Sophie Dramé-Maigné

Maryline Laurent

Laurent Castillo

Augmented chain of ownership : Configuring IoT devices with the help of the blockchain

The blockchain, distributed and unalterable ledger, has lowered the cost of ownership records. This mechanism that required the involvement of notaries and was once reserved for expensive items can now be used for every type of assets. We propose to use it to keep track of the ownership of IoT devices. By registering devices and their transfer into the blockchain, we build a chain of ownership that can be used to guarantee ownership before a sell or to track and warn owners of security threats faced by their devices. Two extensions are proposed. The first one facilitates remote configuration and key management when a single owner must configure a great number of devices. The second one offers potential users information about an IoT device. That information can be used to create a discovery service, inform access decisions or keep track of a device's state.

Introduction

In 2008, Nakamoto [START_REF] Nakamoto | Bitcoin: A peer-to-peer electronic cash system[END_REF] introduced the concept of the blockchain, a public shared unforgeable ledger allowing participants to register transactions in a persistent and decentralized manner. If it was intimately linked with cryptocurrencies at first, the blockchain has since been used in other applications such as voting[4], online games [START_REF]First blood[END_REF], ride sharing[1], and many others 3 . With properties such as desintermediation, unforgeability, and decentralization, the blockchain is also very attractive to the Internet of Things (IoT). Its usability in such setting has been studied with mixed results [START_REF] Christidis | Blockchains and smart contracts for the internet of things[END_REF][START_REF] Conoscenti | Blockchain for the internet of things: A systematic literature review[END_REF][START_REF] Kshetri | Can blockchain strengthen the internet of things?[END_REF][START_REF] Huh | Managing iot devices using blockchain platform[END_REF].

Another common blockchain use case is the tracking of ownership of assets such as houses, cars or artwork [START_REF] Rosenfeld | Overview of colored coins[END_REF]. Such initiatives have the power to combat corruption in countries where acquiring a land can be discussed with corrupt officials instead of the actual owner. Countries such as Sweden4 , Georgia 5 , and Ukraine6 are each at different stages of implementing a land title project using the blockchain.

In this paper, we propose to use the blockchain to track IoT devices' ownership. The blockchain is a cheaper alternative to ownership records when compared to traditional methods that involve an outside authority such as notaries. It is also a simpler and faster process. Thanks to the decreased cost and added usability, ownership records can then be used for more humble assets such as IoT devices.

This mechanism can also be used to exchange device-related secrets, enabling remote configuration and efficient key management. IoT use cases can involve many devices deployed in various physical locations. This makes manual configuration inefficient. Smart grids are a good example of hundreds of devices that need to be deployed to cover the entirety of electricity grids. The deployment speed is highly impacted by the configuration method, as many devices need to be configured at once. By leveraging the chain of ownership published in the blockchain, we propose to facilitate remote configuration. Additionally, owners can use the same mechanism to efficiently manage the multiple keys used to remotely manage their devices.

The mechanism used to register device ownership can also be used to publish information related to IoT devices, either for safekeeping or for advertizing their characteristics to potential users. IoT use cases depart from classical ones that involve only a small number of known actors. The list of clients for one IoT device can be dynamic. In order for that device to be trusted with a task, potential clients may require some guarantees. For instance, a user could demand that the device be running the latest version of a software, hence ensuring that security patches have been applied, or simply that its list of communication protocols intersect with her own. Such information can be published on the blockchain.

Related Work

Ownership tracking via a blockchain has already been implemented. On the Bitcoin blockchain, Colored Coins [START_REF] Rosenfeld | Overview of colored coins[END_REF] can be used to track asset exchanges. On the Ethereum blockchain [START_REF] Vitalik Buterin | Ethereum white paper[END_REF][START_REF] Wood | Ethereum: A secure decentralised generalised transaction ledger[END_REF], smart contracts [START_REF] Szabo | Formalizing and securing relationships on public networks[END_REF] can be programmed to do similar things. Other blockchains such as NXT [START_REF]Nxt[END_REF] provide a native asset exchange. There are also front-end applications [START_REF]Melonport[END_REF]2] that bridge several blockchains together to facilitate interoperation. These implementations are not IoT-specific but their general-purpose tokens are IoT-compatible. They are however only focussed on ownership record and cannot be used for key management or to share configurations.

In the academic literature, the transfer of ownership is addressed at the device level [START_REF] Osaka | An efficient and secure rfid security method with ownership transfer[END_REF][START_REF] Biplob R Ray | Universal and secure object ownership transfer protocol for the internet of things[END_REF]. Ownership transfer is defined [START_REF] Rekleitis | How to protect security and privacy in the iot: a policy-based rfid tag management protocol[END_REF] as "the capability to pass ownership of a tag to a third party without compromising backward untraceability for the said party or forward untraceability for the previous owner." The focus is on key management and domain boundaries. The devices that are con-cerned by these protocols are RFID tags. No record is kept of past owners. This article presicely focuses on these ownership records. Organization Security assumptions and threat models are presented in Section 2.

Based on the notations of Table 1, Section 3 introduces our tracking of ownership using the blockchain. Section 4 proposes an extension of the approach to configure IoT devices and manage keys for the sake of the owner. Section 5 turns to the users and explores what they can gain from it.

Security Considerations

Security assumptions

We operate under the following assumptions:

A1 Secured blockchain keys: Blockchain keys cannot be stolen, lost or otherwise compromised. This implies good key management. A2 Solid cryptographic primitives: Our proposal uses cryptographic primitives such as signatures, hashes, or encryption. We assume these primitives cannot be broken. A3 Blockchain consistency: Fundamental blockchain properties include consistency amongst nodes and consistency over time [START_REF] Pass | Analysis of the blockchain protocol in asynchronous networks[END_REF]. This implies that all nodes in the network will agree on blockchain history, the few last blocks excluded, and that accepted transactions cannot be modified. We assume these properties are verified and the blockchain history cannot be altered. A4 Blockchain capability: All actors (M , R, O, and U ) own a blockchain address, the corresponding public and private key pair, and the means of submitting or retrieving a transaction to or from the blockchain.

Threat model

Across our three proposals, we consider three types of attackers : a malicious new owner, a malicious previous owner, and a malicious uninvolved third party. We detail 8 possible threats involving these actors. These threats are summarized in Table 2.

Malicious previous owners This attacker's goal is to either fool a potential buyer by not providing the device after the sell has been concluded or to retain access to said device and gain access to sensitive data belonging to the new owner. As the previous owner, the attacker is in possession of the credentials that, at the time of the sell, allow access to the device. She also has the possibility to provision anything unto the device and is also able to produce a proof of ownership. When a device is sold, the previous owner can use her knowledge to gain access to sensitive information and maybe use the device as an entry point into the new owner's network. This defines Threat T 1. A prospective owner can also be fooled by the previous owner and buy a device that will not be delivered. This defines Threat T 2.

