Internet DRAFT - draft-party-mimi-user-private-discovery
draft-party-mimi-user-private-discovery
More Instant Messaging Interoperability (mimi) G. Hogben
Internet-Draft F. Olumofin
Intended status: Informational Google
Expires: 7 May 2024 4 November 2023
Interoperable Private Identity Discovery for E2EE Messaging
draft-party-mimi-user-private-discovery-03
Abstract
This document specifies how users can privately discover each other's
Service Specific Identifiers (SSIs) when using end-to-end encrypted
messaging services across multiple providers. Users can retrieve
SSIs without revealing their social graphs to service providers they
are not delivering messages through, using their phone numbers,
email, user IDs, or other Service Independent Identifiers (SIIs).
Our specification can be based on private information retrieval or
associative private sets membership schemes, both of which provide
reasonable tradeoffs between privacy and cost.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://datatracker.ietf.org/doc/giles-interop-user-private-
discovery/. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-party-mimi-user-private-
discovery/.
Discussion of this document takes place on the mimi Working Group
mailing list (mailto:mimi@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/mimi/. Subscribe at
https://www.ietf.org/mailman/listinfo/mimi/.
Source for this draft and an issue tracker can be found at
https://github.com/femigolu/giles-interop-user-private-discovery.
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Table of Contents
1. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Threat actors . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Privacy requirements . . . . . . . . . . . . . . . . . . . . 5
4.1. Requirements by threat actor . . . . . . . . . . . . . . 5
5. Privacy non-requirements . . . . . . . . . . . . . . . . . . 6
6. Other Non-functional Requirements . . . . . . . . . . . . . . 6
7. SSI Discovery . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Private Information Retrieval (PIR) . . . . . . . . . . . 9
7.1.1. Cost estimates . . . . . . . . . . . . . . . . . . . 9
7.2. Private Set Membership (PSM) . . . . . . . . . . . . . . 10
7.2.1. Cost estimates . . . . . . . . . . . . . . . . . . . 10
7.3. Cross-service identity spoofing . . . . . . . . . . . . . 10
8. Thoughts on open questions from 10/10/2023 Interim
MeetingMIMI20231010 . . . . . . . . . . . . . . . . . . . 11
8.1. Trusted Authorities for Mapping SIIs to SSIs . . . . . . 11
8.2. Discovery Scaling . . . . . . . . . . . . . . . . . . . . 12
8.3. Acceptable leakage for discovery . . . . . . . . . . . . 12
8.4. Rate Limiting . . . . . . . . . . . . . . . . . . . . . . 13
8.5. SII Mappings . . . . . . . . . . . . . . . . . . . . . . 13
8.6. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9.1. Appendix . . . . . . . . . . . . . . . . . . . . . . . . 14
10. Normative References . . . . . . . . . . . . . . . . . . . . 14
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
A *service specific identifier* (SSI) is a unique identifier for a
user within a single service provider's service, and encodes the
service provider in the identifier. For example, a user's account
handle and provider identifier is an SSI.
A *service independent identifier* (SII) is a unique identifier for a
user that is independent of any specific service provider. For
example, a user's E.164 phone number or email address are SIIs, since
they can be used to identify the user across multiple different
services.
2. Problem statement
The *discovery problem* is resolving a user's SII into one SSI for
that user, while preserving user privacy in the process.
3. Threat actors
* Alice, Bob, and Carol: Three users within the interoperable E2EE
messaging ecosystem.
* Sender Messaging Platform: A messaging service provider platform
where a registered user has an account and has established a
mapping of SII to SSI. Examples from Fig. is Platform 1 for Alice
and Carol, and Platform 2 for Bob.
* Potential Recipient Messaging Platform: A messaging service
provider platform where a discovered SSI is registered. An
example from Fig. 1 is the role of Platform 2 when Alice resolves
Bob's SSI using Bob's SII. This has three variants in the threat
model:
1. Recipient platform with SSI - the sender sends a message (so
this platform will learn the sender identity).
2. Non-recipient platform with SSI that the recipient SII has an
account with but does not send a message to.
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3. Non-recipient platform without SSI - potential recipient does
not have an SSI registered with this platform.
