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Versions: 00 01

Internet Engineering Task Force                         J. Pouwelse, Ed.
Internet-Draft                            Delft University of Technology
Intended status: Informational                          January 18, 2018
Expires: July 22, 2018

                          TrustChain protocol


   TrustChain is a protocol for a networked datastructure, designed to
   act as a trust ledger.  This protocol acts as a decentral alternative
   to platforms like eBay, Airbnb, and Uber.  It is specifically
   designed to record transactions among strangers without central
   control, support high transaction volumes, be application neutral,
   and avoid vendor lock-in.  The protocol defines recording
   transactions in an ordered list using an append-only datastructure, a
   new communication overlay, and a horizontally scalable consensus
   protocol based on checkpoint consensus, called CHECO.  Trustchain has
   resistance to traditional blockchain attacks, such as the 51 percent
   majority attack.  This is achieved by using a graph-based append-only
   datastructure combined with a personal blockchain for each
   participant with their own genesis block.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 22, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Trustchain Stack: Engineering trust . . . . . . . . . . . . .   5
   3.  Trustchain Fabric: internal data structure  . . . . . . . . .   7
     3.1.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  TxBlock specification . . . . . . . . . . . . . . . . . .   9
     3.3.  Asynchronicity  . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  CHECO: Consensus protocol and block format  . . . . . . .  11
   4.  IPv8: Overlay for identity, discovery and trust . . . . . . .  11
     4.1.  Identity establishment and discovery  . . . . . . . . . .  12
     4.2.  Attestations and trust  . . . . . . . . . . . . . . . . .  12
     4.3.  Peer-to-peer cryptographically signed messaging . . . . .  13
     4.4.  NAT traversal . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Attack resistances  . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Sybil attacks . . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  Double spending attack  . . . . . . . . . . . . . . . . .  15
     5.3.  Replay attack . . . . . . . . . . . . . . . . . . . . . .  15
     5.4.  Whitewashing attack . . . . . . . . . . . . . . . . . . .  15
     5.5.  Spam attack . . . . . . . . . . . . . . . . . . . . . . .  15
     5.6.  DDoS  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   For the past 10 years various distributed ledgers have been deployed
   and used.  This protocol aims to establish some form of trust using

   Creating trust between strangers is at the core of numerous
   successful Internet companies.  Starting 22 years ago, Craigslist

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   offered an unmoderated mailing list of advertisements and gossip on
   which buyer and seller could be trusted.  eBay formalised this in
   1997 and introduced a star-based rating system that enables traders
   to build a trustworthy profile [resnick2002trust].  The e-commerce
   platform was launched at a time when people were still hesitant to
   use their credit card on a technology called The Internet.  Nowadays,
   people let strangers sleep in their houses using Airbnb (since 2008).
   We trust Uber (since 2009) with our physical security and get into
   cars late at night with a driver that has not undergone a criminal
   background check or given a government license.

   The Trustchain protocol aims to create a generic approach and
   continue this evolution of building trust.  Compared to successfull
   central platforms we propose a distributed open underlying
   infrastructure, based on blockchain inspired technology.  Bitcoin
   created money without the need for banks [nakamoto2008bitcoin].  In
   the past, people were required to trust a central bank and a host of
   other intermediaries when making payments [kokkola2011payment].  The
   fundamental technology of Bitcoin, blockchain, radically reduced the
   need to trust financial middlemen.  It bootstrapped an economy where
   no one can be stopped from spending their money.  Despite widespread
   speculation and ecosystems being worth billions, blockchain in
   general suffers from scalability issues due to inefficient mechanisms
   for fraud prevention.  Bitcoin is theoretically limited to seven
   transactions per second and Ethereum has a throughput of around 20
   transactions per second [vukolic2015quest].  Despite various
   scalability efforts like proof-of-stake and sharding, broader
   adoption of blockchain stays out.  Mt.  Gox was at one point the
   largest Bitcoin exchange worldwide.  While a majority of Internet
   users trust the company behind popular platforms, the events
   involving Mt.  Gox highlighted how digital trust can be established
   and compromised[mcmillan2014inside].  In 2014, hackers stole Bitcoin,
   worth around $460 million at that time.  This event, together with
   major data breaches in 2017 at high-profile companies like Uber and
   Equifax, exposed the weakness of centralized architectures
   [apostle2017uber].  These events motivated this proposed protocol
   around decentralised infrastructures, not owned or operated by a
   single authority.  The generic problem of building trust between
   strangers resides on the edge of technology, sociology and
   behavioural science [yan2008trust].  The question whether someone can
   be trusted, depends on properties like personality, level of
   authority, culture and past behaviour.  In this protocol, we address
   the trust problem from a technological perspective, using tamper-
   proof interactions on a scalable blockchain.  This structure is built
   to help ease the detection of fraudulent behaviour and
   misrepresentation.  Trust calculations are out of scope of this work,
   we provide the enabling mechanisms.