Malicious new owners

The goal of this attacker is to gain access to sensitive information without authorization. As the new owner, the attacker has full access and full control over the device. After the sell, if the device has not been wiped clean, the new owner can extract potentially sensitive information related to the former owner from the device itself. This defines Threat T 3. The new owner can also use the device's identity to gain access to previous owner's data. This can be done by interacting with users or devices that still recognize the device as being owned by the previous owner. This defines Threat T 4.

Malicious third party This attacker goal is to pass as a legitimate device owner to fool a potential buyer, steal and re-sale a device, disturb the sell or gain information about the parties involved in the ownership change. When a public blockchain is used, the attacker has access to all information that transits through the blockchain. She can also produce and submit valid blockchain transactions. First, the attacker can try to clog the blockchain network. In this event, the miners would not be able to process the transaction signaling the ownership change. This defines Threat T 5. Second, when the transfer occurs, the attacker may try to gain knowledge about the involved parties. This defined Threat T 6. Third, the attacker may pretend to be the owner of a device she does not possess. This is Threat T 7. Forth, the attacker may try to steal and resale the device. This threat is similar to Thread T 7. Fifth, the attacker may fabricate a blockchain trace for a device that does not actually exist. This is Threat T 8.

3 Asset ownership

Motivation

As previously mentioned, asset tracking is one of the most straightforward blockchain application. Assets that have been considered for this use case tend to be expensive and relatively immutable (i.e. land, cars, houses, paintings, etc). These objects' ownership will most likely already be tracked using third parties such as notaries or other government-sanctioned entities. The corresponding administrative procedures can be long and costly. By using the blockchain instead, trust in these third parties is no longer required. But that is not the only benefit. The cost of a transaction is also highly reduced. The transfer of ownership is a simple and quick operation. Ownership records are more likely to be up to date. For these reasons, ownership records do not have to be confined to expensive items. We propose to apply this principle to IoT devices.

Benefits of the proposal Keeping ownership records on the blockchain offers the following benefits:

-Desintermediation: Traditionally, changes in ownership must be attested and assisted by third parties. As the blockchain keeps a public proof of the transaction, they are no longer necessary. -Shared architecture: When using a blockchain, one can take advantage of the architecture deployed by others. This use case does not require server deployment or federation of systems. -Decentralized storage: This speaks to one of the fundamental blockchain property, persistence. Blockchain transactions will be stored in a decentralized fashion, protecting ownership record from loss and modifications. -Simplicity: The process by which the ownership is transferred requires a single transaction. Its simplicity makes it highly usable, even to private individuals. -Lower costs: Ownership transfer usually involves a third party. This third party will take a commission on the sale. The desintermediation therefore has the added benefit of lowering costs. -Traceability: Ownership of an object can be traced back to its original owner.

This gives a new buyer information about the life of a device, its age, maybe what it was previously used for, etc. -Proof of ownership: Before buying a device second hand, the buyer can require a proof of ownership. He sends a message over to the vendor. The vendor must then provide a valid signature. The private key used to sign the message must match the blockchain address that officially owns the device. This ensures that the device has not been stolen and that the vendor is allowed to proceed with the sale. -Security alerts: When an incident affecting a big number of devices occurs, it is currently hard to track owners and warn them of the issue. Owners can be private individuals. They are not likely to follow best security practices.

For that reason, in the event of a large scale IoT attack, being able to track and warn device owners could prevent further damage. -Availability: By its distributed nature, the blockchain offers availability guarantees that servers cannot match. -Pseudonymity: Traditionally, ownership records are nominative. This is natural as the proof of ownership is linked to one's identity. When the blockchain is used, such a proof is linked to what one owns, the correct blockchain private key. This enables pseudonymous records.

Proposal

We take our example at the very beginning of the ownership chain with the sell of a device D. We consider the following actors : the device's manufacturer M , and a retailer R that wishes to acquire D for its store. Following Assumption A4, both M and R possess a blockchain address, the corresponding public and private key pair, and the means of submitting or retrieving a transaction to or from the blockchain. The general idea is to link the asset's exchange to a series of blockchain transactions, thus creating a chain of ownership. There are two types of transaction available. The transaction that creates the link between the asset and its digital 3. Table 4 and Table 5 breakdown the construction of inputs and outputs respectively.

For an input to be valid, it must be signed with the key corresponding to the output's destination address: transaction tx 0 has 2 outputs, out 0 0 sent to addr A and out 0 1 sent to addr B . Transaction tx 1 uses out 0 0 as input. To be valid, the input must carry the public key corresponding to addr A along with a valid signature produced using priv A , private key corresponding to addr A . Because outputs only carry blockchain addresses, and because hashes are irreversible, the public key pub A is needed for the signature validation (reminder: addr A = Hash(pub A ). A genesis-type transaction has no input. Each output in a transaction corresponds to a different asset. Going back to our example, M issues a genesis transaction, tx 0 , with a single output sent to addr M , her own blockchain address, creating device D's digital representation and registering herself as the original owner. Before a sell can take place, M must produce a valid proof of ownership to R. To this effect, R sends a challenge message, m to M . Because the challenge is chosen by R, M cannot reproduce an intercept message. M signs the message with priv M and sends (Hash(tx 0 ), 0, Sign priv M (m), pub M ) back to R. Using the transaction identifier, R can check for herself the corresponding transaction in the blockchain. She validates that out 0 0 is not spent and was addressed to M . R then verifies the provided signature. If the signature is valid, the proof of ownership is accepted. It is interesting to note that providing such a proof does not compromise the owner's pseudonymity.

Following Assumption A1, blockchain keys cannot be stolen. Assumption A2 states that the blockchain's cryptographic primitives cannot be broken. This means that the only person capable of producing a valid proof of ownership is the owner of both the blockchain key and the device. Furthermore, since the blockchain history cannot be altered according to Assumption A3, once an ownership record has been published or updated, it cannot be modified. This neutralizes Threat T 7.

Now that M has proven he is the rightful owner of D, the sell can proceed. When R purchases the device, M issues a second transaction of type transfer, tx 1 . Transaction tx 1 has one input, out 0 0 , signed with priv M , and one output, out 1 0 sent to addr R . This second transaction transfers the ownership of D to R.

