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Create ASP.NET Core Web API for Ethereum DApps — Part 3: Restful Api

3. Project Web Api

3.1/ Add references:

We have to add references to our classlib in our project Web API beforehand.

From project folder:

dotnet add reference ../ContractInterface.Common/ContractInterface.Common.csproj ../ContractInterface.ERC20/ContractInterface.ERC20.csproj

3.2/ Configure dependency injections in Startup class

Dependency injection is a nice way to separate usage from creation of objects. By delegating the creation of objects to dependency controller, you no longer have to worry about properly properly instantiating and managing object’s lifetime. If you wish to know more about this design patters, please refer to links at the end of the article.

Design Patterns Explained – Dependency Injection with Code Examples

http://bit.ly/2AprTdK

In Asp.Net Core, implementing dependency injection is trivial. All you have to do is registering your types to service container in Startup.cs.

Startup.cs

As you can see in my Startup.cs, I have configured my ContractFacade, ContractOperation, AccountService and ContractService as services to be manage by the container. Each of these services require different life cycles and thus the service container offers three options to free you from headache:

AddSingleton: created once per app instance

AddScoped: created once per web request

AddTransient: created each time requested

You also have the choice to register either your concrete classes or interfaces with their implementations. The advantage of registering interfaces is that you can change your implementation at any moment without worrying about dependencies breaking your code. Thus each time your service interface is requested by consumer classes, the service container will look for registered implementation and resolves the dependencies.

3.3/ Configure Authentication service

Our application needs an authentication/authorization scheme in order to verify user’s identity.

Asp.Net Core provides authentication service as a very neat way to implement login schema in your application. Using it is as simple as registering a dependency injection in Startup.cs.

In Startup class, I have called AddAuthentication from ConfigureServices method in order to configure a login logic using Json Web Token (JWT).

Configure Authentication service in Startup.cs

By JWT standards, when user sends an authentication request to server, if credentials match the server will response with a token to authorize user’s session. This token is included in each of subsequent requests in order to maintain login session until expires. The token must be digitally signed by server so that it cannot be tampered with. And also remember to give it an expiration to minimize consequent in case of being intercepted, because any one with the token can use it to impersonate you and send requests to the server.

Then you have to add the service into app’s middle-ware pipeline in Configure method, above UseMvc().

Add authentication to pipeline

In AccountService class, you have to configure Authenticate method in a way that it verifies given credentials and issues new token when the verification passes correctly.

AccountService.cs

Look into Authenticate method, I have verified user’s ethereum address and password against two hard-coded accounts. The token issued expires within one hour, and contains a claim of user’s identity. The token is hashed using symmetric keyed hash algorithm HmacSha256 on the payload, header and secret key. This is tamper proof because only the server knows its secret key used against hash function, thus only the server can provide a correct hash for the token. If the token has been changed a bit when returned to server, it will be rejected because the hash does not match.

The SymmetricSecurityKey used to calculate hash for the token is created using a provided passphrase configure in appsettings.Development.json.

appsettings.Development.json

Tips: in production, you should not by any means expose your passphrase as above. Please refer here and here for more guidance on how to protect your cryptographic materials.

To retrieve the key from appsettings, I use GetSection method from IConfiguration service (please refer to AccountService.cs image above).

In your controllers, so as to activate authorization scheme, apply [Authorize] attribute to each controller. If you want to exclude one method from authorization scheme, use [AllowAnonymous].

Use authorization in controllers

So to recap:

In Startup.cs: 1/ configure Authentication service, token validation scheme and the type and characteristics of token to be accepted for validation. 2/ Activate authentication middle-ware.

In AccountService.cs : configure the issue of authentication token by verifying user’s credentials.

In API controllers: apply authorization scheme if needed.

Here I used the bare minimum of Identity service provided with dotnet core to verify user’s ethereum address and password. You can very much improve it by implementing a much more complex scheme such as using Entity Framework to configure a database, or associating user’s identity with their Ethereum transaction account (similar to a wallet).

3.4/ Contract Controller

ContractController.cs contains all methods related to our smart contract such as deploying contract and retrieving information specific to contract’s characteristics.

Services:

IConfiguration: configuration service of Asp.net core to retrieve information from appsetting.json

IContractFacade: type of ContractInterface.Common which interfaces with Nethereum to operate upon any smart contracts.

IContractOperation: type of ContractInterface.ERC20 which interfaces with Nethereum to operate upon ERC20 contract.

IAccountService: service specific to our Web Api which manages authentication and ethereum accounts to make transactions.

ContractService: service specific to our Web Api which manage operations on contracts.

ILogger: logging service of Asp.Net Core

Deploying contract:

There are two deployment methods provided in the controller. Deploy() allows deploying any type of ethereum smart contracts to blockchain, and DeployDefault() helps deploy our previously created smart contract without a request body (mostly used for test).

Tips: I use wrapper types as inputs for request methods because the Asp.Net controllers implicitly derive inputs from request body (the same as using [FromBody] attribute), and it can only deserialize one input per request body.

Routing:

To retrieve information related to a particular contract, I add contract address in the route for request methods. This allows getting information of any contracts ERC20 contract given its address.

I don’t place contract address to controller route because the controller contains methods which operate on new contracts that haven’t yet existed on blockchain.

Exception handling:

Generally it is not a good practice to implement try-catch blocks in controllers because it is exhausting ! You should instead configure a middle-ware to globally handle exception.

3.5/ People Controller:

PeopleController.cs contains request methods related to operations on holders and spenders of our ERC20 contract such as adding, removing people, inquire for their balance or status, and transfer tokens.

All methods in the controller operates on an existing contract, that’s why I place contract address in the controller route.

3.6/ Account Service:

The account service contains methods for authentication and to manage user’s ethereum accounts. All transactions that change contract’s state in Ethereum need to be signed by an unlocked account. Nethereum provides ManagedAccount as a wrapper type for Ethereum account, which manages unlocking scheme.

Source

[Telegram Channel | Original Article ]

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