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Local Exchange Trading Systems

A local exchange trading system (LETS) is a local mutual credit association in which members are allowed to create common credit money individually, written into a common ledger. LETS can be thought of as a mechanism to facilitate the velocity of trade, goods, and services out-with the existing monetary system, allowing us to create sustainable local economies.

As an example, assume that Alice, with zero balance, is willing to buy a litre of raw milk from Bob.

First, they agree on a price; for example, assume that the price is about 2 Euro (as Alice and Bob are living in Ireland). After the deal is written into a ledger, Alice's balance becomes -2 (minus two) Euro, and Bob's balance becomes 2 Euro. Then Bob may spend his 2 Euro, for example, on homemade beer from Charlie. Such systems often impose limits on negative balances, and sometimes even on positive ones, to promote exchange in the community.

Bob can spend his balance with other participants of the LETS, and the creation of credit allows for economic activity and velocity of money even where people have, temporarily, no cash. Of course, borrowing limits can be imposed, even on positive ones, to prevent hoarding within the LETS. Additionally, thanks to Sigma Protocols, this system can be built trustlessly on Ergo. A Trustless LETS has no membership record; therefore, no management committee is needed for enrolment, allowing it to operate with full autonomy.

See this blog post for a longer introduction, or head over to the LETS Discussion Summary on ergoforum to see the latest developments.

Basic Implementation

Our reference implementation is simple and consists of two contracts: a management contract and an exchange contract. We skip Ergo preliminaries, so please read the ICO article and ErgoScript tutorials(basic and advanced) for starters.

Nevertheless, we will introduce a couple of new terms in the following sentences.

If a token is issued with an amount equal to one, we call it the singleton token. Similarly, a box that contains the singleton token is called the singleton box.

  • The management contract controls a singleton box that holds members of the LETS system.
  • The contract enables the adding of new members at the pace of one member per transaction.
  • The box is not storing members but only a small digest of authenticated data structure built on top of the members' directory.
  • A member is associated with a singleton token issued in a transaction that adds the member to the directory.
  • The transaction creates a new member's box containing the member's singleton token.
  • The exchange contract protects the member's box.
  • Also, the newly created member's box has the initial balance written down into the R4 register, and the balance is equal to zero in our example.
  • The transaction creating a new member must provide proof of correctness for directory transformation.

The management contract box is controlled usually by a committee, and the committee could evolve over time. To support that, we allow committee logic to reside in the register R5. For example, assume that a new committee member has been added along with a new LETS member, the input management contract box requires 2-out-of-3 signatures, and the output box requires 3-out-of-4 signatures. In this case, the contents of the R5 register in the input and the output box would differ.

The management contract code in ErgoScript with comments is provided below. Please note that userContractHash is about the exchange contract hash.

    val selfOut = OUTPUTS(0)

    // Management script
    val managementScript = selfOut.R5[SigmaProp].get

    // The management script template is replicating self, and management script is satisfied
    val scriptCorrect = (selfOut.propositionBytes == SELF.propositionBytes) && managementScript

    // A spending transaction is creating boxes for directory, user, fee.
    val outsSizeCorrect = OUTPUTS.size == 3

    // Checks that the management label token is replicating self
    val outTokenCorrect = (selfOut.tokens.size == 1) && (selfOut.tokens(0)._1 == letsToken)

    // Checks that new token is issued, and its amount is correct
    // OUTPUTS(0) tokens already checked via outtokenCorrect
    val issuedTokenId = INPUTS(0).id
    val userOut = OUTPUTS(1)
    val correctTokenAmounts =
      (userOut.tokens.size == 1 &&
        userOut.tokens(0)._1 == issuedTokenId &&
        userOut.tokens(0)._2 == 1 &&
        OUTPUTS(2).tokens.size == 0 &&

    // Checks that the new user has been created with the zero balance
    val zeroUserBalance  = userOut.R4[Long].get == 0

    val properUserScript = blake2b256(userOut.propositionBytes) == userContractHash

    // Checks that the new token identifier has been added to the directory
    val selfTree = SELF.R4[AvlTree].get
    val toAdd: Coll[(Coll[Byte], Coll[Byte])] = Coll((issuedTokenId, Coll[Byte]()))
    val proof = getVar[Coll[Byte]](1).get
    val modifiedTree = selfTree.insert(toAdd, proof).get
    val expectedTree = selfOut.R4[AvlTree].get
    val treeCorrect = modifiedTree == expectedTree

    correctTokenAmounts && scriptCorrect && treeCorrect && zeroUserBalance && properUserScript       
    correctTokenAmounts && scriptCorrect && treeCorrect && zeroUserBalance && properUserScript correctTokenAmounts && scriptCorrect && treeCorrect && zeroUserBalance && properUserScript      

The exchange contract script is fairly straightforward and provided below, along with comments describing its logic. The contract assumes that a spending transaction for an exchange contract box receives at least two inputs, and the first two inputs should be protected by the exchange contract script and contain LETS member tokens. To check that singleton member tokens in the inputs do indeed belong to the LETS system; a spending transaction provides the management contract box as the first read-only data input and also should provide proof that the member tokens do belong to the directory authenticated via the R4 register of the management contract box. "letsToken" in the script is about the singleton token of the management box.