Limitations

Blockchain-related limitations The principal limitation of this solution is that the security of the scheme depends on the security of the underlying blockchain [START_REF] Lin | A survey of blockchain security issues and challenges[END_REF]. Amongst other issues we can cite 51% attacks, propagation delays [START_REF] Decker | Information propagation in the bitcoin network[END_REF], withholding attacks [START_REF] Nicolas | On subversive miner strategies and block withholding attack in bitcoin digital currency[END_REF], the untested scalability of blockchains, their complex governance system, etc. Assumption A2 does not cover these issues as they are not crypto-related but rather network-related. However, despite all these theoretical shortcomings, blockchains like Bitcoin and Ethereum have demonstrated their resilience to attacks and only grown stronger as a result. Another issue that needs addressing is the resistance to DDoS attacks. In the Bitcoin blockchain, the only transactions without inputs are coinbase transactions. First transaction of a block, a coinbase transaction can only be issued when a block is mined. Furthermore, now that Bitcoin miners' payment is moving from block reward to transaction fees, all other transactions have a cost. This mitigates DDoS attacks as the cost is linear in the number of transactions. When a big number of transactions floods the network, miners can temporarily increase transaction fees, thus rendering an attack even more costly. This last argument still applies to our use case. Because genesis transactions are not exempt of fees, trying to flood the network with them has a cost that is at least linear in the number of transaction and can even grow faster as the miners's fees adapt to the situation. This addresses Threat T 5. Valid transactions can also be created by transferring a device's ownership to oneself. But the cost is the same.

We propose two additional means of mitigation. The first solution is to use a private blockchain where the right to issue genesis transactions is limited to pre-approved actors. Manufacturers would need to get registered and only them could then create new devices. A manufacturer that behaves incorrectly, by advertizes non existing devices or issuing too many genesis transaction, would lose it's publication privileges. This has the added advantage of addressing Threat T 8. Private blockchains unfortunately do not offer the same openness and decentralization as public ones.

A second solution consists in increasing the cost of genesis transactions. They would require a nounce as an input. Similarly to Proof of Work, the nounce would be chosen so that the hash of the transaction is lower than a pre-defined threshold. The difficulty does not need to be as high as Bitcoin proof of work's and can be adapted to counter DDoS attacks. The downside is that this increased computational cost will mostly impact manufacturers. This is therefore likely to impact the device's cost in return.

Use-case-related limitations In the above proposal, a genesis transaction creates the digital representation of a device. However, no proof of the existence of this device is required. The production of a valid proof of ownership does not translate to the possession of a real-life IoT device. In case of theft for instance, the owner can still produce a valid proof but will not be able to produce the device itself. This situation is not different from online shopping where the buyer has to rely on pictures, listings, reputation, or other criteria to decide whether to trust the vendor. To strengthen this link between digital and tangible, the data field of genesis transactions' outputs can be used to specify the device's identifier. This addresses Thread T 8.

Similarly, the issuance of a transfer transaction does not force the shipping of the device to the new owner. It means however that the previous owner can no longer prove that she owns the device. This is a deterrent as future prospective buyers are unlikely to commit to the sell if the ownership cannot be proven. Bitcoin multisignature can be used to solve this problem. This refers to transactions that need more than one signatures to be valid. The desired number here is 2 out of 3. The buyer and vendor choose a party that they trust to be impartial. The buyer then sends the funds to the multisignature address. If everything goes smoothly, upon reception of the purchased item, the buyer and seller both sign the transaction and funds are sent to the vendor. When a conflict occurs, the third party decides who should receive the funds and signs the transaction together with the interested party. The same can be done with ownership transaction. This addresses Threat T 2.

An owner could also try to sell the same device to two different people. This is a problem that is similar to the cryptocurrency double spending. In a similar fashion, both transactions cannot co-exist. New owners should therefore be sure to wait for the blockchain transaction to be confirmed. For Bitcoin, the generic rule is to wait for the transaction to be burried under 5 to 6 blocks, which takes around an hour. For such a use case, this delay is not an inconvenience.

Finally, malicious previous owners might want to retain control of their former device after it has been shipped to its new owner. To protect against this risk, the device should be wiped clean upon reception and all credentials should be changed. This addresses Threat T 1. The same applies to a former device owner that want to prevent her sensitive data from being accessed by the new owner. Before the device can be shipped, it should be restored to factory default. This addresses Threat T 3. The necessary steps should also be taken to revoke the device's access to all sensitive services such as a smart home private network. This addresses Threat T 4.

Key management

Motivation

Security rests on the sharing of secrets. Regardless of the scheme one uses to secure IoT applications, some security bootstrap is required. Certificates need to be created and installed, keys need to be distributed, etc. In short, secrets need to be exchanged to secure communications or encrypt data. When a device is manufactured, initial secrets are provisioned to start the security chain. This means that before using a factory-fresh device, the initial secrets need to be retrieved from the manufacturer. The means currently at our disposal to do so lead to slow and cumbersome deployment processes. What is needed therefore is a mean of efficiently retrieving that information in order to be able to remotely and efficiently configure devices in an industrial context.

Currently, physical access to the device is often necessary. When buying a device, the new owner will have it shipped to her location and configure it. The pin or the password may be written down on the device's box, in the configuration manual or otherwise physically attached to the device and its packaging. Buyer and seller might also choose to call on a trusted third party to take care of the configuration and installation of devices.

The need for an initial physical access is a hindrance on the deployment process. Because many devices need to be configured at once, this method that is slow, costly and may require to trust confidential information to a third party is ill-fitted.

Another issue is the management of these secrets. In IoT scenarios, multiple devices may be owned by the same entity. Furthermore, symmetric cryptography is often preferred due to the constrained nature of IoT devices. This is another multiplying factor for the number of keys involved. This multiplicity implies the need for an efficient management of secrets over the life a device. Based on the blockchain ownership records, we propose a solution that both delivers a device's secret to its newest owner and allows her to manage such secrets over the life of the device.

Benefits of the proposal

The benefits brought by the proposed scheme are as follows:

-Simplified deployment process -Cost reduction -Reduction of the number of secret keys -Distributed storage of keys As an extension of the proposal from Section 3, to the benefits described above we add the advantages described in Section 3.1 and are inherent to the use of a blockchain as the underlying mechanism.

Proposal

Once again, we start at the beginning of the ownership chain. The manufacturer M sells a batch of n devices {D i } 0≤i<n to a retailer R. Each device has a unique identifier id i . Additionally, R owns a master key K R . The symmetric key k R,i is derived from K R and id i . Key k R,i will be used to encrypt s i , secret linked to device D i . The blockchain still supports two types of transaction, genesis and transfer. Step 1 M interacts with each D i and retrieves a secret s i . This secret s i can be an administrative password, a private key, a pin, etc.

Step 2 M retrieves {k R,i } from R. That information can be provided along with payment information for instance. Note that when all devices are purchased from the same seller, it is not necessary to use different k R,i . If R prefers using asymmetric cryptography, the key used to encrypt s i can also be retrieved from a registry storing public key records. It is used for applicative purposes and should differ from the keys used for the blockchain protocol. Using {k R,i }, M encrypts each s i .