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1.1.  Purpose

   This draft describes the TrustChain architecture, protocols and the
   used technologies, designed to model application neutral trust
   between interacting parties.  TrustChain relies on a new
   communication layer on top of existing communication networks which
   is designed with carrier-grade NAT infrastructure in mind, as well as
   the network protocol based on the CrawlRequest and TxBlock message
   types.  A consensus protocol called CHECO (Cong et al, 2017
   [cong2017blockchain]) is incorporated into TrustChain, which will be
   discussed but not elaborated in this draft.  It is based on the
   blockchain paradigm where the complete network represents a ledger
   where agents' transactions infer an amount of trust between the
   involved parties, as is described by pouwelse, 2017

   As protocols have slowly been moving towards the business layer in
   the past decade, TrustChain is implemented on top of a networking
   overlay and as such is network agnostic.  Other examples of moving
   networking to the business layer are: R3 Corda (Brown, 2017
   [brown2017introducing]) and IOTA (Atzori, 2016
   [atzori2016blockchain]).  The overlay, audaciously called IPv8,
   provides encrypted communication between public keys.  This overlay
   has integrated NAT puncturing to support, for instance, Android-to-
   Android overlay communication, does not require any central server,
   lacks central authorities, and can run directly on top of UDP, TCP,
   or other protocols.  As such, IPv8 provides a set of communication
   methods and messages that provide the required functionality to let
   TrustChain function properly on both PC networks or smartphone-only

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.3.  Terminology

       The actual user representation, but can not be directly used,
       since all information is identifier based.

       A reference that is owned by a given Identity, referring to this
       Identity.  Any Identity can have multiple Identifiers, whilst
       staying anonymous.

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       A node in the TrustChain network representing an identifier for a
       given identity.

       The basic unit of TrustChain communication.  A message will have
       different representations on the wire depending on the transport
       protocol used.  Messages are typically multiplexed into a
       datagram for transmission.

       A sequence of messages that is offered as a unit to the
       underlying transport protocol (UDP, etc.).

       An interaction between two agents containing information on both
       parties and what has been transacted.  This is application
       agnostic, meaning that any given application can infer what type
       of information it needs based on a collection of transaction.

       A cryptographic function that used the private key to create a
       representing string that can be verified by any other party using
       the signer's public key.

       The result of applying a cryptographic hash function, more
       specifically SHA-256 [FIPS180-4], to a piece of data.

2.  Trustchain Stack: Engineering trust

   Our principle mechanism to establish some form of trust is: if
   everyone keeps their secret keys secure, then no signed transaction
   can be spoofed on the overlay by any significant likelihood.  We
   refer to the Trustchain protocol when discussing the mechanism to
   record interactions which are cryptographically signed by multiple
   parties.  We explicitly do not support transactions signed by only a
   single party, which is the foundation of Bitcoin.  Our foundation is
   a multi-signature agreement, without mono-signature support.  The
   Trustchain stack refers to the full system which also includes the
   upper application layer, the network overlay and self-sovereign
   identity layer.  Together they form a complete solution stack.

   The concept of a Self-Sovereign Identity (SSI) (Abraham, 2017
   [abrahamself]) means that agents have full control over their
   identity data, and provide it to those who need to validate it,
   without relying on a central repository of identity data of any kind.
   A large part of any SSI based system is rooted in the problem of

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   proofs and attestations: proof can be anything, such as a secret
   message that is re-encrypted with a shared secret.  But the most
   challenging part is working with attestations, an attestation is when
   a third party validates that according to their records, the claims
   are true, a more transitive property of trust: C attests to B that A
   is who it claims to be, Azouvi et al, 2017 [azouvi2017secure].  As
   such, an attestation from the right authority could be more
   trustworthy than a proof, which might have been forged.  However,
   attestations can be a burden on the agent as the information can be
   sensitive, the information needs to be maintained so that only
   specific agents can access it.  Our stack does not constrain the
   choice of SSI system, but our implementation is focused on the Boneh-
   Franklin [boneh2004secure] 2-DNF scheme ("Evaluating 2-DNF Formulas
   on Ciphertexts").

