Hey there, let's troubleshoot why your PKCE code verifier isn't passing Auth0's validation. I've noticed a couple of key issues in your current implementation that are likely causing the problem:

1. Incomplete Base64URL Encoding (Missing Padding Removal) #

PKCE requires base64url encoding to strip all trailing = padding characters, along with replacing + with - and / with _. While Python's urlsafe_b64encode handles the character replacements, it leaves the trailing = intact—which violates the PKCE spec. Your encoding function needs to account for this.

Fixed Encoding Function: #

def base64URLEncode(random_bytes):
    # Remove padding and convert bytes to UTF-8 string
    return urlsafe_b64encode(random_bytes).rstrip(b'=').decode('utf-8')

This returns a string (not raw bytes) which is the format Auth0 expects for both the verifier and challenge.

2. Incorrect Challenge Generation Logic #

The challenge needs to be derived from the byte representation of the verifier string, not the raw bytes used to generate the verifier. Here's the complete, correct flow:

Full Working Implementation #

import secrets
import hashlib
from base64 import urlsafe_b64encode

def base64URLEncode(random_bytes):
    return urlsafe_b64encode(random_bytes).rstrip(b'=').decode('utf-8')

def sha256(buffer):
    m = hashlib.sha256()
    m.update(buffer)
    return m.digest()

# Generate a valid 32-byte verifier (base64url encoded, no padding)
verifier = base64URLEncode(secrets.token_bytes(32))

# Generate the challenge: SHA256 hash of the verifier's bytes, then base64url encoded
challenge = base64URLEncode(sha256(verifier.encode('utf-8')))

3. Critical Checkpoints to Verify #

• When requesting the authorization code, make sure you send both code_challenge (the value above) and code_challenge_method=S256 to Auth0's authorization endpoint.
• When exchanging the code for tokens, pass the exact verifier value (not the challenge) as the code_verifier parameter—any mismatch here will fail validation.
• Double-check your Auth0 app settings: ensure PKCE is enabled (it's default for most app types now) and your allowed callback URLs are correctly configured.

内容的提问来源于stack exchange，提问作者mstorkson