,-----. ,-----. ,---------. ,------------------. ,---------.
|Alice| |Carol| |Front End| |Discovery Provider| |Front End|
|-----| |-----| |---------| |------------------| |---------|
|-----|--|-----| |---------| |------------------| |---------|
`-----' `-----' `---------' `------------------' `---------'
| | | |
| | | |
,---. ,------------------. ,------------------------. ,------------------.
|Bob| |Discovery Provider| |Key Distribution Service| |Discovery Provider|
|---| |------------------| |------------------------| |------------------|
|---| |------------------| |------------------------| |------------------|
`---' `------------------' `------------------------' `------------------'
| | |
,------------------------. ,-------------------. ,------------------------.
|Key Distribution Service| |Mappings DB Bob,...| |Key Distribution Service|
|------------------------| |-------------------| |------------------------|
|------------------------| |-------------------| |------------------------|
`------------------------' `-------------------' `------------------------'
| |
,---------------------------. ,-------------------.
|Mappings DB Alice,Carol,...| |Mappings DB Bob,...|
|---------------------------| |-------------------|
|---------------------------| |-------------------|
`---------------------------' `-------------------'
Figure 1: Threat actors and systems
* Third Party Platform: A platform that provides discovery services
but is not a messaging service provider. Bob might register with
such a service directly, or such a service may act as a proxy for
Messaging Platform 2 through contractual business agreement.
* Front End: A service within a platform that receives users'
requests and collaborates with other services to process them.
* Discovery Provider: Works to resolve SII to SSI.
* Key Distribution Service: Manages public key material of
registered users.
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4. Privacy requirements
1. *Social graph*: Discovery service providers should not learn the
SII or SSI a user is querying for unless they are sending or
receiving a message on to that user.
2. *Querying user identity*: A discovery service provider should not
share the querying user identity with other discovery services
when it requires their help for discovery.
3. *Metadata*: Discovery service should not learn the exact timing
of when a message is sent (after discovery).
4.1. Requirements by threat actor
The following table describes the requirements to protect the privacy
of an intended recipient's SSI during discovery broken down by the
various threat actors. The possible list of services that may
resolve a discovery request based on their knowledge of the SSI is
shown in the first column. The second and third columns are the
minimum and possible privacy requirements. The optimal privacy
requirements assume that the two devices in E2EE messaging endpoints
are on different messaging service platforms.
Note that current messaging systems segment a user's social graph
across their contacts' messaging services. Without proper privacy
mitigations, a discovery process for the new interoperable ecosystem
can enable an attacker to aggregate these fragments of the user's
social graph across different services, violating their privacy.
Performing the discovery process for contacts that are never used is
common so that it is very likely that most clients will perform
discovery for SII’s that they never send a message to. This is why
we propose hiding the SII from the sender platform unless a message
is sent. We believe this is possible technically because:
1. Spam prevention requirements only apply to sent messages
(standard IP based techniques can be used to prevent DDoS of the
discovery service itself).
2. Client costs for SII hiding mechanisms scale well enough with
database size + number of services.
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+======================+=================+=================+
| Service | Minimum privacy | Optimal privacy |
| | requirements | requirements |
+======================+=================+=================+
| Sender Platform | Do not hide SSI | Hide SSI |
+----------------------+-----------------+-----------------+
| Recipient Platform | Do not hide SSI | Do not hide SSI |
| with SSI | | |
+----------------------+-----------------+-----------------+
| Non-recipient | Hide SSI | Hide SSI |
| Platform with SSI | | |
+----------------------+-----------------+-----------------+
| Non-recipient | Hide SSI | Hide SSI |
| Platform without SSI | | |
+----------------------+-----------------+-----------------+
| Third party service | Hide SSI | Hide SSI |
+----------------------+-----------------+-----------------+
Table 1
Table 1: Discovery privacy requirements by threat actors
5. Privacy non-requirements
1. *Hiding SII <> service mapping*: Hiding service reachability or
the existence of a mapping between an SII and SSI for a service
provider is an explicit non-goal. All major E2EE messaging
services already publish unACL’d reachability information without
opt-out i.e. +16501234567, reachable on Messages, Whatsapp,
Telegram (not including name or any other info). Therefore this
should not be a privacy goal (and would not be feasible to
implement). *However it may be a business goal to prevent
scraping of the full list of account-holders.*
2. *Contact lookup by name* or anything except an SII.
