an exporter value as the handshake context means that any client authentication from the initial handshake is not cryptographically digested into the exporter value. In light of what we ran into with EAP-TLS1.3, do we want to revisit whether we're still happy with that? We should in practice still benefit from the implicit confirmation that anything the server participates in after receiving the Client Finished means the server properly validated it and did not abort the handshake, but it's a bit awkward, especially since the RFC 8446 post-handshake authentication does digest the entire initial handshake transcript. Would we feel any safer if an exporter variant was available that digested the entire initial handshake transcript and we could use that? [ns] This is a very good question. I didn’t follow the EAP-TLS1.3 issue closely, but this does seem to imply an imperfect match with post-handshake authentication in the case of mutually authenticated connections. I would like to suggest that Jonathan Hoyland (cc'd), who did the formal analysis on the protocol, provide his input. A comment from Yaron Scheffer/Benjamin Kaduk on the IANA considerations: My understanding is that the intent of this document is to only allow "servername" to appear in the ClientCertificateRequest structure that otherwise uses the "CR" notation in the TLS 1.3 column of the TLS ExtensionType Values registry, but not to allow for "servername" in authenticator CertificateRequests or handshake CertificateRequests. Assuming that's correct, the easiest way to indicate that would be if there was a "Comment" column in the registry that could say "only used in ClientCertificateRequest certificate requests and no other certificate requests", but there is not such a column in the registry. I think it could also work to be much more clear in the IANA considerations of this document as to why the "CR" is being added and what restrictions there are on its use, even if that's not quite as clean of a solution. [ns] Proposed solution: Add text to section 8.1 to clarify why the CR is being added and what restrictions there are to its use, as is done in URL Alternate solution: Propose a new column, as done in URL Comments and responses Proposals to address IESG comments below are here: URL Martin Duke (No Objection): I come away from this not being entirely sure if this document applies to DTLS or not. There is the one reference in the intro to DTLS, but it's not in the abstract, nor anywhere else. Assuming that the Sec. 1 reference is not some sort of artifact, the document would benefit from a liberal sprinkling of 's/TLS/(D)TLS' (but this would not apply to every instance) [ns] DTLS does not explicitly define a post-handshake authentication, so not all references made sense to update, but I updated all the relevant references. If (D)TLS 1.2 is REQUIRED to implement, then does this document not update those RFCs? [ns] It doesn’t change existing versions of (D)TLS and it is not mandatory to include this functionality, so I don’t think that it updates them. It’s a separate and optional feature built on top of TLS. In any case, the text is changed to make it explicit that this is a minimum requirement. NITS: Sec 1. I think you mean Sec 4.2.6 of RFC 8446, not 4.6.3. [ns] It’s actually neither, it’s 4.6.2. Sec. 4.2.6 defines the extension that the client uses to advertise support for post-handshake authentication. Sec 4 and 5. "use a secure with..." ? [ns] Added “transport” Sec 4. s/messages structures/message structures [ns] Fixed Erik Kline's No Objection COMMENT: [ sections 4 & 5 ] "SHOULD use a secure with" -> "SHOULD use a secure communications channel with" or "SHOULD use a secure transport with" or something? [ns] Fixed in previous change "as its as its" -> "as its" [ns] Fixed [ section 5.2.3 ] I think the first sentence of the first paragraph may not be a grammatically complete sentence? [ns] Fixed [ section 7 ] "following sections describes APIs" -> "following sections describe APIs" [ns] Fixed Francesca Palombini's No Objection Thank you for the work on this document, and thank you to the doc shepherd for the in-depth background. Please find some comments below. Francesca TLS (or DTLS) version 1.2 [RFC5246] [RFC6347]. or later are REQUIRED to implement the mechanisms described in this document. FP: Does this mechanism applies to DTLS as well? The sentence above, the normative reference to DTLS 1.2, and the IANA section 8.2 would imply so. However, this is the only time DTLS is mentioned in the body of the document, excluding IANA considerations. I would like it to be made explicit that this mechanism can be used for DTLS (if that's the case) both in the abstract and in the introduction, add any useful considerations about using this mechanism with DTLS vs TLS, and possibly add a sentence in the introduction stating that DTLS uses what specified for TLS 1.X. Two examples of why that would be useful are the following sentences: using an exporter as described in Section 4 of [RFC5705] (for TLS 1.2) or Section 7.5 of [TLS13] (for TLS 1.3). For TLS 1.3, the "signaturealgorithms" and "signaturealgorithmscert" extension, as described in Section 4.2.3 of [TLS13] for TLS 1.3 or the "signaturealgorithms" extension from Sections 7.4.2 and 7.4.6 of [RFC5246] for TLS 1.2. which makes it clear what applies for TLS but not for DTLS. [ns] Fixed in previous change used to send the authenticator request SHOULD use a secure with equivalent security to TLS, such as QUIC [QUIC-TLS], as its as its FP: What are the implications of not using such a secure transport protocol? Why is it just RECOMMENDED and not MANDATED? nits: missing word "use a secure with" ; remove one of the duplicated "as its". (Note: this text appears again with the same typos for the authenticator in section 5) [ns] There are no known security implications of using an insecure transport here, just privacy implications. This point was debated on list (see