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Andrew Morgan 53bd384f2e Clarify salting

2019-07-03 09:59:38 +01:00

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MSC2134: Identity Hash Lookups

Issue #2130 has been recently created in response to a security issue brought up by an independent party. 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 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 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 mechanism (without significant resources to reverse the hashes).

This proposal thus calls for the Identity Service API's /lookup endpoint to use a back-and-forth mechanism of passing partial hashed 3PIDs instead of their plain-text counterparts, which should leak mess less data to either party.

Proposal

This proposal suggests making changes to the Identity Service API's lookup endpoints. Instead of the /lookup and /bulk_lookup endpoints, this proposal replaces them with endpoints /lookup and /lookup_hashes. Additionally, the endpoints should 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:

/_matrix/identity/v2/lookup
/_matrix/identity/v2/lookup_hashes

A third endpoint is added for clients to request information about the form the server expects hashes in.

/_matrix/identity/v2/hash_details

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

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 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"

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 endpoint:

GET /_matrix/identity/v2/hash_details

{
  "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.

"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 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.

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 (which may be the case if the pepper was changed right after the client's request for it), then the server must inform the client that they need to query the hash details again, instead of just returning an empty response, which clients would assume to mean that no contacts are registered on that identity 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 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"
}

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 IDs). Once hashing has been performed using the defined hashing algorithm, the client sends the first k characters of each hash in an array, deduplicating any matching entries.

k is a value chosen by the client. It is a tradeoff between leaking the hashes of 3PIDs that the Identity Server doesn't know about, and the amount of hashing the server must perform. In addition to k, the client can also set a max_k that it is comfortable with. The recommended values are k = 4 and max_k = 6 (see below for the reasoning behind this). Let's say the client chooses these values.

NOTE: Example digests, not real hash values.

"email alice@example.commatrixrocks" -> "70b1b5637937ab99f6aad01f694b3665541a5b9cbdfe54880462b3f1ad35d1f4"
"email bob@example.commatrixrocks"   -> "21375b56a47c2cdc41a0596549a16ec51b64d26eb47b8e915d45b18ed17b72ff"
"email carl@example.commatrixrocks"  -> "758afda64cb6a86ee6d540fa7c8b803a2479863e369cbafd71ffd376beef5d5f"
"msisdn 12345678910matrixrocks"      -> "21375b3f1b61c975b13c8cecd6481a82e239e6aad644c29dc815836188ae8351"
"email denny@example.commatrixrocks" -> "70b1b5637937ab9846a94a8015e12313643a2f5323ca8f5b4ed6982fc8c3619b"

Also note that pairs (bob@example.com, 12345678910) and (alice@example.com,
denny@example.com) have the same leading characters in their hashed
representations.

POST /_matrix/identity/v2/lookup

{
  "hashes": [
    "70b1",
    "2137",
    "758a"
  ],
  "algorithm": "sha256",
  "pepper": "matrixrocks"
}

The identity server, upon receiving these partial hashes, can see that the client chose 4 as its k value, which is the length of the shortest hash prefix. The identity server has a "minimum k", which is a function of the amount of 3PID hashes it currently holds and protects it against computing too many per lookup. Let's say the Identity Server's min_k = 5 (again, see below for details).

The client's k value (4) is less than the Identity Server's min_k (5), so it will reject the lookup with the following error:

{
  "errcode": "M_HASH_TOO_SHORT",
  "error": "Sent partial hashes are too short",
  "minimum_length": "5"
}

The client then knows it must send values of at least length 5. It's max_k is 6, so this is fine. The client sends the values again with k = 5:

POST /_matrix/identity/v2/lookup

{
  "hashes": [
    "70b1b",
    "21375",
    "758af"
  ],
  "algorithm": "sha256",
  "pepper": "matrixrocks"
}

The Identity Server sees the hashes are within an acceptable length (5 >= 5), then checks which hashes it knows of that match the given leading values. It will then return the next few characters (n; implementation-specific; lower means less information leaked to clients at the result of potentially more hashing to be done) of each that match:

The identity server found the following hashes that contain the leading
characters:

70b1b5637937ab99f6aad01f694b3665541a5b9cbdfe54880462b3f1ad35d1f4
70b1b1b28dcfcc179a54983f46e1753c3fcdb0884d06fad741582c0180b56fc9
21375b3f1b61c975b13c8cecd6481a82e239e6aad644c29dc815836188ae8351

And if n = 7, the identity server will send back the following payload:

{
  "hashes": {
    "70b1b": ["5637937", "1b28dcf"],
    "21375": ["b3f1b61"]
  }
}

The client can then deduce which hashes actually lead to Matrix IDs. In this case, 70b1b5637937 are the leading characters of "alice@example.com" and "denny@example.com", while 21375b3f1b61 are the leading characters of "+12345678910" and 70b1b1b28dcf does not match any of the hashes the client has locally, so it is ignored. "bob@example.com" and "carl@example.com" do not seem to have Matrix IDs associated with them.

Finally, the client salts and hashes 3PID hashes that it believes are associated with Matrix IDs and sends them to the identity server on the /lookup_hashes endpoint. Instead of hashing the 3PIDs again, clients should reuse the peppered hash that was previously sent to the server. Salting is performed to prevent an identity server generating a rainbow table to reverse any non-Matrix 3PIDs that slipped in. Salts MUST match the regular expression [a-zA-Z0-9]*.

Computed previously:

"email alice@example.commatrixrocks"
becomes
"70b1b5637937ab99f6aad01f694b3665541a5b9cbdfe54880462b3f1ad35d1f4"

The client should generate a salt. Let's say it generates "salt123". This
value is appended to the base64-representation of the hash digest of the
initial 3pid and pepper.

