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# 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'=')
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