Light Client for Exonum Blockchain

A JavaScript library to work with Exonum blockchain from browser and Node.js. Used to sign transactions before sending to blockchain and verify blockchain responses using cryptographic proofs. Contains numerous helper functions. Find out more information about the architecture and tasks of light clients in Exonum.

If you are using Exonum in your project and want to be listed on our website & GitHub list — write us a line to [email protected].

Library compatibility with Exonum core:

| JavaScript light client | Exonum core | |---|---| | 0.18.4 | 1.0.* | | 0.18.3 | 1.0.0-rc.1 | | 0.17.1 | 0.12.* | | 0.16.9 | 0.11.* | | 0.16.9 | 0.10.* | | 0.13.0 | 0.9.* | | 0.10.2 | 0.8.* | | 0.9.0 | 0.7.* | | 0.6.1 | 0.6.* | | 0.6.1 | 0.5.* | | 0.3.0 | 0.4.0 | | 0.3.0 | 0.3.0 | | 0.2.0 | 0.2.0 | | 0.1.1 | 0.1.* |

• Getting started
• Data types
• Hash
• Signature
  • Sign data
  • Verify signature
• Transactions
  • Define transaction
  • Sign transaction
  • Send transaction
  • Send multiple transactions
• Cryptographic proofs
  • Merkle tree proof
  • Map proof
• Integrity checks
  • Verify block
  • Verify table
  • Built-in structures
• Helpers
  • Generate key pair
  • Get random number
  • Converters
    • Hexadecimal to Uint8Array
    • Hexadecimal to String
    • Uint8Array to Hexadecimal
    • Binary String to Uint8Array
    • Binary String to Hexadecimal
    • String to Uint8Array
• Contributing
  • Coding standards
  • Test coverage
• Changelog
• License

Getting started

There are several options to include light client library in the application:

The preferred way is to install Exonum Client as a package from npm registry:

npm install exonum-client

Otherwise you can download the source code from GitHub and compile it before use in browser.

Include in browser:

<script src="node_modules/exonum-client/dist/exonum-client.min.js"></script>

Usage in Node.js:

let Exonum = require('exonum-client')

Data types

Exonum uses protobufjs library to serialize structured data into protobuf format.

Each transaction is signed before sending into blockchain. Before the transaction is signed it is converted into byte array under the hood.

The data received from the blockchain should be converted into byte array under the hood before it will be possible to verify proof of its existence using cryptographic algorithm.

Developer can both define data structures on the fly or use precompiled stubs with data structures.

To define Protobuf structures use protobufjs library.

Example:

const MessageSchema = new Type('CustomMessage')
  .add(new Field('balance', 1, 'uint32'))
  .add(new Field('name', 2, 'string'))
const Message = Exonum.newType(MessageSchema)

Exonum.newType function requires a single argument of protobuf.Type type.

Hash

Exonum uses cryptographic hashes of certain data for transactions and proofs.

Different signatures of the hash function are possible:

Exonum.hash(data, type)
type.hash(data)

| Argument | Description | Type | |---|---|---| | data | Data to be processed using a hash function. | Object | | type | Definition of the data type. | Custom data type or transaction. |

An example of hash calculation:

// Define a data structure
const Message = new Type('User')
  .add(new Field('balance', 1, 'uint32'))
  .add(new Field('name', 2, 'string'))
// Define a data type
const User = Exonum.newType(Message)

// Data to hash
const data = {
  balance: 100,
  name: 'John Doe'
}
// Get a hash
const hash = User.hash(data)

It is also possible to get a hash from byte array:

Exonum.hash(buffer)

| Argument | Description | Type | |---|---|---| | buffer | Byte array. | Array or Uint8Array. |

An example of byte array hash calculation:

const arr = [8, 100, 18, 8, 74, 111, 104, 110, 32, 68, 111, 101]
const hash = Exonum.hash(arr)

Signature

The procedure for signing data using signing key pair and verifying of obtained signature is commonly used in the process of data exchange between the client and the service.

Built-in Exonum.keyPair helper function can be used to generate a new random signing key pair.

Sign data

The signature can be obtained using the secret key of the signing pair.

There are three possible signatures of the sign function:

Exonum.sign(secretKey, data, type)
type.sign(secretKey, data)
Exonum.sign(secretKey, buffer)

| Argument | Description | Type | |---|---|---| | secretKey | Secret key as hexadecimal string. | String | | data | Data to be signed. | Object | | type | Definition of the data type. | Custom data type. | | buffer | Byte array. | Array or Uint8Array. |

The sign function returns value as hexadecimal String.

Verify signature

The signature can be verified using the author's public key.

