huitema
commented
5 years ago On 9/26/2019 11:38 AM, Roman Danyliw wrote:
Hi Christian!
Thanks for all of the updates. I have a remaining items are described inline.
To bring up a new item, there was new text introduced in -06 of Section 5 to which I strongly object. Specifically:
"Replacing clear text SNI transmission by an encrypted variant will
also thwart MITM interferences that are sometimes described as
legitimate. As explained in Section 2.3, alternative solutions will
have to be developed.”I read this paragraph as addressing the operational practices outlined in Section 2.1. I think it is inappropriate to refer to some of these operational practices as being "sometimes described as legitimate".
Anything performed by MITM is by definition controversial. But I get your point. How about
"Replacing clear text SNI transmission by an encrypted variant will break or reduce the efficacy of the operational practices and techniques implemented in middle-boxes as described in Section 2.1. As explained in Section 2.3, alternative solutions will have to be developed."
-----Original Message----- From: Christian Huitema [mailto:huitema@huitema.net] Sent: Wednesday, September 25, 2019 3:47 PM To: Roman Danyliw rdd@cert.org; The IESG iesg@ietf.org Cc: draft-ietf-tls-sni-encryption@ietf.org; tls-chairs@ietf.org; tls@ietf.org Subject: Re: [TLS] Roman Danyliw's No Objection on draft-ietf-tls-sni- encryption-05: (with COMMENT)
Hello Roman,
A lot of the fixes that you suggested are incorporated in the draft-07 that was just released. I think the last version addresses your concerns, but you may of course want to verify.
On 9/25/2019 7:27 AM, Roman Danyliw wrote:
Hi Christian!
Thanks for the detailed responses and the helpful background. Below are a number of proposed text block replacements to clarify my intent (instead of more questions). Roman
-----Original Message----- From: iesg [mailto:iesg-bounces@ietf.org] On Behalf Of Christian Huitema Sent: Wednesday, September 18, 2019 10:14 PM To: Roman Danyliw rdd@cert.org; The IESG iesg@ietf.org Cc: draft-ietf-tls-sni-encryption@ietf.org; tls-chairs@ietf.org; tls@ietf.org Subject: Re: [TLS] Roman Danyliw's No Objection on draft-ietf-tls-sni- encryption-05: (with COMMENT)
Thanks for the feedback, Roman. Comments in line.
On 9/18/2019 4:40 AM, Roman Danyliw via Datatracker wrote:
** Section 1. Per “More and more services are colocated on multiplexed servers, loosening the relation between IP address and web service”, completely agree. IMO, unpacking “multiplexed servers” is worthwhile to explain the subsequent text because it motivates the loss of visibility due to encryption with network only monitoring. “Multiplex’ happens at two levels: -- co-tenants (e.g., virtual hosting) – multiple services on the same server (i.e., an IP/port doesn’t uniquely identify the service)
-- cloud/cdn – a given platform hosts the services/servers of a lot of organization (i.e., looking up to what netblock an IP belongs reveals little) OK, will try to incorporate your text. Thanks. Changes incorporated in first paragraph of section 1. The text -07 works for me. Thanks for adding this extra bit.
** Section 2.1. Per “The SNI was defined to facilitate management of servers, though the developers of middleboxes soon found out that they could take advantage of the information. Many examples of such usage are reviewed in [RFC8404].”,
-- Can’t middleboxes also help facilitate the management of servers? This text seems to take a particular view on middleboxes which doesn't seem appropriate.
It is pretty clear that the load balancer in front of a server farm will need access to the service ID, and must be able to retrieve the decrypted SNI. There may be other examples, such as DoS mitigation boxes. The "unanticipated usage" comes typically from middle-boxes that are not in the same management domain as either the client or the server. Is there an established way to designate those? I'm not sure I understand the original of the requirement that the client and server being in the same management domain. RFC3546's definition of SNI opens with: [TLS] does not provide a mechanism for a client to tell a server the name of the server it is contacting. It may be desirable for clients to provide this information to facilitate secure connections to servers that host multiple 'virtual' servers at a single underlying network address.