Step 3 M issues a genesis transaction, tx 0 , with n outputs where the out 0 i corresponds to D i and is sent to addr M , her own blockchain address. M issues a second transaction of type transfer, tx 1 , with {out 0 i } as inputs, signed with priv M . This transaction yields n outputs, one for each D i , sent to addr R . In addition to addr R , each output carries Enc k R,i (s i ).

Step 4 R retrieves {Enc ki (s i )} from the blockchain and deciphers them, recovering {s i }.

Step 5 Using the s i , R gains access to and configures if need be each D i .

The same process can then be repeated by the new owner to sell the device to somebody else. tx 1 's outputs can be separated allowing for devices to be sold separately.

This scheme involves a lot of keys and secrets but only K R and priv R need to be safeguarded by R. Each s i can be recovered from K R . This greatly simplifies the management of secrets where many devices are involved. ), where Enc is the encryption algorithm. Such a transaction can also be made to hide the link between the owner's identity and her blockchain address (Threat T 6).

Limitations

The first delicate point of this scheme is the transmission of the encryption key(s), {k R,i }, from R to M . If symmetric keys are used, then a secure communication channel should be put in place to allow for the exchange. When a large number of sells involve the same actors, typically M and R, the same key can be used for all encryptions. That way, the symmetric key must be exchanged only once. Another work around is to use asymmetric cryptography. The public key can then be transferred without the need for particular precautions. This mitigates Threat T 6.

When the {k R,i } are symmetric keys, they should of course be changed before a new secret can be uploaded to the blockchain. Otherwise, all s new i are exposed to M (Threat T 1). To achieve this, k R,i can be derived from elements linked to blockchain transactions. Let tx n be the latest transaction that proves M owns D i . Such a transaction must exist with unspent output otherwise M is not D i 's rightful owner. In that case, Hash(tx n ) could be used as an input for key derivation. The transfer transaction from M to R, transaction tx m , will use one of tx n output as input. Similarly, when updating s i , an output from tx m will be used. The hash of the previous transaction is therefore an easy element to recover. It varies with transactions, leading to different k R,i .

Sharing configurations

Motivation

We have seen that information pertaining to the device can be stored into the blockchain for its owner's use. It is also the case for information that would be used by users or other IoT devices willing to interact with it. The first example that comes to mind is the advertizing of a public key. Contrary to static properties such as memory space or power consumption, a device possesses a number of dynamic properties, its public key being one of them. Such properties can be useful to others for any number of reasons such as authentication, evaluation of trust (and risks), discovery of assets, etc. Other examples include advertizing known protocols, the version of the softwares that are running on a device or other application-specific information.

During the life of a device, the owner can share and update that information using the blockchain. Transfer transactions can be assorted with any number of properties that the owner judges relevant to the use of her device. Updates are made by transferring ownership to oneself. The users can then retrieve that information from the blockchain.

Benefits of the proposal The proposed scheme presents the following advantages:

-Distributed storage -Shared Infrastructure -Availability

Proposal

Let O be the owner of a device. O wishes to publish P rop D , set of dynamic properties of D. Let U be a potential user of D. User U needs to consult P rop D before exchanging with D. Let tx n be the latest blockchain transaction designating O As illustrated in Figure 4, O issues a transfer transaction, tx m , with out n l as input, signed with priv O . This transaction yields 1 output sent to addr O that, additionally, carries P rop D . U can then retrieve tx m and extract P rop D .

Limitations

The first question that comes to mind is the alloted size for P rop D . This, once again, depends of the underlying blockchain. In Bitcoin, the maximum size of a block is fixed at 1 MB 7 . Naturally, this limits the size of a transaction. Furthermore, bigger transactions have a higher cost. Since miners are paid by transaction fees, if transaction fees are fixed, they would rather include many small transactions as that will amount to more fees. To compensate for that, bigger transactions should pay a higher fee. In Ethereum, there is no fixed limit to the size of a transaction but the amount of gas per block is limited. Even if this limit augments with time, the size of a transaction is currently limited to around 100.000 non-zero byte. Gas being paid for with ether, bigger P rop D also amount to higher cost. Published data should therefore be kept to a minimum. This is inline with many other IoT requirement however.

Privacy concerns might arise from the publication of device information in such public fashion (Threat T 6). They can be tackled by either using a private blockchain or encrypting the published content to restrict its viewing. Group keys [START_REF] Harney | Group key management protocol (gkmp) architecture[END_REF] or attribute-based encryption [START_REF] Goyal | Attribute-based encryption for fine-grained access control of encrypted data[END_REF] can be used to efficiently control access to that information.

Finally, IoT devices may want to retrieve P rop D . Unfortunately, the resources required to maintain a connection to the blockchain network are too much for constrained devices at the moment. This limitation is not due to the blockchain technology but rather to its youth. Light clients specifically designed for IoT devices should emerge before long.

The blockchain has made the tracking of asset's ownership relatively inexpensive. It does not have to be reserved for houses and boats anymore. We therefore propose to use it to track the ownership of IoT devices. The chain of ownership can be augmented by adding additional information to transfer transactions. We present two ways to do so. Firstly, information can be added to help the owner manage its devices. Secondly, information can be added to inform users and other devices of a device's dynamic characteristics that can be relevant in their exchange.

We have defined four security assumptions and eight security threats involving owners and third parties. Limitations of our proposals have been argued. Currently, the requirement that every potential owner possesses a blockchain address is the most unlikely. This is likely to evolve in a near future. The fact that blockchain keys cannot be lost or compromised is also a big assumption. However, the issue of safekeeping a key has been studied extensively and many solutions can be provided.

  address of the output owner Mand Secret See Section 4 Opt Data See Section 5 Opt Table 5. Output formatcounterpart is the asset genesis transaction. Transactions that mark a change in ownership are transfer transactions. Transactions follow the Bitcoin [17] model of input/output, meaning that each transaction uses previous transaction outputs as inputs. Transactions are detailed in Table

Fig. 1 .

 1 Fig. 1. Proof of ownership and ownership transfer

Fig. 2 .

 2 Fig. 2. Transferring ownership and delivering device secret

Figure 2

 2 Figure2illustrates the process by which ownership is transferred and secrets are exchanged :

Fig. 3 .

 3 Fig. 3. Publishing a new secret to the blockchain

Fig. 4 .

 4 Fig. 4. Publishing dynamic properties to the blockchain

Table 1 .

