[messaging] Opportunistic encryption and authentication methods

Tom Ritter tom at ritter.vg
Thu Sep 4 06:17:12 PDT 2014

I don't really have anything to add, but ++, especially on the stealthiness.

I started wondering if an in-band SMP mechanism could work, but
obviously that would be detectable by the person doing an attack.
Perhaps an out-of-band SMP mechanism -bu I'm having a hard time
imagining one that would be simpler than just exchanging fingerprints.
(Unless you don't trust the _second_ channel either, which would be
perfectly reasonable if it was, say, XMPP over your email provider.)


On 3 September 2014 18:37, Trevor Perrin <trevp at trevp.net> wrote:
> We've discussed methods for distributing and authenticating
> public-keys in email-like messaging.  I'll argue that "Opportunistic
> Encryption" (OE) might be a good approach to this type of secure
> messaging, and evaluate authentication methods in that light.
> Background on Opportunistic Encryption
> ------
> OE is an old concept, and people have different ideas about what it
> means [1,2].  My take on the core ideas:
>  * Authenticating a public key is harder than distributing it.
>  * Thus authentication and encryption should be decoupled, so that
> encryption can be deployed on a wide scale even without
> authentication.
> This has traditionally been controversial:  An
> encrypted-but-not-authenticated connection is vulnerable to active
> attack, so OE might not be worth much.  It might even have negative
> value if it gives a false sense of security.
> On the other hand:
> 1) There may be value to resisting large-scale passive eavesdropping
> if switching to large-scale active attack is costly.
> 2) OE provides a foundation on which authentication can be added (e.g.
> TOFU, fingerprints) [3].
> 3) A small number of users performing "stealthy" authentication could
> protect other users by creating uncertainty about which connections
> can be undetectably attacked [4].
> This debate has played out different ways in different protocols.  For example:
>  * STARTTLS between mail servers generally uses OE, and has some good
> deployment between large providers [5].  People are thinking about how
> to add authentication [6,7].
>  * HTTPS is a non-OE protocol.  OE for HTTP (not HTTPS) is being proposed [8,9].
> OE for email-like messaging
> ------
> There's another argument for OE in the person-to-person case:
> 4) In the absence of widespread OE, users who publish their public key
> and encrypt conversations will draw unwanted attention.
> There's a new argument *against* widespread OE in the asynchronous
> messaging case:  A key directory might get out of sync with a user,
> and return a public key that the user has (for example) lost the
> private key for.
> I'll contend that 1-4 make a good case for widespread OE, and the risk
> of messages encrypted to an out-of-sync public key is manageable:
>  * At minimum, a service provider could implement a sort of "half-OE"
> by registering key pairs for users and simply holding the private
> keys.  This would hide to outsiders whether the user had opted for
> full end-to-end encryption, and would provide some confidentiality for
> messages that flow through multiple providers (like email; this is an
> idea from UEE [21]).
>  * A service provider could store most users' private keys encrypted
> by a password, so that even a lost device doesn't result in
> undecryptable messages.  A user could try password cracking in the
> worst case of a forgotten password.
>  * A third option is to simply give every user control of their own
> private key, and if they lose their device(s) then they might lose
> some messages sent before they upload a new key.  That might be
> acceptable, or might not.
> This could be debated more, but if you accept that OE makes sense
> here, some principles follow:
> A) Since we want widespread OE the goal should be for encryption to be
> as frictionless as possible (ideally enabled by default, including
> multiple-device support), scaleable, and reliable.  Users who don't
> care about end-to-end authentication should not be inconvenienced by
> it.
> B) Since widespread OE would limit provider-based spam and malware
> filtering, figuring out how to move these to the client is important
> [10].
> C) Authentication mechanisms should be evaluated on "stealthiness" as
> well as useability and security.  Ideally it should be hard for any
> observer (including service providers) to tell which conversations are
> authenticated and which are not.
> D) Authentication mechanisms will be built on top of OE, so can assume
> that "identity public keys" and "key directories" already exist.
> Evaluating authentication methods for secure messaging with OE
> ------
> We can take the above principles and see whether different
> authentication methods are compatible with widespread OE for
> messaging.
> TOFU: Compatible with OE since users could "stealthily" enable
> notification of TOFU key changes, there's no effect on users who
> don't, and no scaleability issues that would inhibit widespread OE.
> FINGERPRINTS: Compatible with OE since users could "stealthily"
> communicate about fingerprints out-of-band, there's no effect on users
> who don't, and no scaleability issues.  In conjunction with TOFU, this
> is Moxie's "simple thing" argument [11,12].
> KEYS AS IDENTIFIERS: Using public keys or fingerprints directly as
> identifiers, or attaching them to identifiers, has a long history
> (Bitcoin, YURLs / S-Links, SMTorP, CGA, etc.).  The argument is that
> identifiers are being exchanged anyway, so we might as well piggyback
> authentication data.
> I argue this violates the OE concept by inconveniencing users who
> don't care about end-to-end authentication (A).  In particular, it
> adds costs such as:
>  i) useability cost of dealing with long, random-looking identifiers
