🔒 Update HTTP Basic Auth docs fixing timing attacks (#807)

docs/src/security/tutorial007.py

@@ -1,3 +1,5 @@
+import secrets
+
 from fastapi import Depends, FastAPI, HTTPException
 from fastapi.security import HTTPBasic, HTTPBasicCredentials
 from starlette.status import HTTP_401_UNAUTHORIZED
@@ -8,7 +10,9 @@ security = HTTPBasic()
 
 
 def get_current_username(credentials: HTTPBasicCredentials = Depends(security)):
-    if credentials.username != "foo" or credentials.password != "password":
+    correct_username = secrets.compare_digest(credentials.username, "stanleyjobson")
+    correct_password = secrets.compare_digest(credentials.password, "swordfish")
+    if not correct_username and correct_password:
         raise HTTPException(
             status_code=HTTP_401_UNAUTHORIZED,
             detail="Incorrect email or password",

docs/tutorial/security/http-basic-auth.md

@@ -33,8 +33,74 @@ Here's a more complete example.
 
 Use a dependency to check if the username and password are correct.
 
-If the credentials are incorrect, return an `HTTPException` with a status code 401 (the same returned when no credentials are provided) and add the header `WWW-Authenticate` to make the browser show the login prompt again:
+For this, use the Python standard module <a href="https://docs.python.org/3/library/secrets.html" target="_blank">`secrets`</a> to check the username and password:
 
-```Python hl_lines="10 11 12 13 14 15 16 17 21"
+```Python hl_lines="1  13 14 15"
+{!./src/security/tutorial007.py!}
+```
+
+This will ensure that `credentials.username` is `"stanleyjobson"`, and that `credentials.password` is `"swordfish"`. This would be similar to:
+
+```Python
+if not (credentials.username == "stanleyjobson") or not (credentials.password == "swordfish"):
+    # Return some error
+    ...
+```
+
+But by using the `secrets.compare_digest()` it will be secure against a type of attacks called "timing attacks".
+
+### Timing Attacks
+
+But what's a "timing attack"?
+
+Let's imagine an attacker is trying to guess the username and password.
+
+And that attacker sends a request with a username `johndoe` and a password `love123`.
+
+Then the Python code in your application would be equivalent to something like:
+
+```Python
+if "johndoe" == "stanleyjobson" and "love123" == "swordfish":
+    ...
+```
+
+But right at the moment Python compares the first `j` in `johndoe` to the first `s` in `stanleyjobson`, it will return `False`, because it already knows that those two strings are not the same, thinking that "there's no need to waste more computation comparing the rest of the letters". And your application will say "incorrect user or password".
+
+But then the attacker tries with username `stanleyjobsox` and password `love123`.
+
+And your application code does something like:
+
+```Python
+if "stanleyjobsox" == "stanleyjobson" and "love123" == "swordfish":
+    ...
+```
+
+Python will have to compare the whole `stanleyjobso` in both `stanleyjobsox` and `stanleyjobson` before realizing that both strings are not the same. So it will take some extra microseconds to reply back "incorrect user or password".
+
+#### The time to answer helps the attacker
+
+At that point, by noticing that the server took some microseconds longer to send the "incorrect user or password" response, the attacker will know that she/he got _something_ right, some of the initial letters were right.
+
+And then she/he can try again knowing that it's probably something more similar to `stanleyjobsox` than to `johndoe`.
+
+#### A "professional" attack
+
+Of course, the attacker would not try all this by hand, she/he would write a program to do it, possibly with thousands or millions of tests per second. And would get just one extra correct letter at a time.
+
+But doing that, in some minutes or hours the attacker would have guessed the correct username and password, with the "help" of our application, just using the time taken to answer.
+
+#### Fix it with `secrets.compare_digest()`
+
+But in our code we are actually using `secrets.compare_digest()`.
+
+In short, it will take the same time to compare `stanleyjobsox` to `stanleyjobson` than it takes to compare `johndoe` to `stanleyjobson`. And the same for the password.
+
+That way, using `secrets.compare_digest()` in your application code, it will be safe against this whole range of security attacks.
+
+### Return the error
+
+After detecting that the credentials are incorrect, return an `HTTPException` with a status code 401 (the same returned when no credentials are provided) and add the header `WWW-Authenticate` to make the browser show the login prompt again:
+
+```Python hl_lines="16 17 18 19 20"
 {!./src/security/tutorial007.py!}
 ```