[icanhasmath]

rsa 4.5.post1: Python 2.7 security release

rsa 4.5 is the last release that supports Python 2.7. It is still exposed to CVE-2020-25658 (GHSA-xrx6-fmxq-rjj2, high), a Bleichenbacher-style timing oracle in PKCS#1 v1.5 decryption. Upstream fixed it only in 4.7, which needs Python 3. This branch was opened for that fix in 2023 (#1, #2) but was never released: it still says version 4.3.

This PR finishes the backport and cuts the release.

Commit	What
ca9f489	CVE-2020-25658. Brings decrypt() to upstream 4.7's final form. #1 ported upstream dae8ce0 only; this adds 341e5c4 (the public length check raises at once) and f254895 (find() instead of index()/ValueError, so a missing separator takes the same path). Drops the unused import sys from #1. Adds regression tests that every malformed cleartext still raises DecryptionError.
4d9889e	Upstream sybrenstuvel#164 (f878c37): reject padding shorter than 8 bytes (RFC 8017 7.2.2 step 3). Upstream's test, without its Python 3 annotation.
(release)	Version 4.5.post1 in rsa/__init__.py and setup.py. The 4.3 & 4.5 changelog section, which #1 had rewritten, is restored, with a 4.5.post1 section on top.

Advisories against rsa

Advisory	Range	Status in 4.5.post1
CVE-2016-1494 / GHSA-8rjr-6qq5-pj9p	< 3.3	Fixed upstream before the base
CVE-2020-13757 / GHSA-537h-rv9q-vvph	< 4.1	Fixed in the 4.5 base (3283b12)
CVE-2020-25658 / GHSA-xrx6-fmxq-rjj2	>= 2.1, < 4.7	Patched here

OSV lists nothing against 4.7 through 4.9.1, so no later upstream fix needs carrying.

Verification (Python 2.7.18)

• tests/: 97 passed. The base branch has 92; the 5 new tests account for the rest.
• PaddingSizeTest fails without the pkcs1.py change and passes with it.
• The CVE-2020-25658 tests check behaviour: a timing fix cannot be shown red-to-green in a unit test. They pin that the restructured checks still reject a bad marker, a missing separator and an over-long ciphertext, and that a valid block decrypts.
• setup.py sdist bdist_wheel gives rsa-4.5.post1.tar.gz and rsa-4.5.post1-py2.py3-none-any.whl. rsa.__version__ is 4.5.post1.

After merge: tag version-4.5.post1 (the repo's tag style), a GitHub release (--latest=false), inventory rsa 4.5.0.1, and a CVE-scanner override row for CVE-2020-25658.

[icanhasmath]

Superseded by #4: the same release rebuilt on the version-4.5 tag (new base branch version-4.5.x) instead of version-4.3.x, so its history contains upstream 4.5. The final tree is byte-identical to this PR's head (987fe91), and tests/ passes 97 of 97 on Python 2.7. The 4.5.post1-security branch is left in place for reference.