"70b1b5637937ab99f6aad01f694b3665541a5b9cbdfe54880462b3f1ad35d1f4"
becomes
"70b1b5637937ab99f6aad01f694b3665541a5b9cbdfe54880462b3f1ad35d1f4salt123"

Which is then hashed:

"70b1b5637937ab99f6aad01f694b3665541a5b9cbdfe54880462b3f1ad35d1f4salt123"
becomes
"1f64ed6ac9d6da86b65bcc68a39c7c4d083f77193ec7e5adc4b09617f8d0d81a"

A new salt is generated per hash prefix and applied to each hash individually. Doing so requires the identity server to only rehash the 3PIDs whose unsalted hashes matched the earlier prefixes (in the case of 70b1b, hashes 5637937... and 1b28dcf...). This adds only a small multiplier of additional hashes needing to be performed by the Identity Server (the median number of hashes that fit each prefix, a function of the chosen k value).

An attacker would now need to create a new rainbow table per hash prefix, per lookup. This reduces the attack surface significantly to only very targeted attacks.

POST /_matrix/identity/v2/lookup_hashes

{
  "hashes": {
    "70b1b": {
      "1": "1f64ed6ac9d6da86b65bcc68a39c7c4d083f77193ec7e5adc4b09617f8d0d81a", 
      "2": "a32e1c1f3b9e118eab196b0807443871628eace587361b7a02adfb2b77b8d620"
    },
    "21375": {
      "1": "372bf27a4e7e952d1e794f78f8cdfbff1a3ab2f59c6d44e869bfdd7dd1de3948"
    }
  },
  "salts": {
    "70b1b": "salt123",
    "21375": "salt234"
  }
}

The server reads the prefixes and only rehashes those 3PIDs that match these hashes (being careful to continue to enforce its min_k requirement), and returns them:

{
  "mappings": {
    "70b1b": {
      "2": "@alice:example.com"
    },
    "21375": {
      "1": "@fred:example.com"
    }
  }
}

The client can now display which 3PIDs link to which Matrix IDs.

How to pick k

The k value is a tradeoff between the privacy of the user's contacts, and the resource-intensiveness of lookups for the identity server. Clients would rather have a smaller k, while servers a larger k. A larger k also allows the identity server to learn more about the contacts the client has that are not Matrix users. Ideally we'd like to balance these two, and with the value also being a factor of how many records an identity server has, there's no way to simply give a single k value that should be used from the spec.

Instead, we can have the client and identity server decide it amongst themselves. The identity server should pick a k value based on how many 3PIDs records they have, and thus how much hashes they will need to perform. An ideal value can be calculated from the following function:

C <= N / (64 ^ k)

Where N is the number of 3PID records an identity server has, k is the number of
characters to truncate each hash to, and C is the median number of hashing rounds
an identity server will need to perform per hash (denoted complexity). 64 is the
number of possible characters per byte in a hash, as hash digests are encoded in
url-safe base64.

Identity servers should choose a complexity value they're comfortable with.
Let's say 5 (for reference, HIBP's service has set their k value for a complexity
of 478: https://blog.cloudflare.com/validating-leaked-passwords-with-k-anonymity/)

When C is set (implementation specific), k can then be solved for:

k >= - log(C/N)
     ----------
     - log(64)
     
Taking HIBP's amount of passwords as an example, 600,000,000, as N and solving for k, we get:

k >= 4.47

We round k to 5 for it to be a whole number.

As this is quite a lot of records, we advise clients to start with k = 4, and go from there.

For reference, a very small identity server with only 600 records would produce a
minimum k of 0.628, or 1.

From this we can see that even low k values scale to quite a lot of records.

Clients themselves should pick a reasonable default k, and a maximum value that they are comfortable extending towards if the identity server requests a higher minimum number. If the identity server requests too high of a minimum number, clients will need to inform the user, either with an error message, or more advanced clients could allow users to tweak their k values.

Past what they already knew, from this exchange the client and server have learned:

Client:

Unsalted, peppered partial 3PID hash "70b1b1b28dcf" of some matrix user (harder to crack, and new rainbow table needed)
alice@example.com -> @alice:example.com (required)
+1 234 567 8910 -> @fred:example.com (required)

Server:

Partial hash "758af" (likely useless)
The server knows some salted hash 70b1b5637937ab9846a94a8015e12313643a2f5323ca8f5b4ed6982fc8c3619bf (crackable, new rainbow table needed)

No parameter changes will be made to /bind.

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.

If an identity server is too old and a HTTP 404, 405 or 501 is received when accessing the v2 endpoint, they should fallback to the v1 endpoint instead. However, clients should be aware that plain-text 3PIDs are required, and SHOULD ask for user consent to send 3PIDs in plain-text, and be clear about where they are being sent to.

Tradeoffs

There is a small cost incurred by performing hashes before requests, but this is outweighed by the privacy implications of sending plain-text addresses.
Identity services will need to perform a lot of hashing, however with authentication being added in MSC 2140, effective rate-limiting is possible.

Potential issues

This proposal does not force an identity server to stop handling plain-text requests, because a large amount of the Matrix ecosystem relies upon this behavior. However, a conscious effort should be made by all users to use the privacy respecting endpoints outlined above. Identity servers may disallow use of the v1 endpoint, as per above.

Unpadded base64 has been chosen to encode the value due to use in many other portions of the spec.

Conclusion

This proposal outlines an effective method to stop bulk collection of user's contact lists and their social graphs without any disastrous side effects. All functionality which depends on the lookup service should continue to function unhindered by the use of hashes.

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