  // Minimal balance allowed for LETS trader
  val minBalance = -20000

  val lookupProof = getVar[Coll[Byte]](1).get

  // The read-only box which contains directory of LETS members
  val treeHolderBox = CONTEXT.dataInputs(0)
  val properLetsToken = treeHolderBox.tokens(0)._1 == letsToken
  val membersTree = treeHolderBox.R4[AvlTree].get

  // A spending transaction is taking two boxes of LETS members willing to make a deal,
  // and returns boxes with modified balances.
  val participant0 = INPUTS(0)
  val participant1 = INPUTS(1)
  val participantOut0 = OUTPUTS(0)
  val participantOut1 = OUTPUTS(1)

  //Check that members do indeed belong to the LETS
  val token0 = participant0.tokens(0)._1
  val token1 = participant1.tokens(0)._1
  val memberTokens = Coll(token0, token1)
  val membersExist = membersTree.getMany(memberTokens, lookupProof).forall({ (o: Option[Coll[Byte]]) => o.isDefined })

  // Check that LETS member balance changes during the deal are correct
  val initialBalance0 = participant0.R4[Long].get
  val initialBalance1 = participant1.R4[Long].get
  val finishBalance0  = participantOut0.R4[Long].get
  val finishBalance1  = participantOut1.R4[Long].get
  val diff0 = finishBalance0 - initialBalance0
  val diff1 = finishBalance1 - initialBalance1
  val diffCorrect = diff0 == -diff1
  val balancesCorrect = (finishBalance0 > minBalance) && (finishBalance1 > minBalance) && diffCorrect

  // Check that member boxes save their scripts.
  // todo: optimization could be made here
  val script0Saved = participantOut0.propositionBytes == participant0.propositionBytes
  val script1Saved = participantOut1.propositionBytes == participant1.propositionBytes
  val scriptsSaved = script0Saved && script1Saved

  // Member-specific box protection
  val selfPubKey = SELF.R5[SigmaProp].get

  selfPubKey && properLetsToken && membersExist && diffCorrect && scriptsSaved

Note that we can modify both contracts in many ways to get new systems with different properties.

A Local Exchange Trading System (LETS) aims to develop the local economy and is usually used by people of a locality in the vicinity of each other. See this link, which also describes an ErgoScript implementation of a committee-managed LETS, for a brief overview of LETS. We call such a system a managed or _permissioned_since it depends on a committee of trusted members to enrol new members into the LETS. Here we describe a trustless LETS, i.e., one where there is no management committee needed for enrolment.

Trustless LETS

LETS involves several parties that agree to use some form of "local currency", usually pegged to the country's main currency at a 1:1 rate. Assume that our LETS is based in a European country where the currency is Euros, and the exchange is done in "local Euros", which are considered equivalent to national Euros.

Each user in LETS has an account, which contains the LETS balance of that user (in Local Euros). On joining, each user has a balance of zero. The balance is stored in a (possibly decentralized) ledger. An interesting feature of LETS is that a user with zero balance can also "withdraw" money, but only for paying another LETS user. At any time, the sum of LETS balances of all the users is zero.

As an example, Alice, with zero balance, wishes to purchase one litre of milk for 2 Euros from Bob, a member of LETS with zero balance. She transfers 2 Euros from her account to Bob's, making her balance -2 and Bob's +2. Bob can then transfer some or all of his balance to another LETS user in exchange for goods or services.


Since we desire a trustless LETS, we cannot depend on any trusted group of people to admit users. Note that we will still have a committee to perform tasks such as setting up the LETS parameters (local currency, the maximum number of members, etc.) and consuming any joining fee.

We will only assume a trusted pricing oracle that gives the current rate of euros to ergs identified by some global id (rateTokenID) and a singleton box containing exactly one token with this id. A singleton box, described here, is a box containing a singleton token, i.e., a token with only one quantity in existence. This box also contains the rate of ergs to euros at any given period. The rate is updated by spending this box and creating another singleton box with the new rate.

At any instance, our LETS is uniquely defined by a global token box that contains some membership tokens with id letsTokenID. This box defines the LETS parameters such as the location, the currency unit, rateTokenID, etc. The token box is initially started with, say, 10000 membership tokens. Users can spend this box and create their individual LETS boxes as outputs of the transaction, such that each such output has exactly one membership token. The balance membership tokens are put in a newly created token box.

