learning-website-framework1-9  Exercise 2: Cookies and Password Storage
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PART A: Decide how the login cookie is shared (or not)
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Two cookie designs, built and checked by script:
- SHARED: Domain=osztromok.com. One login for every subdomain, but the cookie
  is sent to EVERY subdomain, so one weak subdomain weakens all of them.
- HOST-ONLY: a __Host- cookie. Browsers only accept it with Secure, Path=/ and
  NO Domain attribute, so it can never be shared, and each site logs in
  separately (or gets its own session through a central login).

PART B: Store passwords safely
------------------------------
Never store a password, only a salted, deliberately slow hash, and compare in
constant time. (The live Anime Vault uses PHP's password_hash with bcrypt; this
example uses scrypt from Python's standard library.)

Save as sessions.py:

import hashlib, hmac, os, time

DOMAIN = "osztromok.com"

def session_cookie(value, shared):
    """Build a Set-Cookie header value.
    shared=True : one login for every subdomain (Domain attribute, name without prefix)
    shared=False: host-only cookie with the __Host- prefix (this one site only)"""
    if shared:
        return (f"sid={value}; Domain={DOMAIN}; Path=/; Secure; HttpOnly; SameSite=Lax; Max-Age=86400")
    return (f"__Host-sid={value}; Path=/; Secure; HttpOnly; SameSite=Lax; Max-Age=86400")

def check_cookie(header):
    """Return a list of problems with a Set-Cookie header value."""
    parts = [p.strip() for p in header.split(";")]
    name = parts[0].split("=", 1)[0]
    attrs = {p.split("=", 1)[0].lower(): (p.split("=", 1)[1] if "=" in p else True) for p in parts[1:]}
    problems = []
    if "secure" not in attrs:   problems.append("missing Secure")
    if "httponly" not in attrs: problems.append("missing HttpOnly")
    if "samesite" not in attrs: problems.append("missing SameSite")
    if name.startswith("__Host-"):
        if "domain" in attrs:   problems.append("__Host- cookies must not set Domain")
        if attrs.get("path") != "/": problems.append("__Host- cookies need Path=/")
    return problems

def hash_password(password, salt=None):
    salt = salt or os.urandom(16)
    digest = hashlib.scrypt(password.encode(), salt=salt, n=2**14, r=8, p=1, dklen=32)
    return salt.hex() + "$" + digest.hex()

def verify_password(password, stored):
    salt_hex, digest_hex = stored.split("$")
    candidate = hash_password(password, bytes.fromhex(salt_hex)).split("$")[1]
    return hmac.compare_digest(candidate, digest_hex)       # constant-time comparison

if __name__ == "__main__":
    shared = session_cookie("abc123", shared=True)
    host_only = session_cookie("abc123", shared=False)
    print("shared   :", shared)
    print("  problems:", check_cookie(shared) or "none")
    print("host-only:", host_only)
    print("  problems:", check_cookie(host_only) or "none")
    for bad in ("sid=abc; Path=/", "__Host-sid=abc; Domain=osztromok.com; Secure; HttpOnly; SameSite=Lax; Path=/"):
        print("bad      :", bad)
        print("  problems:", ", ".join(check_cookie(bad)))
    print()
    t = time.perf_counter()
    stored = hash_password("correct horse battery staple")
    ms = (time.perf_counter() - t) * 1000
    print(f"stored format: <32 hex salt>$<64 hex hash>  (length {len(stored)}), hashing took about {ms:.0f} ms")
    print("same password twice gives different stored values:",
          hash_password("pw") != hash_password("pw"))
    print("right password verifies:", verify_password("correct horse battery staple", stored))
    print("wrong password verifies:", verify_password("wrong", stored))

Run it:

  python sessions.py

Output (checked by running it):

shared   : sid=abc123; Domain=osztromok.com; Path=/; Secure; HttpOnly; SameSite=Lax; Max-Age=86400
  problems: none
host-only: __Host-sid=abc123; Path=/; Secure; HttpOnly; SameSite=Lax; Max-Age=86400
  problems: none
bad      : sid=abc; Path=/
  problems: missing Secure, missing HttpOnly, missing SameSite
bad      : __Host-sid=abc; Domain=osztromok.com; Secure; HttpOnly; SameSite=Lax; Path=/
  problems: __Host- cookies must not set Domain

stored format: <32 hex salt>$<64 hex hash>  (length 97), hashing took about 57 ms
same password twice gives different stored values: True
right password verifies: True
wrong password verifies: False

Notes
-----
- Secure sends the cookie only over HTTPS; HttpOnly hides it from page
  scripts (a stolen cookie via script injection is the usual attack);
  SameSite=Lax stops it being sent on most cross-site requests.
- A shared cookie ties the security of every subdomain together. If you choose
  it, every site must be equally careful, and no subdomain may be handed to a
  third party. Otherwise prefer host-only cookies.
- The same password gives a different stored value each time because the salt
  is random, which defeats pre-computed tables.
- Hashing takes tens of milliseconds on purpose, which makes guessing slow.
  Tune the cost so that a login takes a noticeable fraction of a second.

WHY THIS WORKS AS AN ANSWER
---------------------------
Whether to share a login is a security trade-off, and writing the two cookie
headers side by side, with a checker, makes the difference concrete. The
password example shows the three habits that matter: random salt, slow hash and
constant-time comparison.