There are two possible signatures of the verifySignature function:

Exonum.verifySignature(signature, publicKey, data, type)
type.verifySignature(signature, publicKey, data)

| Argument | Description | Type | |---|---|---| | signature | Signature as hexadecimal string. | String | | publicKey | Author's public key as hexadecimal string. | String | | data | Data that has been signed. | Object | | type | Definition of the data type. | Custom data type. |

The verifySignature function returns value of Boolean type.

An example of signature creation and verification:

// Define a data structure
const Message = new Type('User')
  .add(new Field('balance', 1, 'uint32'))
  .add(new Field('name', 2, 'string'))
const User = Exonum.newType(Message)

// Define a signing key pair
const keyPair = Exonum.keyPair()

// Data that has been hashed
const data = {
  balance: 100,
  name: 'John Doe'
}
// Signature obtained upon signing using secret key
const signature = Exonum.sign(keyPair.secretKey, data, User)
// Verify the signature
const result = Exonum.verifySignature(signature, keyPair.publicKey, data, User)

Transactions

Transaction in Exonum is an operation to change the data stored in blockchain. Transaction processing rules is a part of business logic implemented in a service.

Sending data to the blockchain from a light client consist of 3 steps:

1. Describe the fields of transaction using custom data types
2. Sign data of transaction using signing key pair
3. Send transaction to the blockchain

Read more about transactions in Exonum, or see the example of their usage.

Define transaction

An example of a transaction definition:

const Transaction = new Type('CustomMessage')
  .add(new Field('to', 2, 'string'))
  .add(new Field('amount', 3, 'uint32'))

const SendFunds = new Exonum.Transaction({
  schema: Transaction,
  service_id: 130,
  method_id: 0
})

Exonum.Transaction constructor requires a single argument of Object type with the next structure:

| Property | Description | Type | |---|---|---| | schema | Protobuf data structure. | Object | | service_id | Service ID. | Number | | method_id | Method ID. | Number |

schema structure is identical to that of custom data type.

Sign transaction

An example of a transaction signing:

// Signing key pair
const keyPair = Exonum.keyPair()

// Transaction data to be signed
const data = {
  from: 'John',
  to: 'Adam',
  amount: 50
}

// Create a signed transaction
const signed = SendFunds.create(data, keyPair)

Send transaction

To submit transaction to the blockchain send function can be used.

Exonum.send(explorerBasePath, transaction, attempts, timeout)

| Property | Description | Type | |---|---|---| | explorerBasePath | API address of transaction explorer on a blockchain node. | String | | transaction | Signed transaction bytes. | String, Uint8Array or Array-like | | attempts | Number of attempts to check transaction status. Pass 0 in case you do not need to verify if the transaction is accepted to the block. Optional. Default value is 10. | Number | | timeout | Timeout between attempts to check transaction status. Optional. Default value is 500. | Number |

The send function returns a Promise with the transaction hash. The promise resolves when the transaction is committed (accepted to a block).

An example of a transaction sending:

// Define transaction explorer address
const explorerBasePath = 'http://127.0.0.1:8200/api/explorer/v1/transactions'
const transactionHash = await Exonum.send(explorerBasePath, signed.serialize())

Send multiple transactions

To submit multiple transactions to the blockchain sendQueue function can be used. Transactions will be sent in the order specified by the caller. Each transaction from the queue will be sent to the blockchain only after the previous transaction is committed.

Exonum.sendQueue(explorerBasePath, transactions, attempts, timeout)

| Property | Description | Type | |---|---|---| | explorerBasePath | API address of transaction explorer on a blockchain node. | String | | transactions | List of transactions. | Array | | attempts | Number of attempts to check each transaction status. Pass 0 in case you do not need to verify if the transactions are accepted to the block. Optional. Default value is 10. | Number | | timeout | Timeout between attempts to check each transaction status. Optional. Default value is 500. | Number |

The sendQueue function returns a Promise with an array of transaction hashes. The promise resolves when all transactions are committed.

Cryptographic proofs

A cryptographic proof is a format in which a Exonum node can provide sensitive data from a blockchain. These proofs are based on Merkle trees and their variants.

Light client library validates the cryptographic proof and can prove the integrity and reliability of the received data.

Read more about design of cryptographic proofs in Exonum.

Merkle tree proof

const proof = new Exonum.ListProof(json, ValueType)
console.log(proof.entries)

The ListProof class is used to validate proofs for Merkelized lists.

| Argument | Description | Type | |---|---|---| | json | The JSON presentation of the proof obtained from a full node. | Object | | ValueType | Data type for values in the Merkelized list. | Custom data type |

The returned object has the following fields:

| Field | Description | Type | |---|---|---| | merkleRoot | Hexadecimal hash of the root of the underlying Merkelized list | String | | entries | Elements that are proven to exist in the list, together with their indexes | Array<{ index: number, value: V }> | | length | List length | Number |

See an example of using a ListProof.