Yaron Sheffer’s comments) and it was agreed that because there may be alternate deployment scenarios that involve transport outside the original tunnel, TLS-equivalent security is not a mandated requirement. Typos fixed in previous change. extensions (OCSP, SCT, etc.) FP: Please consider adding informative references. [ns] Added Zaheduzzaman Sarker's No Objection Thanks for the work on this document. I found it well written and I have minor comments and Nits Comment : As this document asked for a IANA registration entry with DTLS-OK, hence this mechanism is OK to be used with DTLS. I understand the heavily references to TLS 1.3 as it relay on the mechanisms described there. However, I found it odd not find any reference to DTLS1.3 (we had it on the last formal IESG telechat, it is quite ready to be referenced). Is this intentional? is it supposed to be that this mechanism defined in this document on can be used with DTLS1.2? [ns] This was a common concern, the text has been updated. Section 7.3 & 7.4: is "active connection" defined somewhere? it would be good if some descriptive texts are added for clarification as done for the other bullets in the same list. [ns] Wording adjusted. For the API considerations I was expecting a API to generate the certificaterequestcontext. [ns] An API would likely just return a random number, which isn’t very useful. Not defining such an API gives the application freedom to choose to use this value in some other way. I don’t have strong opinions either way. Nits: Post-handshake authentication is not defined in section 4.6.3 of TLS 1.3 [ns] Updated in prior change. Section 4 & 5: likely copy paste error -- s/as its as its/as its [ns] Updated in prior change. Lars Eggert's No Objection All comments below are very minor change suggestions that you may choose to incorporate in some way (or ignore), as you see fit. There is no need to let me know what you did with these suggestions. Section 1, paragraph 9, nit: TLS (or DTLS) version 1.2 [RFC5246] [RFC6347]. or later are REQUIRED - - TLS (or DTLS) version 1.2 [RFC5246][RFC6347] or later are REQUIRED [ns] Updated Éric Vyncke's No Objection Thank you for the work put into this document. Please find below some non-blocking COMMENT points (but replies would be appreciated), and some nits. I hope that this helps to improve the document, Regards, -éric == COMMENTS == I am trusting the WG and the responsible AD for ensuring that the specific section in references are the correct ones. -- Section 2 -- Unsure where the "terminology" part is... Also, should "Client" and "Server" be defined here ? Reading section 3, I am still unsure whether the "client" and "server" are the "TLS client" and "TLS server" ... In the same vein, a small figure with all parties involved would be welcome by the reader. [ns] Terminology updated to reference the terms in TLS 1.3. There are only two parties in this protocol, so I’m not sure a diagram will help. == NITS == In some places, 'TLS13' is used as an anchor rather than 'RFC8446'. [ns] Corrected. Sometimes, the document uses 'octet' and sometimes 'bytes'. [ns] Interesting. This is likely an oversight due to RFC 8446’s use of both terms. I’ve adjusted the text to try to be more consistent with that document (octets for on-the-wire, bytes for string values). -- Section 1 -- Spurious '.' in the last §. [ns] Fixed in previous change. -- Section 4 -- "as its as its" ? [ns] Fixed in previous change. Ben Kaduk's IESG review + Yes You can view, comment on, or merge this pull request online at: URL Commit Summary Editorial suggestions from IESG review COMMENT: I've made some (mostl) editorial suggestions in a github PR: URL Despite the overall YES ballot, I do have some substantive comments that may be worth some further consideration. Do we want to give any insight or examples into how an application might choose to use the certificaterequestcontext? [ns] I put a proposed line here. Thanks to Yaron Sheffer for the multiple secdir reviews. My understanding is that the intent of this document is to only allow "servername" to appear in the ClientCertificateRequest structure that otherwise uses the "CR" notation in the TLS 1.3 column of the TLS ExtensionType Values registry, but not to allow for "servername" in authenticator CertificateRequests or handshake CertificateRequests. Assuming that's correct, the easiest way to indicate that would be if there was a "Comment" column in the registry that could say "only used in ClientCertificateRequest certificate requests and no other certificate requests", but there is not such a column in the registry. I think it could also work to be much more clear in the IANA considerations of this document as to why the "CR" is being added and what restrictions there are on its use, even if that's not quite as clean of a solution. [ns] I agree that this is an open question. Section 1 multiple identities - Endpoints that are authoritative for multiple identities - but do not have a single certificate that includes all of the identities - can authenticate additional identities over a single connection. IIUC this is only new functionality for server endpoints, since client endpoints can use different certificates for subsequent post-handshake authentication events. [ns] Correct. TLS (or DTLS) version 1.2 [RFC5246] [RFC6347]. or later are REQUIRED to implement the mechanisms described in this document. Please clarify whether this is a minimum TLS version required for using EAs, or that this is a binding requirement on implementations of the named protocol versions. (If the latter is intended we may have some discussions about an Updates: tag...) [ns] I meant “minimum version of 1.2” and have changed the text to reflect this. Section 5.1 Using an exporter value as the handshake context means that any client authentication from the initial handshake is not cryptographically digested into the exporter value. In light of what we ran into with EAP-TLS1.3, do