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Add details about why this proposal should exist

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Andrew Morgan 2019-07-23 15:16:27 +01:00
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proposals/2134-identity-hash-lookup.md
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@ -6,22 +6,41 @@ To summarise the issue, lookups (of Matrix user IDs) are performed using
plain-text 3PIDs (third-party IDs) which means that the identity server can plain-text 3PIDs (third-party IDs) which means that the identity server can
identify and record every 3PID that the user has in their contacts, whether identify and record every 3PID that the user has in their contacts, whether
that email address or phone number is already known by the identity server or that email address or phone number is already known by the identity server or
not. not. In the latter case, an identity server is able to collect email
addresses and phone numbers that have a high probability of being connected
to a real person. It could then use this data for marketing or other
purposes.
If the 3PID is hashed, the identity server could not determine the address However, if the email addresses and phone numbers are hashed before they are
unless it has already seen that address in plain-text during a previous call sent to the identity server, the server would have a more difficult time of
of the [/bind being able to recover the original addresses. This prevents contact
mechanism](https://matrix.org/docs/spec/identity_service/r0.2.1#post-matrix-identity-api-v1-3pid-bind) information of non-Matrix users being exposed by the lookup service.
(without significant resources to reverse the hashes). This helps prevent
bulk collection of user's contact lists by the identity server and reduces
its ability to build social graphs.
This proposal thus calls for the Identity Service API's However, hashing is not perfect. While reversing a hash is not possible, it
[/lookup](https://matrix.org/docs/spec/identity_service/r0.2.1#get-matrix-identity-api-v1-lookup) is possible to build a [rainbow
endpoint to use hashed 3PIDs instead of their plain-text counterparts (and to table](https://en.wikipedia.org/wiki/Rainbow_table), which could map many
deprecate both it and known email addresses and phone numbers to their hash equivalents. When the
[/bulk_lookup](https://matrix.org/docs/spec/identity_service/r0.2.1#post-matrix-identity-api-v1-bulk-lookup)), identity server receives a hash, it would then be able to look it up in this
which will leak less data to identity servers. table, and find the email address or phone number associated with it. In an
ideal world, one would use a hashing algorithm such as
[bcrypt](https://en.wikipedia.org/wiki/Bcrypt), with many rounds, which would
make building such a rainbow table an extraordinarily expensive process.
Unfortunately, this is impractical for our use case, as it would require
clients to perform many, many rounds of hashing, linearly dependent on their
address book size, which would likely result in lower-end mobile phones
becoming overwhelmed. Thus, we must use a fast hashing algorithm, at the cost
of making rainbow tables easy to build.
The rainbow table attack is not perfect. While there are only so many
possible phone numbers, and thus it is simple to generate the hash value for
each one, the address space of email addresses is much, much wider. Therefore
if your email address is decently long and is not publicly known to
attackers, it is unlikely that it would be included in a rainbow table.
Thus the approach of hashing, while adding complexity to implementation and
minor resource consumption of the client and identity server, does provide
added difficultly for the identity server to carry out contact detail
harvesting, which should be considered worthwhile.
## Proposal ## Proposal