 1 Our notations

	Symbol	Description
	D	Device
	{Di} 0≤i<n	Family of n devices
	idi	Identifier of device Di
	O	Device owner
	M	Device manufacturer
	R	Retailer
	addrA	Blockchain address of A. addrA = Hash(pubA)
	(pubA, privA) Public/private blockchain key pair linked to addrA
	si	Secret linked to device Di
	KA	Master key of A
	kA,i	Symmetric key derived from KA and idi
	tx k	k th blockchain transaction
	out k j	j th output of tx k
	P ropD	Set of dynamic properties of device D

Table 5 )

 5 Field DescriptionStatus Tx type Possible values are genesis and transfer Mand Nounce Can be made mandatory for genesis tx (see Section 3.3) Opt Inputs Lists all the inputs of the tx (see Table4) Opt Outputs Lists all the outputs of the tx (see Mand

Table 3 .

 3 Transaction format MandPublic key Public key that matches the address that owns the selected output Mand Signature Must be signed with the private key that matches the given public key Mand

	Field	Description

Table 4 .

 4 Input format

https://gomedici.com/30-non-financial-use-cases-of-blockchain-technologyinfographic/, Last checked Feb, 16th 2018

4 https://www.reuters.com/article/us-sweden-blockchain/sweden-tests-blockchaintechnology-for-land-registry-idUSKCN0Z22KV, Last checked Feb, 16th

2018 5 https://cointelegraph.com/news/georgia-becomes-first-country-to-registerproperty-on-blockchain, Last checked Feb, 16th 2018

https://www.bloomberg.com/news/articles/2017-10-03/ukraine-turns-toblockchain-to-boost-land-ownership-transparency, Last checked Feb, 16th 2018

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INTRODUCTION

Colorectal cancer (CRC) is the most common gastrointestinal malignancy and one of the leading causes of cancer-related deaths worldwide. [START_REF] Brenner | Colorectal cancer[END_REF] Factors such as tumour invasion and frequency of lymph node or distant metastases determine the prognosis of the disease. Recently, studies of inflammatory cells in cancer have gained increasing interest, particularly with regard to tumour invasiveness and metastases. The composition of the microenvironment of several tumour types, especially CRC, could serve as prognostic factors, [START_REF] Miller | The prognostic value of cancer stem-like cell markers SOX2 and CD133 in stage III colon cancer is modified by expression of the immune-related markers FoxP3, PD-L1and CD3[END_REF][START_REF] Galon | Type, density, and location of immune cells within human colorectal tumors predict clinical outcome[END_REF] although others advocate that the TNM stage should remain the preferred prognostic system. [START_REF] Laghi | CD3+ cells at the invasive margin of deeply invading (pT3-T4) colorectal cancer and risk of post-surgical metastasis: a longitudinal study[END_REF] More research on the inflammatory infiltrate in the tumour and the microenvironment both in the tumour and lymph node could help to improve our understanding of tumour genesis. [START_REF] Chiba | Intraepithelial CD8+ T-cell-count becomes a prognostic factor after a longer follow-up period in human colorectal carcinoma: possible association with suppression of micrometastasis[END_REF][START_REF] Menon | Immune system and prognosis in colorectal cancer: a detailed immunohistochemical analysis[END_REF] The inflammatory infiltrate consists of different B and T lymphocytes, mast cells, macrophages, antigen-presenting cells (APC), fibroblasts, and endothelial cells. Among these cells, regulatory T lymphocytes (Tregs) play a crucial part in the inflammatory response in CRC [START_REF] Sobhani | Critical role for CD8 + FoxP3 + regulatory cells in colon cancer immune response in humans[END_REF][START_REF] Chaput | Identification of CD8 + CD25 + FoxP3 + suppressive T cells in colorectal cancer tissue[END_REF][START_REF] Salama | Tumor-infiltrating FOXP3-T regulatory cells show strong prognostic significance in colorectal cancer[END_REF][START_REF] Vignali | How regulatory T cells work[END_REF] and can be detected by immunohistochemistry, using forkhead box protein 3 (FOXP3) as a marker. [START_REF] Poitrasson-Rivière | Regulatory CD4+ T cells are crucial for preventing CD8+ T cell-mediated autoimmunity[END_REF] Moreover, considerable evidence now supports the concept that cells expressing indoleamine 2,3-dioxygenase (IDO) can suppress T-cell responses and promote tolerance. [START_REF] Mellor | IDO expression by dendritic cells and tryptophan catabolism[END_REF] In CRC, high expression of IDO by colorectal tumour cells enables certain cancer subsets to avoid immune attack and defeat the invasion of T cells via local tryptophan (Trp) depletion. [START_REF] Brandacher | Prognostic value of indoleamine-2,3-dioxygenase expression in colorectal cancer: effect on tumor-infiltrating T cells[END_REF] The first aim of our study was to describe precisely the proportion of FOXP3+ and IDO+ cells at the centre of the tumour and invasive front, proximal draining lymph node, and healthy mucosa. Our histopathology department has developed a computer-aided analysis of whole slide digital image derived from tissue microarray (TMA). [START_REF] Glimelius | Tissue microarray and digital image analysis: a methodological study with special reference to the microenvironment in Hodgkin lymphoma[END_REF] This new analysis overcomes the considerable challenges of classic methods due to tumour heterogeneity, the extreme difficulty of reproducing semi-quantitative analysis results, and the time-consuming nature of analysing whole-tissue samples. The second aim of this study was to compare IDO+ infiltration in the tissues with plasma IDO concentration, evaluated with serum kynurenine (Kyn) level and the Kyn/Trp ratio.

MATERIALS AND METHODS

Patients

This prospective longitudinal observational study was conducted in a single institution, Pitié-Salpétrière Hospital, Paris, France, with approval of the local ethics committee.

Thirty-one patients who were operated on for CRC in our institution between March 2010 and September 2011 were included in the study. Prior to TMA construction, all diagnoses were established according to classical histopathological criteria. Analysis of mismatch repair status by immunohistochemistry was performed as described elsewhere. [START_REF] Chaput | Identification of CD8 + CD25 + FoxP3 + suppressive T cells in colorectal cancer tissue[END_REF] A patient was considered MSI+ if there was DNA microsatellite instability (MSI), meaning deficiency in the mismatch repair system, and MSI-if no MSI existed. Tumour micro-invasive status was defined by the presence of vascular emboli, lymphatic invasion, and perineural invasion (collectively referred to as VELIPI); patients were scored from 0 to 3 according to the status of each parameter (0 negative, 1 positive). Demographic parameters and the main characteristics of patients' tumours are reported in Table 1.

Construction of TMA

For each paraffin-embedded tissue donor block, five tissue cylinders with a diameter of 0.6 mm were punched from morphologically representative areas at four sites: centre of the tumour (C), invasive front of the tumour (F), a proximal tumour-draining lymph node without metastasis (N-), metastatic tumour-draining lymph node (N+), and healthy mucosa (M) located 10 cm from the tumour. These cylinders were brought into a recipient paraffin block using a semi-automated tissue arrayer (Alphelys, France). Healthy mucosa of patients operated on for diseases other than cancer was also included in the same TMA to analyse IDO and FOXP3 expression in comparison with that of CRC patients.

