FastAPI Day 7: Implementing Clear JWT Authentication with Dependency Injection

1. Registration: Store Passwords as Hashes

Passwords must never be stored in plain text. The article uses bcrypt to hash passwords.

import bcrypt
def get_password_hash(password: str) -> str:
    return bcrypt.hashpw(password.encode(), bcrypt.gensalt()).decode()
def verify_password(plain: str, hashed: str) -> bool:
    return bcrypt.checkpw(plain.encode(), hashed.encode())

The registration endpoint checks for duplicate email, hashes the password, stores the user record, and returns a public user object.

@app.post("/auth/register", status_code=201)
def register(user: UserCreate):
    if user.email in USERS_DB:
        raise HTTPException(status_code=400, detail="Email already registered")
    hashed = get_password_hash(user.password)
    USERS_DB[user.email] = {"email": user.email, "hashed_password": hashed, ...}
    return UserPublic(email=user.email, full_name=user.full_name)

2. Login: Generate JWT Token

When a user logs in, the password is verified and a JWT token with an expiration time is created.

import jwt
from datetime import datetime, timedelta, timezone

SECRET_KEY = "change-this-in-production"
ALGORITHM = "HS256"

def create_access_token(data: dict, expires_delta: timedelta | None = None):
    to_encode = data.copy()
    expire = datetime.now(timezone.utc) + (expires_delta or timedelta(minutes=30))
    to_encode.update({"exp": expire})
    return jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)

The login endpoint accepts form data, validates credentials, and returns the access token.

@app.post("/auth/login")
def login(username: str = Form(), password: str = Form()):
    if username not in USERS_DB:
        raise HTTPException(status_code=401, detail="Incorrect credentials")
    user = USERS_DB[username]
    if not verify_password(password, user["hashed_password"]):
        raise HTTPException(status_code=401, detail="Incorrect credentials")
    access_token = create_access_token(data={"sub": username})
    return {"access_token": access_token, "token_type": "bearer"}

3. Protected Routes: Dependency‑Injected Authentication Chain

Endpoints that require authentication add a single dependency:

def get_current_user_from_token(token: Annotated[str, Depends(verify_token)]):
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        email = payload.get("sub")
        if email is None:
            raise HTTPException(status_code=401, detail="Invalid token")
    except jwt.ExpiredSignatureError:
        raise HTTPException(status_code=401, detail="Token expired")
    except jwt.InvalidTokenError:
        raise HTTPException(status_code=401, detail="Invalid token")
    return email

@app.get("/auth/me")
def get_me(current_email: Annotated[str, Depends(get_current_user_from_token)]):
    user = USERS_DB[current_email]
    return UserPublic(email=user["email"], full_name=user["full_name"])

The verify_token dependency (introduced on Day 5) parses the Authorization header; get_current_user_from_token only decodes and validates the token.

4. What Was Completed on Day 7

bcrypt password hashing (store hash, verify on login)

JWT token generation with expiration

POST /auth/register endpoint

POST /auth/login endpoint (form‑encoded)

GET /auth/me protected endpoint using JWT

All five new test cases pass, bringing the total to 47

5. Next Steps

Day 8 will cover database integration with SQLModel and project structuring into routers, models, and schemas.