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hs/hash-identity
Andrew Morgan 5 years ago
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proposals/2134-identity-hash-lookup.md
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@ -9,10 +9,12 @@ that email address or phone number is already known by the identity server or
not.
If the 3PID is hashed, the identity server could not determine the address
unless it has already seen that address in plain-text during a previous call of
the [/bind
unless it has already seen that address in plain-text during a previous call
of the [/bind
mechanism](https://matrix.org/docs/spec/identity_service/r0.2.1#post-matrix-identity-api-v1-3pid-bind)
(without significant resources to reverse the hashes).
(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
[/lookup](https://matrix.org/docs/spec/identity_service/r0.2.1#get-matrix-identity-api-v1-lookup)
@ -25,7 +27,7 @@ which will leak less data to identity servers.
This proposal suggests making changes to the Identity Service API's lookup
endpoints. Instead, this proposal consolidates them into a single `/lookup`
endpoint. Additionally, the endpoint should be on a `v2` path, to avoid
endpoint. Additionally, the endpoint is to be on a `v2` path, to avoid
confusion with the original `/lookup`. We also drop the `/api` in order to
preserve consistency across other endpoints:
@ -40,11 +42,13 @@ The following back-and-forth occurs between the client and server.
Let's say the client wants to check the following 3PIDs:
alice@example.com
bob@example.com
carl@example.com
+1 234 567 8910
denny@example.com
```
alice@example.com
bob@example.com
carl@example.com
+1 234 567 8910
denny@example.com
```
The client will hash each 3PID as a concatenation of the medium and address,
separated by a space and a pepper appended to the end. Note that phone numbers
@ -52,55 +56,59 @@ should be formatted as defined by
https://matrix.org/docs/spec/appendices#pstn-phone-numbers, before being
hashed). First the client must prepend the medium to the address:
"alice@example.com" -> "email alice@example.com"
"bob@example.com" -> "email bob@example.com"
"carl@example.com" -> "email carl@example.com"
"+1 234 567 8910" -> "msisdn 12345678910"
"denny@example.com" -> "email denny@example.com"
```
"alice@example.com" -> "email alice@example.com"
"bob@example.com" -> "email bob@example.com"
"carl@example.com" -> "email carl@example.com"
"+1 234 567 8910" -> "msisdn 12345678910"
"denny@example.com" -> "email denny@example.com"
```
Hashes must be peppered in order to reduce both the information a client gains
during the process, and attacks the identity server can perform (namely sending
a rainbow table of hashes back in the response to `/lookup`).
In order for clients to know the pepper and hashing algorithm they should use,
Identity Servers must make the information available on the `/hash_details`
Identity servers must make the information available on the `/hash_details`
endpoint:
GET /_matrix/identity/v2/hash_details
```
GET /_matrix/identity/v2/hash_details
{
"lookup_pepper": "matrixrocks",
"algorithms": ["sha256"]
}
{
"lookup_pepper": "matrixrocks",
"algorithms": ["sha256"]
}
```
The name `lookup_pepper` was chosen in order to account for pepper values being
returned for other endpoints in the future. The contents of `lookup_pepper`
MUST match the regular expression `[a-zA-Z0-9]*`.
The client should append the pepper to the end of the 3pid string before
hashing.
```
The client should append the pepper to the end of the 3PID string before
hashing.
"email alice@example.com" -> "email alice@example.commatrixrocks"
"email bob@example.com" -> "email bob@example.commatrixrocks"
"email carl@example.com" -> "email carl@example.commatrixrocks"
"msisdn 12345678910" -> "msisdn 12345678910matrixrocks"
"email denny@example.com" -> "email denny@example.commatrixrocks"
"email alice@example.com" -> "email alice@example.commatrixrocks"
"email bob@example.com" -> "email bob@example.commatrixrocks"
"email carl@example.com" -> "email carl@example.commatrixrocks"
"msisdn 12345678910" -> "msisdn 12345678910matrixrocks"
"email denny@example.com" -> "email denny@example.commatrixrocks"
```
Clients SHOULD request this endpoint each time before performing a lookup, to
handle identity servers which may rotate their pepper values frequently.