Immunohistochemistry

The immunostaining procedure was performed on formalin fixed, deparaffinised, 4 mm thick TMA sections using a biotin-free polymeric visualisation system (Ultravison LP, ref TL-015-HD; Lab Vision, USA). The following primary antibodies were used at the indicated dilutions: mouse monoclonal anti-IDO antibody (dilution 1:100; clone 10.1; Chemicon-Millipore, France); mouse monoclonal anti-human FOXP3 (dilution 1:200; clone 236A/E7; Abcam, UK). Matched isotype control antibody was used to choose the best IDO antibody dilution and minimise staining background. The antigen retrieval reaction was performed in a water bath, using EDTA buffer (pH 8.0) for 20 min at 97 C. The slides were then incubated with the peroxidase block supplied by the manufacturer of the visualisation system for 10 min. Next, the slides were incubated with the primary antibodies for 60 min at room temperature. The incubation time was 10 min for each step of the visualisation system. Chromogen diaminobenzidine supplied by the manufacturer was applied for 5 min, and counterstaining with haematoxylin for 1 min.

Computer-aided morphometric study of whole slide digital images from TMA TMA glass slides stained with FOXP3 or IDO were scanned to produce digital slides (0.25 mm/pixel at 40× magnification) using the Aperio Slide Scanning System (ScanScope CS; Aperio Technologies, USA). The whole slide digital images were viewed on a Barco Coronis fusion 6MP highdefinition screen (Barco NV, Belgium) to determine the detection thresholds, and images were analysed with ICS software (Tribvn SA, France). This software is able to discriminate the immunostained area on the basis of red, green, and blue colour segmentation and to calculate the immunoreactive area. Given that an accurate optical count of immunoreactive cells was not feasible in the presence of very dense infiltrates, we did not attempt to convert area values to number of cells. We manually distinguished two types of IDO positive tissue area: on one hand, dendritic cells located in the lamina propria of normal mucosa or in the stroma of carcinomatous samples (expressed as a fraction of the area occupied by lamina propria or stroma) and, on the other hand, normal or neoplastic epithelial cells (expressed as a fraction of the area occupied by epithelial cells). We determined in C, F and M, the proportion of IDO+ area of each type. FOXP3 was expressed as a fraction of the TMA spot area. Concerning the lymph nodes, FOXP3 and IDO were expressed as a fraction of the TMA spot area. Finally, we obtained for each case, the mean of the results calculated for five TMA spots for each site (C, F, N-, N+, M) for FoxP3+ cells, IDO+ dendritic cells, and IDO+ epithelial (normal or neoplastic) cells, respectively.

Measurements of serum Trp and Kyn

Blood samples (sheltered from light) were obtained using a standard phlebotomy protocol immediately before each patient was anaesthetised. At the time of blood sampling, the patients had not received any treatment, including surgery, chemotherapy, or radiation therapy. No patients had hepatic or renal insufficiency or neuro-psychiatric disease. The samples were rapidly separated, and serum samples were frozen at -70 C until batch processing. The concentrations of Trp and Kyn were measured simultaneously by highperformance liquid chromatography with fluorescence detection, according to the protocol of Yamada et al. [START_REF] Yamada | Simultaneous measurement of tryptophan and related compounds by liquid chromatography/electrospray ionization tandem mass spectrometry[END_REF] The IDO activity was determined by dividing the serum concentration of Kyn by that of Trp (Kyn/Trp).

Statistical analysis

For the between site comparison, within the CRC patient group, we tested the overall effect of the site by a one way analysis of variance (ANOVA) and compared the site two by two with a post-hoc test of Tukey.

For the between group comparison, we used a two sample Wilcoxon test and computed p values adjusted using Bonferroni method. For the association between baseline characteristics and different measures of FOXP3, IDO, Kyn and Try, we used one ANOVA with a post-hoc test of Tukey or two sample Wilcoxon test and computed p values using Bonferroni method.

For the association between paired samples we used Spearman's rho (r) and computed p values adjusted using Bonferroni method.

All tests involved use of R 2.15.2 (R Foundation, Austria).

RESULTS

Demographic and histological characteristics of patients are summarised in Table 1. According to multivariate analysis, there were no differences in the proportion of tissue areas positive for IDO and FOXP3 in any site according to T, N, M stage or VELIPI status.

Patterns of FOXP3 and IDO expression

The Foxp3 marker gave a distinct nuclear staining (Fig. 1A,C,E,G). The proportion of tissue area positive for FoxP3 varied for different samples between 1% and 20%. The majority (97%) of samples had positive area for FOXP3 of <10%. The expression of IDO was detected in the cytoplasm of tumour cells, normal epithelium, and immune cells (Fig. 1B,D,F,H). The proportion of tissue area positive for IDO varied among samples from 0.1% to 100% in epithelium and from 0.04% to 16.8% in lamina propria. The staining of IDO occurred in both the luminal and basal poles of tumour cells and normal epithelial cells. In the same section, staining of IDO occurred both in cancer cells and normal epithelial cells and in dendritic cells (lamina propria) (Fig. 1D,F,H).

Comparison of the expression of IDO and FOXP3 among the different sites in CRC patients

The proportion of tissue area positive for IDO was significantly higher in lymph node than in the lamina propria of normal mucosa, or in the stroma of carcinomatous samples (p < 0.0001) (Table 2). The proportion of tissue area positive for IDO was significantly higher in N+ than in N-(p < 0.01). There was no statistically significant difference in FOXP3 and IDO between the centre of the tumour and the invasive front.

Comparison of the expression of IDO and FOXP3 in healthy mucosa between CRC patients and controls

The proportion of IDO positive tissue area in the lamina propria was not significantly different between healthy mucosa of patients with cancer than without (1.8 ± 3 vs 1.1 ± 0.9). The proportion of IDO positive tissue area in the epithelium was significantly higher in healthy mucosa of patients with cancer than without (5.4 ± 13.8 vs 2.1 ± 2.4, p = 0.001).

The proportion of FOXP3 positive tissue area was significantly higher in healthy mucosa of CRC patients than controls (1.2 ± 0.8 vs 0.8 ± 0.5, p = 0.03).

Association between the proportion of tissue area positive for IDO and FOXP3 in different sites (Table 3)

FoxP3 in tumour was positively correlated with IDO in N-(r = 0.44, p < 0.01) but not in N+.

