feat: create distributed-entropy.md

docs/ideas/distributed-entropy.md

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+# Distributed entropy
+
+Date: 2019-05-24
+Author: Antoine Eddi (aeddi)
+
+## Description
+
+The idea would be to exchange random data blocks between trusted devices (owned by the same account or by different contacts).
+The exchange will take place during device-to-device handshakes, we could add a step to our handshake protocol to exchange 64 bytes of random data.
+All the random data collected from different sources could be mixed up and stored securely so that it could be reused later to generate new cryptographic materials.
+
+NB: It is obviously out of the question to use the data provided by a single peer without mixing it with data from other sources to generate secrets.
+
+
+## Why?
+
+A particular device could have an unsafe PRNG for some reason. Mixing random data provided by different devices should prevent an attack baased on PNRG weaknesses specific to a particular device.
+
+
+## To be considered
+
+We don't know enough about the subject to assess whether it's a bad practice to do this kind of thing.