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2017-02-15-EncryptedMessagingSystem.md

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---
layout:notes
title: Encrypted Messaging Systems 
scribe: Yingqiao Xiong
---

##'end to end encryption'

  • There are three types of encryption: 1. Encryption in Transit 2. end to end encryption 3. end to end encrption(no service provider)
  1. Encryption in Transit:

Alice -----encrypted------ service provider (no encrption) ------encrypted------ Bob

  1. end to end encryption:

Alice -----encrypted------ service provider(encrypted) ------encrypted------ Bob

  1. end to end encrption(no service provider)

Alice -----encrypted------ encrypted ------encrypted------ Bob

##the OTR 3-step Diffie-Hellman ratchet

      Alice                                             Bob
        g^a
[ encrypted message ]            ->       
[ next key advertisement ]                 
                                                        g^b
                                               [ encrypted message ]
                                   <-          [ Key Acknowledge ]
                                               [ next key advertisement ]
         g^c                                      
[ encrypted message ]
[ Key Acknowledge ]                ->
[ next key advertisement ]
                                                     ( k1, k2 ) = g^ab
                                                     ciphertext = ENCk1(msg)
                                                     ack = MACk2(something)

##Asyn hash ratchet

  • K1 = H(m)
  • K2 = H(K1)
  • K3 = H(K2) +That key material is sensitive, however, because it can be used to calculate the key material for every subsequent sequence number. So a client can't hang on to it forever

##Signal's 2-Step DH Ratchet ("Chain Keys")

       Alice                                         Bob

      [ Encrypted Message ]
[ Current Ephememeral key ] ->

      [ Encrypted Message ]
<- [ Current Ephememeral key ]

+1. Alice generates a new ECDH ephemeral key A1 and uses it immediately to send a message. +2. Alice receives a message with Bob's new ECDH ephemeral B1 and can then destroy A1 and generate A2 when sending her next message.

Meta Data

  • leaks of metadata can lead to great troubles.
  • NSA said "we kill people based on metadata"

Anonymous Communication with emphasis on TOR

 ------------------
 |  -------------  |
 |  |  --------- | |

    | | |MESSAGE| | | | | --------- | | ciphertext = Epk1 [Epk2[Epk3[message]]] | ------------- | -----------------

    Sever 1                     Sever 2 Sever 3

    ------------------  
    | ------------- | ------------ | | --------- | | | -------- | -------     | | | M1  | | | | | M2 | | | M1| M2 | | --------- | | | -------- | ------- | ------------- | ------------ -----------------

  ------------------
 |  -------------  |                  -----------
 |  |  --------- | |                 |   ------- |      Decrypt

    | | | M2 | | | Decrypt and | | M3 | | and ------ D and p     | | --------- | | permutate       |   ------- | permutate | M2 | ----> M3     | ------------- | --------------> ------------- ---------> ------ ----------------- ------------------ | ------------- | -------------- | | --------- | | | -------- | -----     | | |   M3 | | |                |   |   M1 | |                     | M3 | M1     | | --------- | |                | -------- | ------ | ------------- | ---------------- -----------------