6. Other Non-functional Requirements
1. No single entity should be financially responsible for resolving
all discovery queries (e.g. even within a geographical region).
2. Costs for each participating entity of storing and resolving SII
should be proportional to their number of participating users.
3. Performance should support each client device resolving users'
contact SIIs at least once every 24 hours.
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7. SSI Discovery
SSI discovery means retrieving the SSI that an SII maps to. There
are two alternative cryptographic techniques to achieve the privacy
properties for the retrieval:
1. Private Information Retrieval (PIR)
2. Private Set Membership (PSM)
The discovery process is illustrated in Figure 2. Optionally,
Alice’s client may encrypt the SSI of interest using PIR or PSM
before forwarding the SII query to the Discovery Provider of the
Sender Messaging Platform.
The DP for the Sender Messaging Platform may either look up or
compute an encrypted response directly, or it may forward the request
to the Potential Recipient or Third Party Discovery Provider
indicated by the provider identifier included in the request.
Regardless of which party processes the request, a DP will compute an
encrypted response and forward it back to Alice. Alice can then
decrypt the encrypted response (if applicable) to obtain the SSI.
Alice’s client may also optionally send the discovery request
directly to a potential recipient or 3p DPs.
We assume a fixed list of DPs for each SMP so that the client does
not have to specify in the query request which DPs to use.
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┌─────────────────────────┐ ┌─────────────────────────────┐
┌─────┐ │Sender Messaging Platform│ │Potential Recipient or Third │
│Alice│ │Discovery Provider │ │Party Discovery Provider │
└──┬──┘ └────────────┬────────────┘ └──────────────┬──────────────┘
────┐ │ │
│ 0. resolve SII │ │
<───┘ │ │
│ │ │
│ 1. SII | Encrypted(SII) │ │
│───────────────────────────────────>│ │
│ │ │
│ 1b. SII | Encrypted(SII) │
│ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─>│
│ │ │
│ ────┐ │
│ │ 2. Lookup | Compute Response │
│ <───┘ │
│ │ │
│ │ 3. SII | Encrypted(SII) │
│ │ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ >│
│ │ │
│ │ ─ ─ ┐
│ │ | 4. Lookup | Compute Response
│ │ < ─ ┘
│ │ │
│ │ 5. SSI | Encrypted(SSI) │
│ │<─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─│
│ │ │
│ 6. SSI | Encrypted(SSI) │ │
│<───────────────────────────────────│ │
│ │ │
────┐ │ │
│ 7. SSI | Decrypt Response =>SSI│ │
<───┘ │ │
┌──┴──┐ ┌────────────┴────────────┐ ┌──────────────┴──────────────┐
│Alice│ │Sender Messaging Platform│ │Potential Recipient or Third │
└─────┘ │Discovery Provider │ │Party Discovery Provider │
└─────────────────────────┘ └─────────────────────────────┘
Figure 2: Discovery with Sender Messaging Platform
*Note:* * Note that the DPs should not learn that Alice is the author
of the request. * Alice is not required to hide discovery requests
when the processor DP is within the Sender Messaging Platform. *
Alice’s client may, but is not required to hide discovery requests
from Potential Recipient DPs. Both of these requests can be sent in
the clear.
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7.1. Private Information Retrieval (PIR)
A PIR protocol enables a client holding an index (or keyword) to
retrieve the database record corresponding to that index from a
remote server. PIR schemes have communication complexities sublinear
in the database size and they provide access privacy for clients
which precludes the server from being able to learn any information
about either the query index or the record retrieved. A standard
single-server PIR scheme provides clients with algorithms to generate
a query and decode a response from the server. It also provides an
algorithm for the server to compute a response.
We proposed a lattice-based PIR framework by Patel et
al[PIRFramework] with sharded databases. This framework is
applicable with any standard PIR scheme such as the open source
implementation here (https://github.com/google/private-retrieval).
Cost estimates suggest this is feasible even for a very large
database with 10 billion records/mappings.
