# Copyright 2016 Google Inc. All rights reserved. # # Licensed under the Apache License, Version 2.0 (the "License"); # you may not use this file except in compliance with the License. # You may obtain a copy of the License at # # http://www.apache.org/licenses/LICENSE-2.0 # # Unless required by applicable law or agreed to in writing, software # distributed under the License is distributed on an "AS IS" BASIS, # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. # See the License for the specific language governing permissions and # limitations under the License. """ Utility functions for implementing Proof Key for Code Exchange (PKCE) by OAuth Public Clients See RFC7636. """ import base64 import hashlib import os def code_verifier(n_bytes=64): """ Generates a 'code_verifier' as described in section 4.1 of RFC 7636. This is a 'high-entropy cryptographic random string' that will be impractical for an attacker to guess. Args: n_bytes: integer between 31 and 96, inclusive. default: 64 number of bytes of entropy to include in verifier. Returns: Bytestring, representing urlsafe base64-encoded random data. """ verifier = base64.urlsafe_b64encode(os.urandom(n_bytes)).rstrip(b'=') # https://tools.ietf.org/html/rfc7636#section-4.1 # minimum length of 43 characters and a maximum length of 128 characters. if len(verifier) < 43: raise ValueError("Verifier too short. n_bytes must be > 30.") elif len(verifier) > 128: raise ValueError("Verifier too long. n_bytes must be < 97.") else: return verifier def code_challenge(verifier): """ Creates a 'code_challenge' as described in section 4.2 of RFC 7636 by taking the sha256 hash of the verifier and then urlsafe base64-encoding it. Args: verifier: bytestring, representing a code_verifier as generated by code_verifier(). Returns: Bytestring, representing a urlsafe base64-encoded sha256 hash digest, without '=' padding. """ digest = hashlib.sha256(verifier).digest() return base64.urlsafe_b64encode(digest).rstrip(b'=')