Serum concentration of Kyn

The concentration of Kyn was not significantly different in patients with CRC (1.68 ± 0.59, median 1.6, range 0.6-3.1) than in patients without (1.57 ± 0.79, median 1.3, range 0.8-2.9; p = 0.07). The Kyn/Trp ratio was not different in patients with CRC than in patients without (0.05 ± 0.02 vs 0.04 ± 0.01; p = 0.05). The serum concentration of Kyn or the Kyn/Trp ratio was not correlated with the proportion of tissue epithelial area positive for IDO in any sites of CRC patients. No difference in Kyn or Kyn/Trp ratio was found according to the TNM or VELIPI stages.

DISCUSSION

Our study, combining TMA and digital whole slide, gave a precise description of IDO and FOXP3 infiltration in different sites of colorectal tissue removed from patients operated on for CRC and healthy mucosa of patients without tumour. Studies of inflammatory cells in cancer have gained increasing interest, particularly with regard to tumour invasiveness. The composition of the microenvironment is of prognostic importance in CRC, [START_REF] Salama | Tumor-infiltrating FOXP3-T regulatory cells show strong prognostic significance in colorectal cancer[END_REF][START_REF] Pagès | Effector memory T cells, early metastasis and survival in colorectal cancer[END_REF] and new drugs targeting the microenvironment are currently under development. [START_REF] Hanna | The tumour microenvironment: a novel target for cancer therapy[END_REF] Because the TMA method allows the simultaneous analysis of selected regions from histological materials assembled on a single microscopy slide, it is a time-and cost-saving method. This method has been validated when investigating markers expressed by tumour cells, [START_REF] Camp | A decade of tissue microarrays: progress in the discovery and validation of cancer biomarkers[END_REF][START_REF] Hedvat | Application of tissue microarray technology to the study of non-Hodgkin's and Hodgkin's lymphoma[END_REF] as well as surrounding cells in the tumour stroma. [START_REF] Glimelius | Tissue microarray and digital image analysis: a methodological study with special reference to the microenvironment in Hodgkin lymphoma[END_REF] Because some evidence exists that cells expressing IDO can suppress T-cell responses and promote tolerance, [START_REF] Uyttenhove | Evidence for a tumoral immune resistance mechanism based on tryptophan degradation by indoleamine 2,3-dioxygenase[END_REF][START_REF] Munn | Indoleamine 2,3-dioygenase and tumor-induced tolerance[END_REF] we were interested in the combined study of IDO+ and FOXP3 cells infiltration. For the morphometric analysis, we manually differentiated epithelial IDO+ cells from dendritic IDO+ cells. In non-metastatic draining tumour lymph node, since IDO+ cells are only dendritic cells, they could be detected automatically by morphometric analysis. Interestingly, the proportion of IDO+ area in the lamina propria was significantly higher in N+ than in N-, and significantly higher in N-than in other studied sites (Table 2). This suggests the importance of the presence of dendritic cells in lymph node, as much as the node is invaded by the cancer. We found that the proportion of IDO+ epithelial cells was higher in healthy mucosa of patients with CRC than in patients without. Such an observation has not been reported previously, because no case-control studies have compared pieces of surgical colectomy from patients with and without CRC. The same observation was made for FOXP3 infiltration, which was greater in healthy mucosa of CRC patients. Thus, healthy FOXP3+ area (%) 2.3 ± 1.8 2.6 ± 2.9 6.0 ± 2.9 a 14.2 ± 2.9 b 1.2 ± 0.8 IDO+ area in lamina propria (%) or in non-epithelial area of lymph node 1.6 ± 3.1 1.1 ± 1.3 3.4 ± 2.5 a 9.1 ± 8.5 b 1.8 ± 3.0 IDO+ area in epithelial cells (%) 7.2 ± 11.9 7.2 ± 13.9 N/A 6.7 ± 5.4 5.4 ± 13.8

Results are expressed as mean ± SD. N/A, not applicable.

a p < 0.001 N-vs C, F and %. b p < 0.001 N+ vs C, F, N-and M. mucosa of patients with CRC seems to be the site of the inflammatory reaction. Contrary to Brandacher et al., [START_REF] Brandacher | Prognostic value of indoleamine-2,3-dioxygenase expression in colorectal cancer: effect on tumor-infiltrating T cells[END_REF] who found a correlation between IDO immunoreactivity of CRC cells and frequency of liver metastases, we did not find any relationship between IDO expression in neoplastic cells and TNM stage. Some explanations can be given for these discrepancies. Firstly, these authors considered only cancer cell lines from which IDO activity was analysed by reverse transcription-PCR. By contrast, in our immunohistochemical study, IDO was expressed by tumour and normal epithelial cells. Secondly, these authors could easily separate samples with high IDO expression from those with low expression using PCR method. In the present study, we could only confirm that infiltration by IDO+ cells is higher in healthy mucosa of CRC patients than in those without cancer and that IDO expression was not higher inside the tumour (with no difference between the centre and invasive front) than in healthy mucosa. Gao et al. [START_REF] Gao | The paradoxical patterns of expression of indoleamine 2,3-dioxygenase in colon cancer[END_REF] also reported the absence of difference in IDO expression between normal adjacent mucosa and primary tumour and suggested that it is unlikely that IDO contributes to tumour progression in primary CRC.

Concerning FOXP3, Salama et al. [START_REF] Salama | Tumor-infiltrating FOXP3-T regulatory cells show strong prognostic significance in colorectal cancer[END_REF] studied the density of tumour-infiltrating lymphocytes bearing FOXP3 (for Tregs) semi-quantitatively using whole slide digital images derived from TMA. They also confirmed a higher density of cells positive for FOXP3 in tumour compared with adjacent normal mucosa. They explored neither lymph node nor IDO infiltration.

The Trp degradation by IDO from CRC cells, activated macrophages, or dendritic cells, [START_REF] Uyttenhove | Evidence for a tumoral immune resistance mechanism based on tryptophan degradation by indoleamine 2,3-dioxygenase[END_REF] does result in an increase in the Kyn level in the tumour microenvironment and could induce tumour tolerance. [START_REF] Munn | Indoleamine 2,3-dioygenase and tumor-induced tolerance[END_REF] Our results are different to those reported by Suzuki et al. [START_REF] Suzuki | Increased serum kynurenine/ tryptophan ratio correlates with disease progression in lung cancer[END_REF] in lung cancer; they found that Kyn concentrations in the blood were significantly higher and Trp concentrations significantly lower in patients with cancer than in healthy controls. In addition, our study failed to show any increase in the Kyn/Trp ratio in more advanced stages. [START_REF] Brandacher | Prognostic value of indoleamine-2,3-dioxygenase expression in colorectal cancer: effect on tumor-infiltrating T cells[END_REF][START_REF] Gao | The paradoxical patterns of expression of indoleamine 2,3-dioxygenase in colon cancer[END_REF][START_REF] Suzuki | Increased serum kynurenine/ tryptophan ratio correlates with disease progression in lung cancer[END_REF] The extraordinary complexity of Trp and Kyn metabolism [START_REF] Kolodziej | Kynurenine metabolism in health and disease[END_REF] and their regulation in the whole body could be the cause of these discrepant results. The Kyn/Trp ratio in the tumour reflects IDO activity in the relatively closed system of the tumour, but perhaps not that in the blood. The lack of statistical power of our study could be another explanation for the discrepancy.

