SLF Authentication: API Key Setup & Security Best Practices (2026)

Authentication overview

SLF's API authentication system is designed to provide secure access to its suite of Japanese geocoding and address services. The primary method for authenticating requests to SLF APIs is through the use of API keys. These keys serve as a unique identifier for your application and are used to authorize your access to specific API endpoints and resources, as detailed in the SLF Geocoding API documentation. Each API key is associated with your SLF developer account and is subject to the usage limits and plan specifications configured for that account. Proper management and protection of these keys are critical to maintaining the security and operational integrity of your applications that rely on SLF's services.

When an API key is included in a request, SLF's servers validate it against registered keys. A valid key grants access, while an invalid or missing key results in an authentication error, typically an HTTP 401 Unauthorized or 403 Forbidden status code. This system ensures that only authorized applications can consume SLF's geocoding data, protecting both the service provider and the data integrity for users.

Supported authentication methods

SLF currently supports API key-based authentication for all its core products, including the Geocoding API, Address Search API, and Reverse Geocoding API. This method is common for web services where server-to-server communication or client-side applications require a straightforward authentication mechanism without user interaction for each request.

The following table summarizes the supported authentication method:

API keys are typically passed as a query parameter in the API request URL. While convenient, this approach necessitates strict security practices to prevent key exposure. For client-side applications, it's recommended to restrict API keys by referrer (e.g., specific domains) to limit their usability if intercepted. For server-side integrations, environmental variables or secure configuration management systems are preferred for storing keys.

Getting your credentials

To obtain your SLF API key, you need to register for an SLF developer account and subscribe to one of their API plans. The process generally involves these steps:

1. Account Registration: Visit the SLF homepage and navigate to the developer or API section to sign up for a new account. You will typically need to provide basic contact information and agree to their terms of service.
2. Plan Selection: Choose an appropriate API plan based on your anticipated usage. SLF offers various plans, starting with the API Basic Plan, which provides a certain number of requests per month.
3. API Key Generation: Once your account is active and a plan is selected, you can access your developer dashboard or console. Within this interface, there will be an option to generate or view your API key. This key is a unique string that you will use to authenticate your API requests.
4. Key Management: The developer console usually provides options to manage your API keys, such as regenerating a key if it's compromised, or setting restrictions (e.g., IP address whitelisting, HTTP referrer restrictions) to enhance security. Consult the specific SLF API reference for detailed instructions on key management features available in your account.

It is important to treat your API key as a sensitive credential. Do not embed it directly into publicly accessible client-side code without appropriate restrictions, and avoid committing it to version control systems. Instead, use environment variables or a secure secrets management service for server-side applications.

Authenticated request example

SLF APIs are RESTful, meaning you interact with them using standard HTTP methods (GET, POST) and pass data typically in JSON format. Authentication is performed by including your API key in the request URL as a query parameter.

Below is an example of an authenticated request to the SLF Geocoding API using curl. Replace YOUR_API_KEY with your actual SLF API key and adjust the address parameters as needed for your specific query.

curl -X GET \
  "https://api.slf.co.jp/geocoding/v1/search?address=東京都千代田区大手町1-5-5&key=YOUR_API_KEY" \
  -H "Accept: application/json"

In this example:

• https://api.slf.co.jp/geocoding/v1/search is the endpoint for the Geocoding API search function.
• address=東京都千代田区大手町1-5-5 is the Japanese address you are querying.
• key=YOUR_API_KEY is where you append your unique API key as a query parameter.
• -H "Accept: application/json" specifies that you prefer the response in JSON format.

Upon successful authentication and a valid request, the API will return a JSON response containing geocoded information for the specified address. If the API key is incorrect or missing, or if there are issues with your account, you will receive an error message indicating an authentication or authorization failure. Always ensure your API key is correctly formatted and included in the request URL for every call to SLF's services.

For client-side JavaScript applications, you might construct the URL similarly:

const apiKey = 'YOUR_API_KEY'; // In a real app, load this securely
const address = '東京都新宿区西新宿2-8-1';
const url = `https://api.slf.co.jp/geocoding/v1/search?address=${encodeURIComponent(address)}&key=${apiKey}`;

fetch(url, {
  method: 'GET',
  headers: {
    'Accept': 'application/json'
  }
})
.then(response => {
  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  }
  return response.json();
})
.then(data => {
  console.log(data);
})
.catch(error => {
  console.error('Error fetching geocoding data:', error);
});

When deploying client-side applications, it's crucial to implement API key restrictions like HTTP referrer limits to prevent unauthorized usage if the key is exposed in the browser's network requests. This adds a layer of security by ensuring the key only works from your approved domains.

Security best practices

Protecting your SLF API keys is paramount to securing your applications and preventing unauthorized usage of your account. Adhering to these best practices will help mitigate common security risks:

• Never hardcode API keys directly into public client-side code: Embedding keys directly in JavaScript or mobile app binaries makes them easily extractable. Even with referrer restrictions, this isn't ideal for highly sensitive operations. For client-side scenarios where direct key exposure is unavoidable, ensure robust restrictions (e.g., HTTP referrer, IP address) are applied to the key in your SLF developer console.
• Use environment variables for server-side applications: For backend services, store your API keys as environment variables (e.g., SLF_API_KEY=YOUR_API_KEY). This keeps them out of your source code and configuration files, making them more secure.
• Implement secrets management systems: For complex deployments or microservices architectures, consider using dedicated secrets management solutions like AWS Secrets Manager, Google Cloud Secret Manager, or HashiCorp Vault. These systems securely store, manage, and distribute sensitive credentials.
• Restrict API key usage: Whenever possible, configure your API keys with specific restrictions in the SLF developer console. This might include:
• HTTP Referrer restrictions: Allow requests only from specific website domains.
• IP Address restrictions: Permit requests only from a list of trusted server IP addresses. This is highly effective for server-to-server communication.
• API restrictions: Limit the key to only access the specific SLF APIs your application needs, preventing it from being used for other services if compromised.
• Rotate API keys regularly: Periodically generate new API keys and deprecate old ones. This practice reduces the window of opportunity for a compromised key to be exploited.
• Monitor API usage: Regularly review your SLF API usage logs and billing details. Unusual spikes in usage could indicate a compromised key or unauthorized access.
• Secure your development environment: Ensure your local development machines and CI/CD pipelines are secure and that API keys are not exposed in logs or build artifacts.
• Use HTTPS exclusively: All communication with SLF APIs should occur over HTTPS (TLS). This encrypts data in transit, protecting your API key and request/response data from eavesdropping. SLF mandates HTTPS for all API interactions.

By following these guidelines, you can significantly enhance the security posture of your applications integrating with SLF's Japanese geocoding services.