14-WAKU2-MESSAGE: Security issue related to unencrypted timestamp of WakuMessage

[jimstir]

Reference issue: vacp2p/rfc#439
Author: staheri14

Problem

While currently, the timestamp is not part of the relay Message, it is always accessible in plaintext even when the WakuMessage is encrypted. This is because encryption only affects the payload of WakuMessage but not other fields like timestamp. As such, a relay node is able to extract the timestamp from a relayed WakuMessage. This timestamp visibility may cause security issues. For example, one can break message unlinkability by having access to the sender timestamp i.e., an attacker links messages published by the same author through the pattern of timestamps. A similar problem has been reported for Waku v1/ Whipser in this issue status-im/nim-eth#105.
@arnetheduck would be great if you could elaborate on the security issues you mentioned in https://github.com/status-im/nim-waku/pull/681/files#r677171720.

Context on the timestamp use case in Waku2

The timestamp is currently used in the store protocol to index waku messages and to enable paging for historical queries.

Acceptance Criteria

• Investigate the use of timestamps and their visibility in other systems like the Status app, Waku v1, libp2p, etc.
• Identifying security implications of exposing timestamp
• Sketching out potential remedies

Potential solutions

• Anonymizing timestamp by using a coarser resolution (use a representative value for each time interval): research directions are 1) generalization to achieve k-anonymity or 2) differential privacy to randomize timestamp