   Based upon the assumption that these identities are persistent and
   secure, the new architecture (or Fabric) is designed to use Peer-to-
   Peer communication to increase the transaction throughput.  This
   communication is based on the new networking overlay: IPv8, which
   handles peer discovery, making connections with them across NAT boxes
   and peer-to-peer cryptographically signed messaging.  As IPv8 is
   transport and application agnostic, it can run over any transport
   protocol: it does not depend on IP and may run on top of NDN, XIA,
   and other new Internet architectures.

   To ensure that the blockchain always is in a valid state, a new
   horizontal scaling consensus protocol is proposed: CHECO.  CHECO is
   specifically designed to counter the vulnerabilities that a
   distributed, permissionless, multi-chain architecture will have to
   cope with (although this also creates innate vulnerabilities to other
   kinds of attacks).  By creating an indication of the state of
   validity for each agent, the responsibility of verification lies with
   the agent itself, a malicious agent in an invalid state can easily be
   detected, and should be avoided for interactions.

   We do not constrain or limit the applications utilising this
   blockchain, each transaction block can contain information of
   arbitrary content and length.  Currently there is a basic
   decentralised market implemented.  More specialised market have also
   be implemented and emulated such as a open Taxi service market and a
   mortgage investment market.  Multiple applications can be used at the
   same time, next to the decentralised market, for instance: it could
   be used for byte accounting in a ad-hoc Manet

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       |                          |
       |   <Application layer>    |
       |                          |
       |                          |
       |           CHECO          |
       |                          |
       |                          |
       |    Trustchain Fabric     |
       |                          |
       |                          |
       |           IPv8           |
       |                          |
       |                          |
       | Self-Souvereign Identity |
       |                          |

                                 Figure 1

3.  Trustchain Fabric: internal data structure

   Trustchain is designed to be a non-blocking format for agents that
   supports simultaneous interactions with other agents.  Non-blocking
   is a requirement rooted in the immutability of the chain and the
   strict ordering of the blocks.  To support this, the blocks are
   designed to be dependent on signing by all participating agents, and
   will be called TxBlocks henceforth, as is described in this section,
   along with the macro data structure in which these blocks are used.

3.1.  Architecture

   In contrast to traditional blockchains, in Trustchain every agent in
   the network has its own genesis node, in essence creating a personal
   blockchain for each agent.  Each interaction creates a new
   transaction block which is based on the last block of the two (or
   more) concerned parties.  This does not only influence the block-
   creation speed, but also the amount of effort needed to verify a
   chain.  Along with some other security properties, this is one of the
   implicit capabilities of this protocol.

   By removing the proof-of-work mechanism needed for classic blockchain
   implementations, Trustchain yields inherent horizontal scalability.

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   However the cost of scalability is that each application requires a
   mechanism to guard against transaction spam and abuse.  Trustchain is
   based on the assumption that both parties agree on the transaction
   before signing it, making tampering inherently easy to detect.  One
   of the aspects that supports this is the fact that TrustChain is
   organised as a set of temporally ordered, intertwining chains, which
   form a Directed Acyclic Graph (DAG).  This is called a "bottom-up
   consensus model", giving the participants the responsibility to
   verify the correctness of the transaction in stead of a central
   (sometimes randomly chosen) elected leadership.