Map proof

const proof = new Exonum.MapProof(json, KeyType, ValueType)
console.log(proof.entries)

The MapProof class is used to validate proofs for Merkelized maps.

| Argument | Description | Type | |---|---|---| | json | The JSON presentation of the proof obtained from a full node. | Object | | KeyType | Data type for keys in the Merkelized map. | Custom or built-in data type | | ValueType | Data type for values in the Merkelized map. | Custom data type |

Keys in a map proof can either be hashed (which is the default option) or raw. To obtain a raw version for KeyType, use MapProof.rawKey(KeyType). The key type is determined by the service developer when the service schema is created. Raw keys minimize the amount of hashing, but require that the underlying type has fixed-width binary serialization.

The returned object has the following fields:

| Field | Description | Type | |---|---|---| | merkleRoot | Hexadecimal hash of the root of the underlying Merkelized map | String | | missingKeys | Set of keys which the proof asserts as missing from the map | Set<KeyType> | | entries | Map of key-value pairs that the are proven to exist in the map | Map<KeyType, ValueType> |

See an example of using a MapProof.

Integrity checks

Verify block

Exonum.verifyBlock(data, validators)

Each new block in Exonum blockchain is signed by validators. To prove the integrity and reliability of the block, it is necessary to verify their signatures. The signature of each validator are stored in the precommits.

The verifyBlock function throws an error if a block is invalid.

| Argument | Description | Type | |---|---|---| | data | Structure with block and precommits. | Object | | validators | An array of validators public keys as a hexadecimal strings. | Array |

Verify table

Exonum.verifyTable(proof, stateHash, fullTableName)

Verify table existence in the root tree.

Returns root hash for the table as hexadecimal String.

| Argument | Description | Type | |---|---|---| | proof | The JSON presentation of the proof obtained from a full node. | Object | | stateHash | Hash of current blockchain state stored in each block. | String | | fullTableName | Name of the table, such as token.wallets. | String |

Built-in structures

The library exports Protobuf declarations from the core crate. Consult Protobuf files included into the library for more details.

Helpers

Generate key pair

const pair = Exonum.keyPair()

{
  publicKey: "...", // 32-byte public key
  secretKey: "..." // 64-byte secret key
}

Exonum.keyPair function generates a new random Ed25519 signing key pair using the TweetNaCl cryptographic library.

Get random number

const rand = Exonum.randomUint64()

Exonum.randomUint64 function generates a new random Uint64 number of cryptographic quality using the TweetNaCl cryptographic library.

Converters

Hexadecimal to Uint8Array

const hex = '674718178bd97d3ac5953d0d8e5649ea373c4d98b3b61befd5699800eaa8513b'
Exonum.hexadecimalToUint8Array(hex)

Hexadecimal to String

const hex = '674718178bd97d3ac5953d0d8e5649ea373c4d98b3b61befd5699800eaa8513b'
Exonum.hexadecimalToBinaryString(hex)

Uint8Array to Hexadecimal

const arr = new Uint8Array([103, 71, 24, 23, 139, 217, 125, 58, 197, 149, 61])
Exonum.uint8ArrayToHexadecimal(arr)

Uint8Array to Binary String

const arr = new Uint8Array([103, 71, 24, 23, 139, 217, 125, 58, 197, 149, 61])
Exonum.uint8ArrayToBinaryString(arr)

Binary String to Uint8Array

const str = '0110011101000111000110000001011110001011110110010111110100111010'
Exonum.binaryStringToUint8Array(str)

Binary String to Hexadecimal

const str = '0110011101000111000110000001011110001011110110010111110100111010'
Exonum.binaryStringToHexadecimal(str)

String to Uint8Array

const str = 'Hello world'
Exonum.stringToUint8Array(str)

Contributing

The contributing to the Exonum Client is based on the same principles and rules as the contributing to exonum-core.

Coding standards

The coding standards are described in the .eslintrc file.

To help developers define and maintain consistent coding styles between different editors and IDEs we used .editorconfig configuration file.

Test coverage

All functions must include relevant unit tests. This applies to both of adding new features and fixing existed bugs.

Changelog

Detailed changes for each release are documented in the CHANGELOG file.

Other languages support

Light Clients for Java and Python

License

Exonum Client is licensed under the Apache License (Version 2.0). See LICENSE for details.