It seems to me that if we are trying to channel original intent, then only the virtual server use case applies. I'd propose: OLD The SNI was defined to facilitate management of servers, though the developers of middleboxes soon found out that they could take advantage of the information. Many examples of such usage are reviewed in [RFC8404]. NEW The SNI was defined to facilitate secure connections to servers that host multiple 'virtual' servers at a single underlying network address [RFC3546]. However, addition management and security practices emerged making use of this information. Examples of such usage are reviewed in [RFC8404]. This language would let you distinguish all of the middle box behaviors done by operators and enterprises from a possible [RFC7258] attacker. As I noted in my reply to Ben, I don't follow the current language for anticipated/unanticipated.
https://mailarchive.ietf.org/arch/msg/tls/54cEpjUcIZqNW_y7Eb344XLQp0Y
-- RFC8404 describes a number of middlebox practices, but only Section 6.2 explicitly discusses SNI, and of the examples list here, only one comes from RFC8404. A few of the examples also come in the "deep packet inspection" sections of 8404. But rather than going in a long discussion, I would rather rewrite the sentence as: Many examples of such usage are reviewed in [@?RFC8404], other examples came out during discussions of this draft. The -07 text works for me. Thanks.
** Section 2.1. The “monitoring and identification of specific sites” isn’t symmetric to the other examples – it is rather generic. The other examples, identify a what/who (e.g., ISP, firewall) + action (e.g., block, filter). Also, to implement most of the other example, “monitoring and identification of specific sites” needs to be done. I still think this needs to be cleaned up in some way. IMO, I'd drop it. Was rewritten in new section 2.1 as "Filtering or censorship of specific services for a variety of reasons." I'm still struggling with the specificity here and the overlap. Dropping the censorship and it reads "filtering of specific services for a variety of reasons". That's exactly what bullet 2 (content filtering by operators) and bullet 3 (enterprise firewalls for NSFW)
What's unique about this bullet -- is it who is doing this filter/censorship? where it is happening? What tech is being used?
For example, arbitrary filtering of web sites competing with other services preferred by the network provider. Some filtering and censorship is more controversial than others. But I don't think that listing every controversial usage would be appropriate. I would rather keep the somewhat vague phrasing than try to have a debate about which practice is controversial and which is not.
....
** Section 2.1. Per “The SNI is probably also included in the general collection of metadata by pervasive surveillance actors”, I recommend against speculation and instead simply stating that SNI would be interesting meta-data for a RFC7258 attacker. Yes, Mirja made a similar comment. Proposed replacement:
The SNI is probably also included in the general collection of metadata by pervasive surveillance actors, for example to identify services used by surveillance targets. IMO, explicitly linking it to the draft would help.
OLD: The SNI is probably also included in the general collection of metadata by pervasive surveillance actors, for example to identify services used by surveillance targets.
NEW: The SNI could be included in the general collection of metadata by pervasive monitoring attacker [RFC7258], for example to identify services used by surveillance targets. I missed the reference to 7258. Will add it in the next version. Recommend adding the RFC7258 reference. As I noted above, the semantics of "probably" vs. "could". "probably" suggests a confidence to me.
Yes, there is some confidence, based on the Snowden revelation of systems like Quantum Insert, comments like "recording the whole haystack", or the Korean regulation explicitly targeting the SNI field. It is more probable than not that SNI data is being recorded, and I think that "probably" is just fine.
** Section 2.2. Per “One reason may be that, when these RFCs were written, the SNI information was available through a variety of other means”, what would those “other means” be? The list includes at a minimum:
Clear text exchanges amenable to deep packet inspection (DPI), server certificates send in clear text during TLS/SSL exchanges, DNS names of servers in clear text DNS queries, and server specific IP addresses in packet headers.
I guess I could write that all, but it makes the text a bit redundant, since the following paragraphs do discuss server certificates, DNS names and IP addresses. I understand. I didn't read it that way. My recommendation isn't to describe the "other means" (as it is described below), but to be clear on the obvious, what is the SNI information. OLD: One reason may be that, when these RFCs were written, the SNI information was available through a variety of other means.