   Trustchain depends on signatures from all participants in a
   transaction, creating a n-to-n node.  This system is extendible, as
   mentioned before, by extending the transaction description to provide
   specific properties.  Each transaction is stored in a block, in
   agreement-block format, each block has parts that are signed and
   submitted by all participating parties, where the initial request is
   called the block-proposal and a completely signed and validated block
   is called a agreement-block (where pair indicates the cooperation,
   not the limitation to two participating parties).  These are signed
   and sequenced so that each sequence number is unique in its accessory
   chain, as can be seen below in the general structure diagram.

       |                      |
       | Transaction A with D |
       |                      |
       |                      |
       | sequence number A: 3 |
       | signed by A          |
       | sequence number D: 49|
       | signed by D          |
                 |  |
                 |  +----------------------+
                 |                         |  |
       +---------+--+---------+  +---------+--+---------+
       |                      |  |                      |
       | Transaction A with C |  | Transaction D with B |
       |                      |  |                      |
       |                      |  |                      |
       +----------------------+  +----------------------+
       | sequence number A: 2 |  | sequence number D: 48|

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       +----------------------+  +----------------------+
       | signed by A          |  | signed by D          |
       +----------------------+  +----------------------+
       | sequence number C: 4 |  | sequence number B: 12|
       +----------------------+  +----------------------+
       | signed by C          |  | signed by B          |
       +---------+--+---------+  +---------+--+---------+
                 |                         |  |
                 |  +----------------------+
                 |  |
       |                      |
       | Transaction A with B |
       |                      |
       |                      |
       | sequence number A: 1 |
       | signed by A          |
       | sequence number B: 11|
       | signed by B          |

                                 Figure 2

3.2.  TxBlock specification

   Using the proposal-block and agreement-block formats means singing
   the blocks on the current views of the respective parties: the
   requester and the responder(s).  Each party signs and fills the block
   with the information that it has at that specific point in time.  The
   requester fills the structure with his own previous hash and his own
   part of the transaction data, signs it and sends it to the
   responder(s), which in turn construct the other sections of the
   block, if it agrees with the content before sending it back.  This
   nullifies any ordering and asynchronicity issues, since the requester
   constructs the block with the information that he has, and keeps it
   in memory while it waits on the responder to send the finished block

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   | Number |        Description         |     Type     | Size (bytes) |
   |   1    |    Requester public key    |  Char array  |      74      |
   |   2    | Requester sequence number  | Unsigned int |      4       |
   |   3    |    Responder public key    |  Char array  |      74      |
   |   4    | Responder sequence number  | Unsigned int |      4       |
   |   5    |  Requester previous hash   |  Char array  |      32      |
   |   6    |         Signature          |  Char array  |      64      |
   |   7    | Transaction block size (n) | Unsigned int |      4       |
   |   8    |     Transaction block      |  Char array  |      n       |
   |        |          *Total:*          |              |   256 + n    |

                    Table 1: TxBlock fields description

3.3.  Asynchronicity

   Because there is the need to communicate between the requester and
   responder(s), there will be a delay which may be significant.  To
   have a high level of asynchronicity and enable multiple peers
   interacting simultaneously, extending the chain should be able while
   waiting for a response.  In order to do this, the block refers to the
   previous block using the hash of the requesters part, since this is
   the only stable reference at that point.  The other hash reference
   (the "previous hash responder") can then be either the "hash
   requester" or "hash responder" part of the head-block of the
   responder chain.  Which one is used depends on whether the responder
   was the requester or responder in its previous interaction.  This
   mechanic is also used for the "previous hash requester" field, but
   this reference is known when the block is created.  In effect, this
   results to theoretically unlimited horizontal scalability: the more
   actors are active on the chain, the more throughput can be achieved.
   Though this is in fact limited by the memory speed, or database
   slowdown when the chain grows.

   One of the drawbacks of this mechanic is when the responders does not
   sign and respond, whether because it will/can not, there will be an
   orphan block.  While this is not a vulnerability on itself, it might
   be the starting point of a certain type of attack (the other "normal"
   types of attacks used for blockchains can be mitigated, at least to a
   certain level, as is described in resistances (Section 5).).  The
   adversary might let someone initiate a transaction an block creation,
   after which it will create an orphan.  Doing this multiple times in a
   short time span will force the requester to use a considerable amount
   of processing power and memory, all the while injecting orphan block
   into his chain.  As mentioned before, this is not a vulnerability in
   itself, but might be a launchpad for a more elaborate attack.  Though

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   one coping method could be to split larger, more vulnerable
   transactions up into multiple smaller transactions.  This way the
   consequences stay the same for the malicious actor, but the losses
   are smaller.

