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@ -2,21 +2,21 @@
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[Issue #2130](https://github.com/matrix-org/matrix-doc/issues/2130) has been
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recently created in response to a security issue brought up by an independent
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party. To summarise the issue, lookups (of matrix user ids) are performed using
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non-hashed 3pids (third-party IDs) which means that the identity server can
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identify and record every 3pid that the user wants to check, whether that
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party. To summarise the issue, lookups (of Matrix user IDs) are performed using
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non-hashed 3PIDs (third-party IDs) which means that the identity server can
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identify and record every 3PID that the user wants to check, whether that
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address is already known by the identity server or not.
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If the 3pid is hashed, the identity service could not determine the address
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If the 3PID is hashed, the identity server could not determine the address
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unless it has already seen that address in plain-text during a previous call of
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the /bind mechanism.
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Note that in terms of privacy, this proposal does not stop an identity service
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from mapping hashed 3pids to users, resulting in a social graph. However, the
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identity of the 3pid will at least remain a mystery until /bind is used.
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from mapping hashed 3PIDs to users, resulting in a social graph. However, the
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identity of the 3PID will at least remain a mystery until /bind is used.
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This proposal thus calls for the Identity Service’s /lookup API to use hashed
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3pids instead of their plain-text counterparts.
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3PIDs instead of their plain-text counterparts.
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## Proposal
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@ -137,10 +137,10 @@ ask for user consent accordingly.
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## Potential issues
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This proposal does not force an identity service to stop handling plain-text
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requests, because a large amount of the matrix ecosystem relies upon this
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This proposal does not force an identity server to stop handling plain-text
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requests, because a large amount of the Matrix ecosystem relies upon this
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behavior. However, a conscious effort should be made by all users to use the
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privacy respecting endpoints outlined above. Identity services may disallow use
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privacy respecting endpoints outlined above. Identity servers may disallow use
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of the v1 endpoint.
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Unpadded base64 has been chosen to encode the value due to its ubiquitous
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@ -152,7 +152,7 @@ address will have to be encoded when used as a parameter value.
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Ideally identity servers would never receive plain-text addresses, however it
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is necessary for the identity server to send email/sms messages during a
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bind, as it cannot trust a homeserver to do so as the homeserver may be lying.
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Additionally, only storing 3pid hashes at rest instead of the plain-text
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Additionally, only storing 3PID hashes at rest instead of the plain-text
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versions is impractical if the hashing algorithm ever needs to be changed.
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Bloom filters are an alternative method of providing private contact discovery,
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@ -162,7 +162,7 @@ are explored in https://signal.org/blog/contact-discovery/ Signal's eventual
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solution of using SGX is considered impractical for a Matrix-style setup.
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While a bit out of scope for this MSC, there has been debate over preventing
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3pids as being kept as plain-text on disk. The argument against this was that
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3PIDs as being kept as plain-text on disk. The argument against this was that
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if the hashing algorithm (in this case SHA-256) was broken, we couldn't update
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the hashing algorithm without having the plaintext 3PIDs. Well @toml helpfully
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added that we could just take the old hashes and rehash them in the more secure
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@ -176,4 +176,3 @@ This proposal outlines an effective method to stop bulk collection of user's
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contact lists and their social graphs without any disastrous side effects. All
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functionality which depends on the lookup service should continue to function
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unhindered by the use of hashes.
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