How to Generate PKCE Code Challenge in Python

import base64
import hashlib
import os
import secrets
import string

def generate_code_verifier(length=64):
    alphabet = string.ascii_letters + string.digits + "-._~"
    return "".join(secrets.choice(alphabet) for _ in range(length))

def generate_code_challenge(code_verifier, method="S256"):
    if method == "S256":
        digest = hashlib.sha256(code_verifier.encode("utf-8")).digest()
        return base64.urlsafe_b64encode(digest).rstrip(b"=").decode("utf-8")
    elif method == "plain":
        return code_verifier
    else:
        raise ValueError(f"Unsupported method: {method}")

def generate_pkce_pair(method="S256", verifier_length=64):
    verifier = generate_code_verifier(verifier_length)
    challenge = generate_code_challenge(verifier, method)
    return {"code_verifier": verifier, "code_challenge": challenge, "method": method}

if __name__ == "__main__":
    # Simple demonstration
    verifier = generate_code_verifier()
    challenge = generate_code_challenge(verifier)
    print(f"Code Verifier: {verifier}")
    print(f"Code Challenge (S256): {challenge}")
    print(f"Code Challenge length: {len(challenge)} chars")

    # Example with custom length
    pair = generate_pkce_pair("S256", 43)
    print(f"\nCustom 43-char verifier: {pair['code_verifier']}")
    print(f"Corresponding challenge: {pair['code_challenge']}")