3.4.  CHECO: Consensus protocol and block format

   The final part of the Trustchain Fabric is CHECO, a horizontally
   scaling consensus protocol specifically designed for multi-chain
   implementation and completely application agnostic.  CHECO is based
   on three separate protocols: A consensus protocol, a transaction
   protocol and a validation protocol, as wel as an extension of the
   architecture by introducing a new type of block next to the TxBlock:
   CpBlock.  Every round a set of so-called facilitators is selected at
   random, which collect the CP and TX blocks, to feed to the validation
   protocol, after which te results are broadcast before a new round

   CHECO is designed to create an internal state ledger for each chain,
   without having to rely on separate methods or instances.  This is
   achieved by introducing a new block: the Checkpoint block (CpBlock),
   which contains a hash pointer to the previous block, along with a
   hash of the consensus result, round and sequence number and, lastly,
   a signature.  The consensus result is defined as a tuple containing
   the validity states of the blocks agreed on by the facilitators of
   that round and the round number.  If your chain is deemed valid, a
   CpBlock is injected, thus validating the state of the chain.  While
   the content of the injected CpBlock differs from the TxBlock, these
   blocks to not interfere with the transaction protocol, since they fit
   in the architecture without modifying it.

   Instead of requiring a proof-of-work (as seen in more conventional
   blockchain implementations), CHECO is round based, creating a
   consensus state every so often, creating a fully asynchronous and
   horizontally scaling protocol.  These facilitators are chosen
   randomly each round, and will collect the CP blocks from all other
   nodes since their last CpBlock.  Validation is done using
   Asynchronous Common Subset (ACS) algorithm based on HoneyBadgerBFT
   (Miller et al, 2016 [miller2016honey]), a byzantine consensus

4.  IPv8: Overlay for identity, discovery and trust

   To enable this new platform to function properly, a new method to
   find, connect to and manage agents was needed.  Additionally, new
   models for identity verification, network discovery and inter-peer
   trust were required to enable these agent methods.  It is a network
   stack, a set of protocols and models, that separates concerns and

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   enables applications (such as Trustchain) to use the needed methods
   and protocols, without giving up interoperability and upgradeability.
   On top of this interface lies the actual TrustChain overlay, which
   uses the components and protocols of the IPv8 interface to create the
   specific functionalities needed for TrustChain to function.

   IPv8 is built keeping the Unix Philosophy of creating small
   components that are easy to understand and test.  This is mainly why
   the complete networking system used in Tribler was re-written as a
   generic networking interface, enabling modifications and additions
   without losing any functionality.  Although agents might use
   different protocols based on their capabilities, IPv8 is a basic
   layer over a multitude of networking components and subsystems,
   creating a means to communicate with other agents regardless of
   networking capabilities.  An open source reference implementation
   based on Python is available on Github, called py-ipv8.
   Interoperable open implementations in Java and Kotlin are still only
   partially functional.

4.1.  Identity establishment and discovery

   Identities are created, attested and distributed over the Trustchain
   using IPv8 as the communication interface.  These are all public and
   self sovereign, leading to distributed when an agent creates a new
   identity, and are organised using a Public Key Infrastructure (PKI).
   This distribution is done to agents who have (or might have) interest
   in having the identity of this agent, since they are in the same
   cluster or due to other factors.  Spreading information this way is
   called Gossiping, since agents learn of new identities, and spreading
   them among directly connected agents like a gossip would spread.

   Discovering identities is done based on Distributed Hash Tables
   (DHT), somewhat similar to the Domain Name System (DNS) currently
   used for the web, using Random-Walk and Live-Edge-Walk: discovery
   protocols for DHTs, respectively based on making random DHT queries
   in order to learn about a large number of identities quickly and
   making pseudo-randomised queries about the agents with the highest
   trust scores in the network.  Using the random walks causes the DHTs
   to converge much faster, whilst having a small load at the very
   beginning, but by traversing the live edges, TrustChain is more spam
   and Sybil attack resistant.

4.2.  Attestations and trust

   The primary problem with identities and proofs is falsification, and
   need to be verified to prevent this.  However, even proofs can be
   forged, leading to the problem of needing trust in a proving party,
   which can not be solved by having a more centralised proving party.