>  ii) switching cost of replacing widely-distributed identifiers with
> new ones (in address books, memory, published materials, etc.)
>  iii) operational cost of redistributing identifiers whenever the
> private key changes.  If users change keys frequently due to new
> devices, software reinstallation, lost passwords, etc., it would be
> inconvenient to change email addresses every time [11,15].
> proposes that users register a name for their public key in a
> cryptocurrency-type blockchain.  Once the public key is registered, it
> can only be changed by expiration or a chain of signatures (signing a
> new key, which can sign another key, etc.)
> There are some questionable design decisions in Namecoin [13,14], but
> the general idea of first-come first-serve names for public keys that
> are widely witnessed seems potentially useful.
> If these names are the user's primary identifier, then this is similar
> to the "keys as identifiers" approach except keys are given better
> names by a public infrastructure.  So while this improves (i), it
> still violates the OE concept due to (ii) the cost of switching to new
> names and (iii) the operational cost of having your identifier tied to
> a key.  Additionally, publishing all names and relying on a new
> infrastructure raises hard-to-answer questions about privacy,
> reliability, and scaleability.
> If these names aren't primary identifiers, but are instead exchanged
> out-of-band to authenticate a specific public key, then this is
> similar to fingerprints except keys are given better names:
>  * my public key is "trevor_perrin_1970_email_2014 at Namecoin"
>  * my public key is "gacuqk - aqoq - ecsag - biza - sjebre" (base32 fingerprint)
> But this trades off "stealth" (C), as users with named keys are
> advertising that they care about end-to-end authentication and might
> be comparing keys out-of-band.  Users without named keys can probably
> be attacked with impunity.
> It's possible that the useability benefit of "named keys" instead of
> fingerprints might justify the infrastructure cost and loss of
> stealthy authentication, but the tradeoff is hard to evaluate.
> idea of a "transparency log", inspired by Certificate Transparency,
> which is being explored by Keybase and Google's End-to-End [16,17,18].
> Compared to a "provider-immutable" log, this accepts a more modest
> security goal (notify on key changes) so that it works with existing
> identifiers.  Moxie argues this goal is not much different than what
> TOFU + fingerprints can achieve [19].  That's worth exploring more,
> but to me this seems different enough that it would add security.
> In any case, this doesn't suffer from (ii) or (iii), so the main
> questions regarding compatibility with OE are privacy and
> infrastructure cost.
> Privacy:  Hashing identifiers won't be that effective [20], so this is
> asking service providers to publish identifiers for a large portion of
> their userbase.
> Infrastructure cost:
>  - Instead of just looking up Bob's public key, Alice needs to lookup
> a proof-of-inclusion, which might increase the response size to 1 KB+
> for large providers.
>  - Storage of all the log data, and recalculating new logs, might be
> significant, depending on (frequency of log publication, frequency of
> key changes, size of userbase, etc).
>  - To be practical, new keys would probably be batched into a new log
> every 24 hours or so, which adds a delay that's not trivial deal with.
>  - To be effective, third-party monitors would need to download and
> review log entries, and it's not clear who these are and what costs
> they'd have to pay to keep up.
> ANONYMIZED LOOKUP AND AUDITING:  Some projects (e.g. Nyms [22]) have
> suggested key lookups be performed via anonymized connections (e.g.
> Tor, or a similar chain of proxies).  Then users could audit their own
> key directory just by looking up their own key.
> For widespread OE these lookups would be frequent.  Whether the
> latency, reliability, and infrastructure cost of anonymizing them is
> acceptable seems like an open question.
> Conclusions
> -----
> Not sure.  The TL;DR is that there might be value to deploying
> end-to-end encryption at scale, even without end-to-end authentication
> (OE), so it would be good to have authentication methods that enhance
> the value of that instead of impeding it.
> Trevor
> [1] http://en.wikipedia.org/wiki/Opportunistic_encryption
> [2] https://datatracker.ietf.org/doc/draft-dukhovni-opportunistic-security/?include_text=1
> [3] http://www.ietf.org/mail-archive/web/uta/current/msg00311.html
> [4] https://moderncrypto.org/mail-archive/messaging/2014/000229.html
> [5] https://www.eff.org/encrypt-the-web-report
> [6] https://github.com/jsha/starttls-everywhere/blob/master/README.md
> [7] https://datatracker.ietf.org/doc/draft-ietf-dane-smtp-with-dane/
> [8] http://httpwg.github.io/http-extensions/encryption.html
> [9] http://lists.w3.org/Archives/Public/ietf-http-wg/2014JulSep/1727.html
> [10] https://moderncrypto.org/mail-archive/messaging/2014/000727.html
> [11] https://moderncrypto.org/mail-archive/messaging/2014/000718.html
> [12] https://moderncrypto.org/mail-archive/messaging/2014/000723.html
> [13] https://moderncrypto.org/mail-archive/messaging/2014/000679.html
> [14] https://moderncrypto.org/mail-archive/messaging/2014/000685.html
> [15] https://moderncrypto.org/mail-archive/messaging/2014/000234.html
> [16] https://moderncrypto.org/mail-archive/messaging/2014/#226
> [17] https://moderncrypto.org/mail-archive/messaging/2014/000706.html
> [18] https://moderncrypto.org/mail-archive/messaging/2014/#708
> [19] https://moderncrypto.org/mail-archive/messaging/2014/000723.html
> [20] https://moderncrypto.org/mail-archive/messaging/2014/000766.html
> [21] https://github.com/tomrittervg/uee/blob/master/proposal.md
> [22] http://nyms.io/
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