NEW: One reason may be that when the RFCs were written, the name of the server the being contacted by the client (i.e., the SNI) was evident through other means. The new text in -07 reads more clearly to me. Thanks for this change.
** Section 2.3. Per “Deploying SNI encryption will help thwarting most of the ‘unanticipated’ SNI usages described in Section 2.1, including censorship and pervasive surveillance.”:
-- Why the quotes around "unanticipated" SNI usage? Removing the quotes. Otherwise, you will be convinced that the authors believe that all middle-boxes are the spawn of the devil... Thanks.
-- One person’s censorship is another person’s threat mitigation, policy enforcement for a network they own, or parental controls (per the list in Section 2.1) – recommend being more precise on the order of “Deploying SNI encryption will {break | reduce the efficacy of } the operational practices and techniques used in middleboxes described in Section 2.1”. OK. I will try to make the text just stick to the facts:
Deploying SNI encryption thwarts most of the unanticipated SNI usages described in (#snileak). It reduces the efficacy of the operational practices and techniques implement in middle-boxes. Most of these functions can, however, be realized by other means. Thanks for this.
Works for me. However, I'd drop "Most of these functions can, however, be realized by other means" because this opens the debate on how exactly, etc. I still recommend dropping the "other means" text because that opens debate.
** Section 2.3. Per “It will also thwart functions that are sometimes described as legitimate”, what functions are those? I’d recommend eliminating this sentence as it reads like a value judgement on existing practices (which doesn’t seem germane for discussing requirements). Now that censorship got added to the list in Section 2.1, you likely want to:
s/including censorship and pervasive surveillance/including pervasive surveillance/
Otherwise, LGTM. Thanks.
OK.
.... ...
"This document does not present the design of a solution, but provides guidelines for evaluating proposed solutions." However, the current text in Section 4 is explicitly states it is providing a solution. The sub-section of Section 4.x assume the solution in Section 4.0 and describe the follow-on work. Section 2 - 3 do lay out the means for evaluation nicely. Perhaps, something on the order of:
OLD: This document does not present the design of a solution, but provides guidelines for evaluating proposed solutions. NEW: This provides guidelines on evaluating solution and proposes an architecture to mitigate the threats created by an unencrypted SNI using existing approaches.
I added the reference to a paper by Fified et al. that describes the "domain fronting" solution and some of its deployments.
Note that the intent is not to recommend the domain fronting solution. That solution, and the HTTPS tunnel solution, are examined as example of proposed solutions that meet some of the requirement but not all. But I understand that you perceived the text as some kind of endorsement.
....
Draft 07 added text describing actual deployments. The use of the [domfront] citation works for me and addresses my concerns. Two nits:
I'd recommend using the URL to the paper from conference site itself: https://petsymposium.org/2015/papers/03_Fifield.pdf OK. I'd also recommend adding a sentence to the last paragraph of Section 1, "This document does not present the design ...", to foreshadow that you'll discuss an alternative approach even if encrypted SNI isn't realized yet.
OK, will try make clear that this is not an endorsement or a proposal.
-- Christian Huitema
Roman Danyliw has entered the following ballot position for draft-ietf-tls-sni-encryption-05: No Objection
When responding, please keep the subject line intact and reply to all email addresses included in the To and CC lines. (Feel free to cut this introductory paragraph, however.)
Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html for more information about IESG DISCUSS and COMMENT positions.
The document, along with other ballot positions, can be found here: https://datatracker.ietf.org/doc/draft-ietf-tls-sni-encryption/
COMMENT:
** Section 1. Per “More and more services are colocated on multiplexed servers, loosening the relation between IP address and web service”, completely agree. IMO, unpacking “multiplexed servers” is worthwhile to explain the subsequent text because it motivates the loss of visibility due to encryption with network only monitoring. “Multiplex’ happens at two levels:
-- co-tenants (e.g., virtual hosting) – multiple services on the same server (i.e., an IP/port doesn’t uniquely identify the service)
-- cloud/cdn – a given platform hosts the services/servers of a lot of organization (i.e., looking up to what netblock an IP belongs reveals little)
** Section 2.1. Per “The SNI was defined to facilitate management of servers, though the developers of middleboxes soon found out that they could take advantage of the information. Many examples of such usage are reviewed in [RFC8404].”,
-- Can’t middleboxes also help facilitate the management of servers? This text seems to take a particular view on middleboxes which doesn't seem appropriate.