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   This is where attestations are used, witness reports that the
   identity in question is actually valid, and requires some level of
   trust between the agents.  Attestations are a method to enable agents
   to validate interactive zero-knowledge proofs (ZKPs) within a network
   of agents, a so-called web-of-trust [azouvi2017secure], but the
   transitive property of trust is used to prevent the need for every
   agent to verify an identity itself as is noted in Section 2.

   In this case, the probabilistic homomorphic asymmetrical encryption
   scheme of Boneh-Franklin [boneh2004secure] is used to validate these
   proofs, meaning a form of randomness is used in the encryption and
   computations on the ciphertext result in a valid plaintext.  Using
   Boneh-Franklin leads to these ZKPs to be hardened agains chosen
   plaintext attacks (an attacker can encrypt the suspected plaintext to
   see if the ciphertexts match) with the probabilistic aspect.  These
   attestations are tied to metadata, which can be verified separately
   and the related identities are stored internally in a database, and
   these metadata attributes are gossiped around the network using

4.3.  Peer-to-peer cryptographically signed messaging

   Most messages that are sent between peers are encrypted, the keys are
   retrieved from either the PKI, and verified based on the key
   attestations, or from the internal database after verification
   already has been done.  Encryption is done using the ECDSA encryption
   scheme [johnson2001elliptic], which is an Elliptic Curve based
   cryptography system.  This enables IPv8 to send messages effectively
   to a multitude of agents when (at the least) their identities are
   known, preferably also attested for.  Having such a low bar to
   encrypt the messages discourages the use of unencrypted
   communications channels, making interactions secure almost as early
   as the handshake, as the first message sent to any agent can be
   already encrypted with its respective key (which is retrieved from
   the database, other agents or the initial bootstrapping list).

4.4.  NAT traversal

   Network Address Translation (NAT) is omnipresent in the modern
   Internet, mostly due to networks being separated and the limited
   amount of global IP addresses available.  Most consumer devices are
   behind a number of layers of NAT, but data center nodes can be behind
   NAT for security or virtualisation reasons.  Containerised
   deployments are making things worse, making every peer based
   communication scheme must have a way to traverse NATs, otherwise
   operations will be affected.  Even nodes meant to run with real IP
   addresses must implement NAT traversal techniques, as they may need
   to establish connections to peers behind NAT.  Messages puncturing

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   based on UDP is key to this overlay.  It conducts a random network
   walk to preserve connectivity under churn.  Participants helps each
   other to puncture NAT infrastructure.  Each participant will
   periodically introduce and connect some of its neighbors.  When their
   random neighbors do not yet know each other, a new participant is
   discovered.  Carefully timed concurrent UDP messages are used to
   traverse carrier-grade NAT infrastructure.  Implementation,
   deployment and measurements of smartphones users has show that it is
   possible to build a healthy overlay without servers, even if nearly
   100 percent of users are behind a NAT (Android-to-android overlay

5.  Attack resistances

   For blockchain implementations, attack resistance is an important
   requirement, especially with horizontal scalability.  Therefore it
   will have to cope with the same difficulties and attacks that other
   blockchain implementations have to, but due to the novel structure
   are be countered.  With this novel structure, validation,
   uncertainty, and tamper proofing can be handled in a more intuitive
   manner while throughput does not need to suffer.

5.1.  Sybil attacks

   One of the most difficult attacks to repel for a blockchain
   implementation is the sybil attack, where many agents are injected
   into the chain (and authenticity cannot easily be verified) to
   subvert a large portion of the systems voting power or trust (see
   section 3.3).  Usually peer verification is used to resist this kind
   of attacks (for instance: proof-of-work), usually resulting in slow
   systems.  But when the influence of the attacker is large enough,
   even these methods will not be able to stop such an attack.

   Trustchain deal with this problem by having an inherently different
   structure, where each peer has its own origin.  On top of that,
   transaction injection can only be done with two valid signatures,
   meaning a sybil attacker can only create trust with himself.  This
   results in a network of disconnected agents that have no relation
   outside of its own cluster, which can easily be identified.  Even
   when the sybils acquire some degree of trust outside of their
   cluster, using accounting mechanisms the profit from such an attack
   can only be weakly profitably beneficial with bounded profit (using
   Netflow, not discussed in this paper) (Otte et al, 2017

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5.2.  Double spending attack

   Using control over the blockchain to create a fork and creating two
   different transaction branches is called double spending.  This kind
   of attack can be applied with relative ease to single chain
   implementations of the blockchain by injecting two conflicting
   transactions at the same time.  Trustchain counters this kind of
   attack by having the chain verified with each CHECO round, during
   with the hidden transaction can be easily found.  By broadcasting
   both block as a proof-of-fraud the malicious agent will have
   decreased trust and can be blacklisted or refused service.

