| # 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'=') |