Clients MUST choose one of the given hash algorithms to encrypt the 3PID during
lookup.
Note that possible hashing algorithms will be defined in the Matrix
specification, and an Identity Server can choose to implement one or all of
them. Later versions of the specification may deprecate algorithms when
necessary. Currently the only listed hashing algorithm is SHA-256 as defined by
[RFC 4634](https://tools.ietf.org/html/rfc4634) and Identity Servers and
clients MUST agree to its use with the string `sha256`. SHA-256 was chosen as
it is currently used throughout the Matrix spec, as well as its properties of
being quick to hash. While this reduces the resources necessary to generate a
rainbow table for attackers, a fast hash is necessary if particularly slow
mobile clients are going to be hashing thousands of contact details.
At a minimum, clients and identity servers MUST support SHA-256 as defined by
[RFC 4634](https://tools.ietf.org/html/rfc4634), identified by the
`algorithm` value `"sha256"`. SHA-256 was chosen as it is currently used
throughout the Matrix spec, as well as its properties of being quick to hash.
While this reduces the resources necessary to generate a rainbow table for
attackers, a fast hash is necessary if particularly slow mobile clients are
going to be hashing thousands of contact details. Other algorithms can be
negotiated by the client and server at their discretion.
When performing a lookup, the pepper and hashing algorithm the client used must
be part of the request body. If they do not match what the server has on file
@ -112,20 +120,23 @@ server.
If the algorithm does not match the server's, the server should return a `400
M_INVALID_PARAM`. If the pepper does not match the server's, the server should
return a new error code, 400 `M_INVALID_PEPPER`. A new error code is not
return a new error code, `400 M_INVALID_PEPPER`. A new error code is not
defined for an invalid algorithm as that is considered a client bug.
Each of these error responses should contain the correct `algorithm` and
`lookup_pepper` fields. This is to prevent the client from needing to query
`/hash_details` again, thus saving a round-trip. An example response to an
incorrect pepper would be:
{
"error": "Incorrect value for lookup_pepper",
"errcode": "M_INVALID_PEPPER",
"algorithm": "sha256",
"lookup_pepper": "matrixrocks"
}
The `M_INVALID_PEPPER` error response should contain the correct `algorithm`
and `lookup_pepper` fields. This is to prevent the client from needing to
query `/hash_details` again, thus saving a round-trip. `M_INVALID_PARAM` does
not include these fields. An example response to an incorrect pepper would
be:
```
{
"error": "Incorrect value for lookup_pepper",
"errcode": "M_INVALID_PEPPER",
"algorithm": "sha256",
"lookup_pepper": "matrixrocks"
}
```
Now comes time for the lookup. Note that the resulting hash digest MUST be
encoded in URL-safe unpadded base64 (similar to [room version 4's event
@ -133,36 +144,40 @@ IDs](https://matrix.org/docs/spec/rooms/v4#event-ids)). Once hashing has been
performed using the defined hashing algorithm, the client sends each hash in an
array.
