Constant-time `if-else` in python

Question

I'd like to know if there is an easy way (maybe a library) to write constant-time programs in Python. In particular, I'd like to be able to specify that an if-else flow must always last the same time either the if condition is True or False.

For instance:

if condition:
    foo1()
else:
    foo2()
foo3()

The idea of constant-time is that in the execution, the time that takes until it hits f3() should take the same time independently of the result of the evaluation of condition. This would prevent leakage of time as a side-channel to reveal other information (cf. timing attacks).

Answer 1

Since your question is about security, I assume we can leave aside performance and very naively introduce a minimal time to spend on all possible branches.One way to achieve this is with context managers:
Your problem could be then written as:

with ConstantTime(0.1):
    if condition:
        foo1()
    else:
        foor2()
foo3()

Using a context manager defined like this:

import threading
import time

class ConstantTime():
    def __init__(self, length):
        self.length = length

    def __enter__(self):
        self.timer = threading.Thread(target=time.sleep, args=[self.length])
        self.timer.start()

    def __exit__(self, exc_type, exc_value, traceback):
        self.timer.join()

You would of course have to adjust the actual time to realistic values, depending on what you are doing.

In general you can't be sure your minimal time won't be exceeded by the chosen branch, since Python is a very high level language and you're probably not running it in a real-time OS, but if you manage to cover the average runtime, you should significantly reduce the information gathered from timing analysis.