Authentication and Authorization

Authentication (AuthN) answers “who are you?” — it verifies identity.

Authorization (AuthZ) answers “what can you do?” — it enforces permissions.

These are distinct concerns that are often conflated. A user can be authenticated (their identity is Verified) but not authorized (they lack permission for a specific action). Conversely, a system Might authorize a request without authentication (anonymous access).

Use a dedicated password hashing function with a unique random salt per password:

# Argon2id (recommended)
from argon2 import PasswordHasher
ph = PasswordHasher(time_cost=3, memory_cost=65536, parallelism=4)
hash = ph.hash("user_password")  # $argon2id$v=19$m=65536,t=3,p=4$...

# bcrypt (widely supported)
import bcrypt
hash = bcrypt.hashpw(b"user_password", bcrypt.gensalt(rounds=12))

# scrypt (memory-hard alternative)
import hashlib
hash = hashlib.scrypt(b"user_password", salt=os.urandom(16), n=2**14, r=8, p=1, dklen=64)

A password hash must contain:

1. Algorithm identifier: Which function was used (allows migration)
2. Parameters: Cost factor, memory, parallelism (allows increasing work factor)
3. Salt: Unique per password (prevents rainbow tables and identical password detection)
4. Hash output: The actual derived key

Example formats:

# Argon2id
$argon2id$v=19$m=65536,t=3,p=4$c29tZXNhbHQ$RdescudvJCsgt3ub+b+dWRWJTmaaJObG

# bcrypt
$2b$12$R9h/cIPz0gi.YJJFsRyuPOYfGJTCGijPylCPvmFbzXuKJ5Xo1GZ6u

# scrypt (PHC format)
$scrypt$ln=16,r=8,p=1$c2FsdHNhbHQ$E4JnHDMfM/4R5U6YbGzbVg==

When you need to upgrade from bcrypt to Argon2id:

1. On login, verify the password against the existing hash
2. If verification succeeds and the hash uses the old algorithm, re-hash with the new algorithm
3. Store the new hash
4. Over time, all active users migrate to the new algorithm

This avoids forcing a password reset for all users.

The NIST Digital Identity Guidelines represent the current best practice for password policies, and They contradict many traditional policies.

Do:

• Require minimum 8 characters (15+ for higher security)
• Allow all printable characters and spaces
• Check passwords against breached password databases (HaveIBeenPwned API)
• Use rate limiting to prevent brute-force attacks
• Allow password managers and paste functionality
• Implement secure password reset (time-limited, single-use tokens)

Do Not:

• Force periodic password rotation (leads to predictable patterns: Password1!``Password2!…)
• Require composition rules (uppercase + lowercase + digit + special character — users just capitalize the first letter and add 1!)
• Require passwords to be changed after a breach unless compromise is confirmed
• Use knowledge-based authentication (security questions are guessable)
• Store password hints
• Set maximum password length under 64 characters

Check passwords against known breached password databases at creation and authentication time:

import requests
import hashlib
import sys

def check_pwned_password(password):
    """Check if password appears in HaveIBeenPwned database using k-anonymity."""
    sha1 = hashlib.sha1(password.encode()).hexdigest().upper()
    prefix, suffix = sha1[:5], sha1[5:]
    response = requests.get(f"https://api.pwnedpasswords.com/range/{prefix}")
    for line in response.text.splitlines():
        hash_suffix, count = line.split(":")
        if hash_suffix == suffix:
            return int(count)
    return 0

MFA requires two or more independent factors from different categories:

TOTP (RFC 6238) generates a 6-8 digit code based on a shared secret and the current time. The server And client both compute:

Where $K$ is the shared secret and T = \lfloor \mathrm{current\_time / 30 \rfloor.

Limitations: TOTP codes are phishable. An attacker can proxy the login page and forward the TOTP Code to the real service in real time. TOTP is not a replacement for phishing-resistant MFA.

FIDO2 (Fast Identity Online 2) is the gold standard for phishing-resistant authentication. It uses Public-key cryptography with a hardware authenticator.

sequenceDiagram
    participant U as User
    participant C as Client (Browser)
    participant S as Server

    S->>C: Send challenge + allowed credentials
    C->>U: Prompt for biometric/PIN
    U->>C: Unlock authenticator
    C->>S: Send authenticator assertion (signature over challenge)
    S->>S: Verify signature with stored public key
    S->>C: Authentication success

Key properties:

• Phishing-resistant: The authenticator binds to the relying party (origin), so a phishing site cannot replay the credential.
• Public-key based: The server stores a public key, not a shared secret. Compromising the server does not allow impersonation.
• Hardware-bound: Private key never leaves the authenticator (YubiKey, Touch ID, Windows Hello).
• Multi-device: Passkeys (synced WebAuthn credentials) allow cloud-synced FIDO2 credentials.