Create index.md

fastapi/index.md

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+## Authentication with API Key in Header
+
+FastAPI makes it easy to authenticate users by passing an API key through HTTP headers.
+
+### Example: API Key Authentication
+
+The following is a simple example where an API key is passed through the request header for authentication:
+
+```python
+from fastapi import FastAPI, Header, HTTPException, Depends
+
+app = FastAPI()
+
+# Dependency to check API key in header
+def get_api_key(x_api_key: str = Header(...)):
+    if x_api_key != "mysecretapikey123":
+        raise HTTPException(status_code=400, detail="Invalid API Key")
+    return x_api_key
+
+@app.get("/secure-data")
+async def get_secure_data(api_key: str = Depends(get_api_key)):
+    return {"message": "This is secure data."}
+
+curl -X 'GET' \
+  'http://127.0.0.1:8000/secure-data' \
+  -H 'x-api-key: mysecretapikey123'