fix: handle URL-safe base64 decoding for JWT

[baku2san]

Description

This pull request addresses an issue with JWT decoding where URL-safe base64 decoding was not being used, leading to UTF-8 decoding errors. The changes ensure that the base64 encoded string is properly padded and decoded using URL-safe base64 decoding.

RFC4648

Changes:

• Replaced base64.decodebytes with base64.urlsafe_b64decode.
• Added logic to calculate and append necessary padding to the base64 string.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Thank you for your contribution @baku2san! We will review the pull request and get back to you soon.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[pvaneck]

Thanks for the PR and improving the JWT payload decoding! Left a suggestion inline.

Do you mind also adding this change to the async decorator as well?

[baku2san]

@microsoft-github-policy-service agree

[pvaneck]

Thanks for the updates. Seems like the black reformatting might be causing some conflicts. To be sure that we use the same black versioning/configuration that the CI uses, try running it via tox (from inside the sdk/identity/azure-identity directory):

pip install tox
tox run -e black -c ../../../eng/tox/tox.ini --root . -- azure

[baku2san]

Thanks for the guidance on using tox for Black formatting. I've followed your instructions and reformatted the code accordingly.

I have verified and confirmed that the expression padding_needed = -len(base64_meta_data) % 4 is correct for determining the necessary padding in Base64 encoding. The results clearly show that this method accurately calculates the required padding, ensuring the data length is a multiple of 4.

Thank you for your assistance in clarifying this!

len(base64_meta_data)	padding_needed = -(len(base64_meta_data) % 4)	padding_needed = -len(base64_meta_data) % 4	base
0	0	0	4
1	-1	3	3
2	-2	2	2
3	-3	1	1
4	0	0	4

import pandas as pd

lengths = range(11)  # This will create a range from 0 to 10
results = []
for length in lengths:
    expr1 = -(length % 4)
    expr2 = -length % 4
    expr3 = 4 - (length % 4)
    results.append((length, expr1, expr2, expr3))

df = pd.DataFrame(results, columns=["len(base64_meta_data)", "padding_needed = -(len(base64_meta_data) % 4)", "padding_needed = -len(base64_meta_data) % 4", "base"])
print(df)