A LETS box represents a LETS member and must be used in every transaction. For simplicity, this article restricts all LETS transactions to involve exactly two members, one being the sender and the other the receiver, such that the sender transfers some positive amount of the LETS currency (local euros) to the receiver. Such a transaction consumes the member's boxes and recreates them as output with the updated balance.


To prevent spam and DDoS attacks, we require at least a minimum number of ergs (minErgsToJoin) to be locked in the newly created member's box. The ergs will be locked until at least the minWithdrawTime number of blocks has been mined. A box can have a negative LETS balance up to the amount that can be covered by the locked ergs (using the rate at the time of trade).

// a tokenBox stores the membership tokens and has this script
val tokenBox = OUTPUTS(0) // the first output must also be a tokenBox
// first output contains remaining LETS tokens

def isLets(b:Box) = { // returns true if b is a LETS box
   // A LETS box must have exactly 1 membership token in tokens(0)
   b.tokens(0)._1 == letsTokenID && b.tokens(0)._2 == 1 &&
   blake2b256(b.propositionBytes) == memberBoxScriptHash &&
   SELF.R4[Long].get == 0 && // start the box with zero LETS balance
   b.value >= minErgsToJoin && // the box must contain some minimum ergs
   b.R6[Long].get <= HEIGHT // store the creation height in R6

// how many lets boxes creared in the tx
val numLetsBoxes = OUTPUTS.filter({(b:Box) => isLets(b)}).size

// In the transaction following is preserved for the token box ...
tokenBox.tokens(0)._1 == SELF.tokens(0)._1 &&                //  token id
tokenBox.tokens(0)._2 == SELF.tokens(0)._2 - numLetsBoxes && //  quantity
tokenBox.propositionBytes == SELF.propositionBytes           //  script

A LETS member's box is protected by the script below, whose hash memberBoxScriptHash is used above. The script requires exactly one (sender, receiver) pair per transaction.

val validRateOracle = CONTEXT.dataInputs(0).tokens(0)._1 == rateTokenID
val rate = CONTEXT.dataInputs(0).R4[Int].get
val inBalance = SELF.R4[Long].get    // LETS balance of current input
val pubKey = SELF.R5[SigmaProp].get  // owner of the current input
val createdAt = SELF.R6[Long].get    // height at which current input was mined

val index = getVar[Int](0).get       // index of the corresponding output
val out = OUTPUTS(index)
val outBalance = out.R4[Long].get    // LETS balance of the output

// A LETS box is one that has the same script as the current box
val isMemberBox = {(b:Box) => b.propositionBytes == SELF.propositionBytes}
val letsInputs = INPUTS.filter(isMemberBox)    // all LETS input boxes
val letsOutputs = OUTPUTS.filter(isMemberBox)  // all LETS output boxes

// The current input belongs to the receiver if its LETS balance increases
// There may be some ergs in receiver's input box. We need to ensure that
// the receiver's output box also contains the same amount of ergs as input
val receiver = outBalance > inBalance && out.value == SELF.value

val getBalance = {(b:Box) => b.R4[Long].get} // returns LETS balance of a box

val letsBalIn =, {(l:Long, r:Long) => l + r})
val letsBalOut =, {(l:Long, r:Long) => l + r})

// sender box can contain less amount of ergs (sender may withdraw ergs provided 
// that any negative LETS balance of sender in out is backed by sufficient ergs)
val correctErgs = out.value >= -outBalance * rate && (
  out.value >= SELF.value || SELF.R6[Long].get + minWithdrawTime > HEIGHT

// for the receiver, we don't touch the erg balance, 
// since a receiver is not actively involved in the transaction

inBalance != outBalance && // some transaction should occur; balance must change
SELF.tokens(0)._1 == letsTokenID && // the current input has the right token
out.tokens(0)._1 == letsTokenID && // corresponding output has the right token
validRateOracle &&          // oracle providing rate has the correct "rate token"
letsBalIn == letsBalOut &&  // total LETS balance is preserved in the transaction
letsInputs.size == 2 && letsOutputs.size == 2 &&  // only two LETS inputs, outputs
out.propositionBytes == SELF.propositionBytes &&  // out is a LETS box ...
out.R5[SigmaProp].get == pubKey &&                // ... with the right pub key
out.R6[Long].get == SELF.R6[Long].get &&          // ... and creation height
(receiver ||              // either current input belongs to receiver ...
  (pubKey && correctErgs) // ... or out has correct ergs and tx has signature

The transaction spending a box with the above script requires: - The sum of the LETS balance of inputs and outputs is preserved - There are two LETS inputs and two LETS outputs - The public keys (stored in R5) are preserved in the corresponding output - The creation height (stored in R6) be preserved in the corresponding output

Some public key is the receiver if the LETS balance of its output is higher than that of its input.