-- RFC8404 describes a number of middlebox practices, but only Section 6.2 explicitly discusses SNI, and of the examples list here, only one comes from RFC8404.
** Section 2.1. The “monitoring and identification of specific sites” isn’t symmetric to the other examples – it is rather generic. The other examples, identify a what/who (e.g., ISP, firewall) + action (e.g., block, filter). Also, to implement most of the other example, “monitoring and identification of specific sites” needs to be done.
** Section 2.1. Why is parental controls in quotes? In RFC8404, it is not. The quotes could be read as a judgement on the practice.
** Section 2.1. Per “The SNI is probably also included in the general collection of metadata by pervasive surveillance actors”, I recommend against speculation and instead simply stating that SNI would be interesting meta-data for a RFC7258 attacker.
** Section 2.2. Per “One reason may be that, when these RFCs were written, the SNI information was available through a variety of other means”, what would those “other means” be?
** Section 2.3. Per “Deploying SNI encryption will help thwarting most of the ‘unanticipated’ SNI usages described in Section 2.1, including censorship and pervasive surveillance.”:
-- Why the quotes around "unanticipated" SNI usage?
-- One person’s censorship is another person’s threat mitigation, policy enforcement for a network they own, or parental controls (per the list in Section 2.1) – recommend being more precise on the order of “Deploying SNI encryption will {break | reduce the efficacy of } the operational practices and techniques used in middleboxes described in Section 2.1”.
** Section 2.3. Per “It will also thwart functions that are sometimes described as legitimate”, what functions are those? I’d recommend eliminating this sentence as it reads like a value judgement on existing practices (which doesn’t seem germane for discussing requirements).
** Section 3. Per “Over the past years, there have been multiple proposals to add an SNI encryption option in TLS.”, can these past proposals be cited so future readers can learn from them.
** Section 3.4. The existence of designs were alluded to but not cited. Be specific with citation.
** Section 3.7.1. The rational for including this discussion about ALPN isn’t clear as it doesn’t suggest new requirements for SNI encryption.
** Section 4. I got hung-up on the description of Section 4 describing a “solution”. Is Section 4 (and the related subsections) describing an operational practice or a notional reference architecture? The text reads one part “people are doing” and another part “people could do”.
** Section 4. Per “In the absence of TLS-level SNI encryption, many sites rely on an "HTTP Co-Tenancy" solution”, this seems like a strong of a statement about utilization of this architecture explicitly to hide the hidden.example.com SNI. Can you provide a citation for a sense penetration.
** Section 4. Per the bullet “since this is an HTTP-level solution”, I recommend citing that it fails on the requirement identified in Section 3.7 (instead of enumerating a list of protocols)
** Section 4. The opening of this section noted that “many sites” rely on the architecture described in this section. Later, it is noted that “a browser extension that support[s] HTTP Fronting” is a necessary architecture component. Can a few citations be made to the popular extensions.
** Section 4.2. Per "to reach example hidden.example.com, use fake.example.com as a fronting server", shouldn’t this read: “to reach hidden.example.com, use fake.example.com as a fronting server"?
** Editorial Nits -- Section 2. Typo. s/mutiple/multiple.
-- Section 2.1. Typo. s/fradulent/fraudulent/
-- Section 3.1. “Hidden Service” is capitalized in a few places like it is a proper noun? Is it meant to be? If so, define or cite it.
-- Section 3.6. s/the the/
-- Section 4.1. s/tunnelling/tunneling/
-- Section 4.2. s/ferver/server/
-- Section 7. s/Martin Rex Martin Thomson/Martin Rex, Martin Thomson/
TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls