Security: RC4 encryption primitives are exposed for active use

[tuanaiseo]

Problem

The cryptography provider implements and exposes ARC4/RC4 encrypt/decrypt paths. RC4 is cryptographically weak and should not be used for new data protection. If selected for PDF encryption, confidentiality is significantly weaker than modern AES-based modes.

Severity: low
File: pypdf/_crypt_providers/_cryptography.py

Solution

Restrict RC4 to legacy-read compatibility only, disable RC4 for new writes by default, and require explicit insecure opt-in with clear warnings; prefer AES-256-based PDF encryption profiles.

Changes

• pypdf/_crypt_providers/_cryptography.py (modified)

Testing

• Existing tests pass
• Manual review completed
• No new warnings/errors introduced

[stefan6419846]

Thanks for the PR. Please follow the usual code style and do not use custom approaches while ensure that all existing tests pass (which is not checked on your to-do list as well).

Additionally, such a change should be discussed first as it might have further implications - especially given that changing a default requires a proper deprecation period. From the user side, having to maintain an environment variable feels a bit complicated just to enable this; all other choices are done by monkey-patching a constant.

Edit: Why is this an issue for the cryptography provider only, but not for the pycryptodome or fallback provider?

[stefan6419846]

Closing due to the lack of response and unanswered open questions. Please open a issue outlining a proper migration path if you think that this is an actual issue.