5.3.  Replay attack

   Using the transaction signature of the counterpart, a malicious agent
   can try to replay a transaction on the blockchain, which results in
   increased trust or could be used to gain credits.  CHECO and the
   novel structure mate it trivial to find the conflicting blocks when
   verifying the counterparties chain.  The two blocks with the same
   outgoing pointer together make the proof-of-fraud which then can be
   used to decrease the trust in the malicious party and can be
   blacklisted or refused service.

5.4.  Whitewashing attack

   Abusing the permissionless structure of Trustchain to create
   additional identities at any given point can negate the effect of
   having trust, so this kind of attack differs from a sybil attack.
   When an agent suffers from reputation loss, it can simply discard his
   current identity and take on a new one.  Since refusing service to
   agents with little trust will affect usability and willingness to
   join the network, and adequate solution can be prioritising
   strategies.  One method for implementing this is described in the
   paper discussing Netflow (not discussed in this paper) (Otte et al,
   2017 [otte2017trustchain]).

5.5.  Spam attack

   Since the TxBlocks have a dynamic size, spam (in the sense of useless
   data in the transaction field) can be used to clog an agents network
   or database with excessively large messages, slowing down its
   operations or bringing them to a complete halt due to memory/network
   being full.  This kind of attack can be coped with by having a
   throttle per connection to keep some bandwidth available and a limit
   on the size of the message.  If large messages are to be expected, a
   file based buffer will enable large message transfer without
   exceeding the memory capacity.

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   Another type of spam can be identified as a collection of useless
   messages, clogging the network and database with large amounts of
   empty messages, which is possible since transaction do not require an
   agent to pay transaction costs (as BitCoin does).  Though this is
   easily countered by not accepting those messages, leading to the
   malicious agent having a lot of orphan block.

5.6.  DDoS

   When massive quantities of useless or empty messages are sent over a
   network, it might get congested, leading to dropped operations,
   network congestion and unreachability of agents, or in the most
   extreme cases even to system failure due to overload.  Due to the
   distributed, peer based communications architecture, this is not
   feasible without flooding the network of the malicious agent itself,
   as it has to send messages to each target individually.

6.  Acknowledgements

   We very much thank the European Union for providing us the required
   funding for this work.  Through EU FP6 and FP7 funding instruments we
   have been developing and deploying our own distributed ledger fabric
   since August 2007.  An estimated 3.4 million Euro has been granted
   through these specific projects and leading directly to this work
   ([P2P-Fusion], [P2P-Next],[QLectives]).

   We thank master student Stijn for his help with writing of this

7.  IANA Considerations

   This memo includes no request to IANA.

   All drafts are required to have an IANA considerations section (see
   Guidelines for Writing an IANA Considerations Section in RFCs
   [RFC5226] for a guide).  If the draft does not require IANA to do
   anything, the section contains an explicit statement that this is the
   case (as above).  If there are no requirements for IANA, the section
   will be removed during conversion into an RFC by the RFC Editor.

8.  Security Considerations

   From a security perspective, the usage of novel structures such as
   Trustchain might lead to new kind of attacks.  We consider this risk
   of less importance for a private and consortium network, where all
   participants are known to the operator and authentication mechanisms
   are used to restrict access to the network.

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   For the public blockchain networks, the usage of Trustchain might
   lead to new kind of attacks.  For instance, an attacker might be able
   to pollute the chain with refusal to sign attacks to decrease trust.
   The scope of such attacks and security violations needs to be
   investigated and is not part of this draft.

9.  References

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              Gallagher, P., "Secure hash standard (shs)", 2008,

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              "Next Generation Peer-to-Peer Content Delivery Platform",
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Author's Address

   Dr. J.A. Pouwelse (editor)
   Delft University of Technology

   Phone: +31 15 2782539
   Email: j.a.pouwelse@tudelft.nl

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