"email alice@example.commatrixrocks" -> "y_TvXLKxFT9CURPXI1wvfjvfvsXe8FPgYj-mkQrnszs"
"email bob@example.commatrixrocks" -> "r0-6x3rp9zIWS2suIque-wXTnlv9sc41fatbRMEOwQE"
"email carl@example.commatrixrocks" -> "ryr10d1K8fcFVxALb3egiSquqvFAxQEwegXtlHoQFBw"
"msisdn 12345678910matrixrocks" -> "c_30UaSZhl5tyanIjFoE1IXTmuU3vmptEwVOc3P2Ens"
"email denny@example.commatrixrocks" -> "bxt8rtRaOzMkSk49zIKE_NfqTndHvGbWHchZskW3xmY"
POST /_matrix/identity/v2/lookup
{
"hashes": [
"y_TvXLKxFT9CURPXI1wvfjvfvsXe8FPgYj-mkQrnszs",
"r0-6x3rp9zIWS2suIque-wXTnlv9sc41fatbRMEOwQE",
"ryr10d1K8fcFVxALb3egiSquqvFAxQEwegXtlHoQFBw",
"c_30UaSZhl5tyanIjFoE1IXTmuU3vmptEwVOc3P2Ens",
"bxt8rtRaOzMkSk49zIKE_NfqTndHvGbWHchZskW3xmY"
],
"algorithm": "sha256",
"pepper": "matrixrocks"
}
```
"email alice@example.commatrixrocks" -> "y_TvXLKxFT9CURPXI1wvfjvfvsXe8FPgYj-mkQrnszs"
"email bob@example.commatrixrocks" -> "r0-6x3rp9zIWS2suIque-wXTnlv9sc41fatbRMEOwQE"
"email carl@example.commatrixrocks" -> "ryr10d1K8fcFVxALb3egiSquqvFAxQEwegXtlHoQFBw"
"msisdn 12345678910matrixrocks" -> "c_30UaSZhl5tyanIjFoE1IXTmuU3vmptEwVOc3P2Ens"
"email denny@example.commatrixrocks" -> "bxt8rtRaOzMkSk49zIKE_NfqTndHvGbWHchZskW3xmY"
POST /_matrix/identity/v2/lookup
{
"hashes": [
"y_TvXLKxFT9CURPXI1wvfjvfvsXe8FPgYj-mkQrnszs",
"r0-6x3rp9zIWS2suIque-wXTnlv9sc41fatbRMEOwQE",
"ryr10d1K8fcFVxALb3egiSquqvFAxQEwegXtlHoQFBw",
"c_30UaSZhl5tyanIjFoE1IXTmuU3vmptEwVOc3P2Ens",
"bxt8rtRaOzMkSk49zIKE_NfqTndHvGbWHchZskW3xmY"
],
"algorithm": "sha256",
"pepper": "matrixrocks"
}
```
The identity server, upon receiving these hashes, can simply compare against
the hashes of the 3PIDs it stores. The server then responds with the Matrix
IDs of those that match:
{
"mappings": {
"y_TvXLKxFT9CURPXI1wvfjvfvsXe8FPgYj-mkQrnszs": "@alice:example.com",
"c_30UaSZhl5tyanIjFoE1IXTmuU3vmptEwVOc3P2Ens": "@fred:example.com"
}
}
```
{
"mappings": {
"y_TvXLKxFT9CURPXI1wvfjvfvsXe8FPgYj-mkQrnszs": "@alice:example.com",
"c_30UaSZhl5tyanIjFoE1IXTmuU3vmptEwVOc3P2Ens": "@fred:example.com"
}
}
```
The client can now display which 3PIDs link to which Matrix IDs.
@ -173,7 +188,7 @@ as part of this proposal.
## Fallback considerations
`v1` versions of these endpoints may be disabled at the discretion of the
implementation, and should return a 403 `M_FORBIDDEN` error if so.
implementation, and should return a `403 M_FORBIDDEN` error if so.
If an identity server is too old and a HTTP 400 or 404 is received when
accessing the `v2` endpoint, they should fallback to the `v1` endpoint instead.
@ -186,11 +201,13 @@ be clear about where they are being sent to.
* There is a small cost incurred by performing hashes before requests, but this
is outweighed by the privacy implications of sending plain-text addresses.
## Potential issues
## Security Considerations
Hashes are still reversible with a rainbow table, but hopefully the provided
pepper, which can be rotated by identity servers at will, should help mitigate
this to some extent.
Hashes are still reversible with a rainbow table, but the provided pepper,
which can be rotated by identity servers at will, should help mitigate this.
Phone numbers (with their relatively short possible address space of 12
numbers), short email addresses, and addresses of both type that have been
leaked in database dumps are more susceptible to hash reversal.
Additionally, this proposal does not stop an identity server from storing
plain-text 3PIDs. There is a GDPR argument in keeping email addresses, such

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