The last condition requires that either the input and output boxes belong to the receiver (so that the ergs are preserved), or, if they belong to the sender, a signature is provided. The required number of ergs backs the output if its LETS balance is negative. Furthermore, it requires that the sender's ergs balance cannot be reduced until at least the minWithdrawTime number of blocks have been mined after the ergs were locked.

Compared to the managed LETS, the above system has the following differences:

  • No membership record: Unlike the managed LETS, We don't store any membership information here.
  • Multiple-boxes: A person can create multiple membership boxes, which is permitted. We only require that any negative balance be backed by the corresponding number of ergs locked.
LETS-1: Zero Sum, Collateral

The above is the basic variant, which we call LETS-1. It has the following features:

  • Time-locked Joining-Fee: To prevent spam attacks, a member has to pay a certain minimum fee in ergs at the time of joining. This fee is refundable but only after a predefined number of blocks.
  • Zero-Sum: The sum of the LETS balances of all member boxes is zero. Member boxes can have a negative balance as long as it is within a certain limit.
  • Collateral: For the sender's output, ergs are used as collateral to cover the negative LETS balance at the current exchange rate.

The following are some variations of LETS-1.

LETS-2: Zero Sum, No collateral

(A slight variation of LETS-1)

  • Non-refundable joining fee: Like LETS-1, a joining fee is needed to prevent spam attacks. However, unlike LETS-1, this fee is non-refundable and must be sent to some predefined management committee.
  • Zero-Sum: As in LETS-1.
LETS-3: Positive-Sum, Collateral

The above two variants require the total LETS balance to be always zero. Here we consider a positive value for this sum. In particular, this variant has the following properties:

  • Time-locked Joining Fee: As in LETS-1.
  • Positive Sum: The LETS balance of every member must always be non-negative, which ensures that the sum of the LETS balances of all member boxes is positive. The initial LETS balance is set to a positive value based on the joining fee at the current rate, capped to some maximum value.
  • Collateral: Any reduction in the ergs balance of the sender must be accompanied by a reduction of the corresponding LETS balance at the current exchange rate.

We can also allow topping up the LETS balance during a transaction by adding the equivalent amount of ergs.

LETS-4: Positive-Sum, No collateral

This is similar to LETS-3 but with some small variations:

  • Non-refundable Joining Fee: As in LETS-2
  • Positive-Sum: As in LETS-3

The following table summarizes the variants:

Zero Sum Positive Sum
Collateral LETS-1 LETS-3
No collateral LETS-2 LETS-4

We considered LETS transactions involving a single sender-receiver pair. More advanced models can allow multiple senders and receivers and need not be in pairs.

Crossing the last mile

The need to reform the global financial system has been clear since the last crisis in 2008. Now, COVID-19 has forced our hands. We cannot delay, and the best way to start is from the grassroots.

As lockdowns across the world effectively shut down whole sectors and a large part of the economy, we are entering a new era at a breakneck pace. Central banks are taking unprecedented action in the form of money printing, but, just like last time around, they will almost certainly fail at the 'last mile' (intentionally or otherwise): while their efforts will benefit the financial sector and the wealthy, the funds will not reach those small businesses and individuals who need them the most.

Technology like the Ergo platform can address this problem efficiently, building bridges into and within our local economies without the need for commercial or central banks. A key principle of Ergo, and one stated in the white paper, is that it is for regular people. The developers have spent considerable time building technology that can be applied to real-world use cases. (This is the idea behind 'Smart contracts for the people', for example.)

Crisis measures

Systems like this have historically become popular during times of crisis. Michael Linton established the first system of this kind in a Canadian town stuck in depression back in 1981, and LETS were also popular during the 1998-2002 Argentine Great Depression.

Most LETS groups consist of 50 to 250 members, with paper-based credit notes and ledgers maintained by a committee. However, it is unsurprising that paper-based LETS have suffered from problems such as counterfeit notes, fraudulent activity by administrators, and so on (much like centralized crypto exchanges). A blockchain-based LETS could be vastly superior to any previous system.

Moreover, building lots of small credit systems on the same blockchain enables interoperability and novel financial products designed to strengthen the system. Hundreds of different LETS could exist, for individuals and small businesses, with different participation criteria, credit limits, collateralization requirements and other parameters. And yet, they could still be connected by gateways allowing liquidity to move between different LETS if required – while avoiding exposure to toxic debt.

What use cases do you have for Ergo to address the broken financial system? Let us know.