7.1.1. Cost estimates
Use database shards each of ~1 million mappings. For 1.28 TB (10
billion records), breaking this down into 10,000 shards each of size
1 million records gives a cost estimate for each query as below:
+================================================+===============+
| Parameter/Metric | Cost estimate |
+================================================+===============+
| Server Storage Per Device | 14 MB |
+------------------------------------------------+---------------+
| Client Device Storage (for 10 billion records) | 5 MB |
+------------------------------------------------+---------------+
| Upload Bandwidth Per Query | 14 KB |
+------------------------------------------------+---------------+
| Download Bandwidth Per Query | 21 KB |
+------------------------------------------------+---------------+
| Client Time Per Query | 0.1s |
+------------------------------------------------+---------------+
| Server Time Per Query (Single Thread) | 0.8-1s |
+------------------------------------------------+---------------+
Table 2
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7.2. Private Set Membership (PSM)
The discovery provider holds a set of SIIs that maps to an associated
set of SSI. A PSM protocol enables a client with an SII to lean the
associated SSI held by the server with the following privacy
guarantees:
1. The discovery provider does not learn the SII held by the client.
2. The discovery provider does not learn whether a matching SII was
found or not.
3. The client does not learn any information about the other SIIs
and associated SSIs held by the discovery provider.
An open source implementation is available here
(https://github.com/google/private-membership).
7.2.1. Cost estimates
For a database with 1.28 TB (10 billion associated records of SSI),
using 1,000 shards each of size 10 million records, the cost estimate
for each query is:
+=======================================+===============+
| Parameter/Metric | Cost estimate |
+=======================================+===============+
| Communication | 2.8 MB |
+---------------------------------------+---------------+
| Client Time Per Query | 0.1s |
+---------------------------------------+---------------+
| Server Time Per Query (Single Thread) | 1-2s |
+---------------------------------------+---------------+
Table 3
7.3. Cross-service identity spoofing
Today, a messaging service may support one or more ways of
identifying a user including email address, phone number, or service
specific user name.
Messaging interoperability introduces a new problem that
traditionally has been resolvable at the service level: cross-service
identity spoofing, where a user on a given E2EE may or may not be
addressable at the same ID on another service due to a lack of global
uniqueness constraints across providers.
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As a result, a user may be registered at multiple services with the
same handles, e.g. if Bob's email is bob@example.com
(mailto:bob@example.com) and his phone number is 555-111-2222 and he
is registered with Signal and iMessage, he would be addressable at
bob@example.com (mailto:bob@example.com):iMessage,
555-111-2222:iMessage, and 555-111-2222:Signal. In this case, the
same userId on iMessage and Signal is acceptable as the phone number
can map to only one individual who proves their identity by
validating ownership of the SIM card.
On services where a user can log in with a username _alone_, however
e.g. Threema and FooService, the challenge becomes:
* Alice messages Bob at Bob's preferred service (bob@Threema)
* Eve messages Alice impersonating Bob using bob@FooService
* Alice needs some indicator or UI to know that bob@Threema isn't
bob@FooSercice and that when bob@FooService messages, it should
not be assumed that bob@FooService is bob@Threema.
Options for solving this are: 1. Storing the supported services for
a contact in Contacts and if a recipient receives a message from an
unknown sender, to treat it as spam or otherwise untrusted from the
start. 2. Requiring the fully qualified username for services that
rely on usernames only - e.g. bob@threema.com vs bob.
8. Thoughts on open questions from 10/10/2023 Interim
Meeting[MIMI20231010]
8.1. Trusted Authorities for Mapping SIIs to SSIs
_Which actors should be trusted authorities for mapping SIIs to
SSIs?_
In general, this should be considered out of scope for this proposal,
however we expect that by default, Messaging Service Providers (MSP)
should be trusted authorities for creating these mapping. Users may
"own" their SIIs, but messaging service providers own SSIs. MSP
should verify ownership of SIIs (one time password code to phone via
text or call, or to email).
An MSP may share established mapping data with 3P discovery providers
to facilitate lookups, or may delegate establishing new mappings to
these providers under contractual agreements between them.
Preferably, delegate discovery providers should be lookup providers
only and should not create or update existing mappings unless the
delegate is a reputable/trusted certification authority.
