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Introduction

Overview​

The e-Invoicing API is an online backend service designed to streamline the process of issuing, managing, and processing tax payer's electronic invoices in compliance with the Federal Inland Revenue Services (FIRS) requirements. This API enables businesses to automate their invoicing workflows, ensuring efficient, accurate, and compliant financial transactions. This section defines the architectural requirements and API designs for the FIRS e-invoicing Service.

Sequence Diagram​

Sequence Diagram

Authentication​

Authentication will use an API Key and HMAC-SHA256 signature computed with the client’s secret in the request header. This ensures every request is uniquely signed and verified. Each API request must include the following headers:

  • X-API-Key: Identifies the client application.
  • X-API-Signature: HMAC-SHA256 signature generated from (request body + timestamp) using the client’s secret key.
  • X-API-Timestamp: ISO-20022 timestamp of the request (±5 min allowed) to prevent replay attacks.

If any of these headers are missing, invalid, or the timestamp is outside the allowed window, the API will return an appropriate error message from the server.

Sandbox BaseURL: https://firseinvoice.etranzactng.com/​

Sample Header Request:​

POST
curl -X POST
  -H "X-API-Key: CLIENT_APP_001"
  -H "X-API-Signature: 9f4e3a8d7c9b12345abcdef67890fedcba9876543210abcdef98765"
  -H "X-API-Timestamp: 2025-09-28T12:30:00+01:00"
  -H "Content-Type: application/json"
  -d '{"key": "value"}'
'