we want to revisit whether we're still happy with that? We should in practice still benefit from the implicit confirmation that anything the server participates in after receiving the Client Finished means the server properly validated it and did not abort the handshake, but it's a bit awkward, especially since the RFC 8446 post-handshake authentication does digest the entire initial handshake transcript. Would we feel any safer if an exporter variant was available that digested the entire initial handshake transcript and we could use that? [ns] Addressed in the open question section above. Section 5.2.1 Certificates chosen in the Certificate message MUST conform to the requirements of a Certificate message in the negotiated version of TLS. In particular, the certificate chain MUST be valid for the signature algorithms indicated by the peer in the "signaturealgorithms" and "signaturealgorithmscert" extension, as described in Section 4.2.3 of [TLS13] for TLS 1.3 or the "signaturealgorithms" extension from Sections 7.4.2 and 7.4.6 of [RFC5246] for TLS 1.2. This seems awkward. Do "the requirements of a Certificate message in the negotiated version of TLS" include the actual message structure? Because the rest of the document suggests that we always use the TLS 1.3 Certificate, but this line would set us up for using a TLS 1.2 Certificate. [ns] They do not include the actual message structure. The constraints are about the certificatelist. I’ve updated the text. START Also, per §1.3 of RFC 8446, "signaturealgorithmscert" also applies to TLS 1.2, so the last sentence seems incorrect or at least incomplete. [ns] Updated Section 5.2.1 Otherwise, the Certificate message MUST contain only extensions present in the TLS handshake. Unrecognized extensions in the authenticator request MUST be ignored. At least the first sentence seems redundant with the previous section (but I did not propose removing this in my editorial PR). The second sentence arguably follows from the RFC 8446 requirements, though I don't mind mentioning it again as much as I do for the first sentence. [ns] I don’t mind being explicit here considering there was some confusion about this previously. Section 5.2.2 The authenticator transcript is the hash of the concatenated Handshake Context, authenticator request (if present), and Certificate message: Hash(Handshake Context Certificate) Where Hash is the authenticator hash defined in section 4.1. If the authenticator request is not present, it is omitted from this construction, i.e., it is zero-length. This construction is injective, because the handshake messages start with a message HandshakeType. Should we say so explicitly in the document? [ns] I’m not sure I understand what injective means in this context or how it helps the reader. If the party that generates the exported authenticator does so with a different connection than the party that is validating it, then the Handshake Context will not match, resulting in a CertificateVerify message that does not validate. This includes situations in which the application data is sent via TLS-terminating proxy. Given a failed CertificateVerify validation, it may be helpful for the application to confirm that both peers share the same connection using a value derived from the connection secrets before taking a user-visible action. Is it safe to reuse the Handshake Context itself as the "value derived from the connection secrets"? [ns] Yes, it should be safe. Updated. Section 7.4 It returns the identity, such as the certificate chain and its extensions, and a status to indicate whether the authenticator is valid or not after applying the function for validating the certificate chain to the chain contained in the authenticator. I strongly recommend not returning the identity in the case when validation fails. [ns] Fair. Updated. The API should return a failure if the certificaterequest_context of the authenticator was used in a previously validated authenticator. Is the intent for this to be a "distinct previously validated authenticator" (i.e., that the API returns the same value when given the same inputs subsequently)? That does not seem to be the meaning conveyed by the current text. [ns] Yes, this is about distinct validators. Updated. Section 8.2 IANA is requested to add the following entries to the registry for Exporter Labels (defined in [RFC5705]): "EXPORTER-server authenticator handshake context", "EXPORTER-client authenticator finished key" and "EXPORTER-server authenticator finished key" with Don't we also need "EXPORTER-client authenticator handshake context"? [ns] Yes, added. Section 9 I think we should more explicitly incorporate the TLS 1.3 security considerations by reference. [ns] Yes. Section 11.1 I don't think [QUIC-TLS] or RFCs 7250 and 7540 are normative references. [ns] Updated Murray Kucherawy's No Objection Why are the SHOULDs in Section 4 only SHOULDs? Why might an implementer reasonably not do what this says? [ns] Addressed after discussion on list and in reply above. Similarly, the SHOULDs in Section 7 seem awkward, as they have little to do with interoperability. [ns] It’s true that this is about interoperability. I’m open to changing these to lower-case shoulds since they reflect implementation recommendations rather than interoperability. Nick

Revised text:
The signatures generated with this API cover the context string "Exported Authenticator" and therefore cannot be transplanted into other protocols. <ins> In TLS 1.3 the client can not explicitly learn from the TLS layer whether its Finished message was accepted.  Because the application traffic keys are not dependent on the client's final flight, receiving messages from the server does not prove that the server received the client's Finished.  To avoid disagreement between the client and server on the authentication status of EAs, servers MUST verify the client Finished before sending an EA or processing a received EA. </ins>