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If a 3p discovery service is used, it may also authenticate the
mapping independently or it may act as a pass-through for a signed
mapping by an MSP or another identity provider.
SSL is sufficient to authenticate the mapping assertion.
8.2. Discovery Scaling
_Does discovery need to scale to accommodate 10s, 100s, or 1000s of
service?_
A discovery request should be sent to a specific MSP or 3P discovery
provider. It is up to those providers if they want to fan out the
discovery to other providers or answer the discovery request from its
own mapping only. It will be costly to fork out discovery requests
to a large number of discovery providers while completely hiding the
SSI from these providers. We do not want forking to fit DDoS
patterns on these services.
However the protocols should be feasible (in terms of computation and
communication cost) for 1000s of services.
8.3. Acceptable leakage for discovery
_What is it acceptable for queries to reveal about the social graph,
and to whom?_
A query should not reveal the SII in a user's query to discovery
providers unless the discovery provider is also within the Sender's
platform or the Recipient's platform with the SSI mapping. For an
encrypted query and *since discovery precedes E2EE messaging*, a
discovery provider won't be able to tell if the SSI maps to an SSI in
its service. It is okay to take the no-leakage approach for all
providers.
Alice may use the different provider owning each SSI that her phone
maps to. Bob may use different email addresses to map to multiple
SSI with the same provider.
Returning an SSI set of different cardinalities leaks information to
a discovery provider about the likely sets of SSIs that are of
interest for a query. A one-to-one mapping of SII to SSI does not
leak such information. A discovery provider cannot tell when a
privacy-preserving discovery returns an empty result or a single SII.
However, it will be able to tell when a large number of SSIs are
returned.
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8.4. Rate Limiting
_Is rate limiting useful to prevent scraping?_
It is up to a discovery provider to rate-limit given the potential
computational cost of responding to batch queries from a single user.
Nonetheless, we should require that a user should be able to look up
no less than 50 SII per discovery provider for each messaging
provider in a given 24 hours period. Third party discovery providers
are under obligation to messaging service providers and are excluded
from the minimum discovery load per user.
8.5. SII Mappings
_An SII may map to multiple SSIs. Should the requestor learn all of
them, and if so, how?_
* _One service that returns all SSIs for an SII?_
* _Query each service provider independently?_
* _User figures out out-of-band what service provider to query?_
SII mapping to multiple SSIs within a single provider
1. This is a choice that MSPs will have to make, if they want to
allow it.
2. Having multiple SSIs per SII makes preserving the privacy of
discovery more challenging because of the side channel leakage of
response size. The tradeoff is acceptable if on the average
users have multiple SSI with a MSP.
3. For privacy reasons (i.e., protecting the association of multiple
SSIs), the user may not want to group multiple SSIs together.
4. We may devise a scheme where an SII could be suffixed with an
index during registration and discovery of the SSI to retrieve
from the set. For example, given an SII +1234567890, a user may
map +12345678900 to the first Whatsapp SSI, and +1234567891 to
the second Whatsapp SSI and so on.
The user should figure out out-of-band what discovery provider to
query, and discovery providers should not be required to fork out
discovery requests to other providers given the computational cost
impact.
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8.6. Notes
9. IANA Considerations
This document has no IANA actions.
9.1. Appendix
10. Normative References
[MIMI20231010]
Geoghegan, T., "Discovery requirements", MIMI Virtual
interim October 10, 2023 , n.d..
[PIRFramework]
Patel, S., Seo, J. Y., and K. Yeo, "Don't be Dense:
Efficient Keyword PIR for Sparse Databases", 32nd USENIX
Security Symposium, USENIX Security 2023 , n.d..
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
Acknowledgments
The technical description of the private information retrieval
framework is based on Sarvar Patel, Joon Young Seo and Kevin Yeo's
USENIX Security '23 paper titled "Don't be Dense: Efficient Keyword
PIR for Sparse Databases "
(https://www.usenix.org/conference/usenixsecurity23/presentation/
patel).
Authors' Addresses
Giles Hogben
Google
Email: gih@google.com
Femi Olumofin
Google
Email: fgolu@google.com
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