An interesting result of our study is the positive correlation between the infiltration of FOXP3+ cells in tumour and IDO+ cells in N-but not in N+. Tregs develop in peripheral lymphoid tissues from naïve T cells at a very low frequency, and the local environment is the key to the generation of Tregs. [START_REF] Mougiakakos | Regulatory T cells in cancer[END_REF] Tumour cells can directly initiate the induction of Tregs through several factors including IDO, IL-10, and TGFb, and modulate APCs, whether tissue-resident or recruited from lymph node to become tolerogenic, which thereby strongly contribute to the induction of Tregs (FOXP3+) within the tumour microenvironment, normal mucosa, or draining lymph nodes. [START_REF] Mougiakakos | Regulatory T cells in cancer[END_REF] Our results in CRC illustrated the classical theory of Treg recruitment to the lymph node draining the cancer after the interaction of APC with tumour antigens. In N+, the great quantity of cancer cells prevents recruitment of APC, which could explain the lack of correlation.

This study showed that immunohistochemistry performed on TMA is a good technique for analysing the CRC environment. It is much simpler that immunostaining performed on whole tissue samples. [START_REF] Laghi | CD3+ cells at the invasive margin of deeply invading (pT3-T4) colorectal cancer and risk of post-surgical metastasis: a longitudinal study[END_REF][START_REF] Salama | Tumor-infiltrating FOXP3-T regulatory cells show strong prognostic significance in colorectal cancer[END_REF][START_REF] Sincrope | A low intraepithelial effector (CD3)/regulatory (FoxP3) Tcell ratio predicts adverse outcome of human colon carcinoma[END_REF] Moreover, five cylinders of tissue were analysed for each site, allowing for minimal sampling error related to tissue heterogeneity. Computerised images of whole TMA glass slides permit standardisation of the analysis, which is impossible with the human eye. Contrary to the technique of gene expression profiling and flow cytometry in which the architecture of tissues is destroyed, immunohistochemical techniques applied to TMA allow analysis of the spatial relations between different cell types in lymph nodes for a large number of samples, which is a promising research method. [START_REF] Gao | The paradoxical patterns of expression of indoleamine 2,3-dioxygenase in colon cancer[END_REF][START_REF] Zlobec | Nodenegative colorectal cancer at high risk of distant metastasis identified by combined analysis of lymph node status, vascular invasion, and Raf-1kinase inhibitor protein expression[END_REF][START_REF] Matera | Low FOXP3 expression in negative sentinel lymph nodes is associated with node metastases in colorectal cancer[END_REF] Moreover, gene profiling and flow cytometry require living cells extracted from tumour and depend on the quality of extraction procedure and time of preservation. One limitation of our study is the small size of the sample, which could explain the large confidence interval observed for some of our results.

In conclusion, this study showed for the first time that the TMA technique permits simultaneous analysis of FOXP3+ and IDO+ cells at different anatomical sites, including tumour, draining non-metastatic and metastatic lymph nodes, and normal mucosa. The strong correlation between FOXP3+ areas and IDO+ area in N-but not in N+, constitutes an argument in favour of interactions between Tregs and APCs within the tumour and the draining lymph nodes. This method, which allows for better analysis of the inflammatory infiltrate in different anatomical sites, both within and outside the tumour, should be used for larger studies.

Fig. 1

 1 Fig. 1 An example of immunohistochemical expression of FOXP3 and IDO in colorectal cancer, expressed as area tissue percentage at different sites (for each site, an immunostaining image and the corresponding digital mark-up are presented). Tumour-draining lymph node without metastasis: (A,B) 5.8% for FOXP3, and (C,D) 2% for IDO. Healthy mucosa: (E,F) 1.1% for FOXP3, and (G,H) 100% of epithelial cells area for IDO. Tumour centre: (I,J) 10.3% for FOXP3, and (K,L) 100% of neoplastic cells for IDO. Tumour front: (M,N) 6.6 % for FOXP3, and (O,P) 83% of neoplastic cells for IDO. For digital mark-up, red corresponds to positive staining and blue negative. Scale bars = 100 mm.

  areas in central tumour (C), invasive front of the tumour (F), tumour-draining lymph node without metastasis (N-), tumour-draining lymph node with metastasis (N+), and normal mucosa (M)

Table 1

 1 Demographic and histological characteristics of the 31 patients with CRC

	Characteristics	No. patients
	Age, years, mean ± SD	63 ± 12
	Sex ratio (M/F)	19/12
	T stage
	T1	1
	T2	2
	T3	19
	T4	9
	N stage
	N0	15
	N1	8
	N2	8
	M stage
	M0	23
	M1	8
	MSI status
	+	3
	-	28
	VELIPI status
	0	1 3
	1	8
	2	3
	3	7
	TNM staging
	I	4
	IIA	12
	IIC	1
	IIIB	3
	IIIC	3
	IVA	4
	IVB	4
	TNM staging according the AJCC 8th edition.

MSI, microsatellite instability; VELIPI, venous embolism, lymphatic invasion, perineural invasion.

Table 2

 2 Proportions of IDO+ and FOXP3+ tissue

Table 3

 3

	Correlations of immunohistochemical expression of IDO and
	FOXP3 between different sites
		r	p
	FOXP3 (F)/IDO (N+)	0.1	0.7
	FOXP3 (F)/IDO (N-)	0.44	0.005
	FOXP3 (N-)/IDO (F)	0.26	0.16
	FOXP3 (N+)/IDO (F)	0.35	0.11
	FOXP3 (M)/IDO (N+)	0.19	0.63
	FOXP3 (M)/IDO (N-)	0.41	0.05

F, invasive front of the tumour; M, normal mucosa; N-, tumour-draining lymph node without metastasis; N+, tumour-draining lymph node with metastasis; p, value of statistical significance; r, coefficient of correlation of Spearman.

Conflicts of interest and sources of funding: The authors state that there are no conflicts of interest to disclose.

Address for correspondence: Dr Daniel Eyraud, Service d'Anatomie Pathologique, Hôpital Pitié-Salpêtrière, 47-83 Boulevard de l'Hôpital, 75013 Paris, France. E-mail: daniel.eyraud@aphp.fr