Observable Timing Discrepancy in cryptography

Summary

cryptography is a package designed to expose cryptographic primitives and recipes to Python developers. In versions 44.0.0 prior to 50.0.0, `pkcs7_decrypt_der`, `pkcs7_decrypt_pem`, and `pkcs7_decrypt_smime` reported the outcome of decrypting a `RecipientInfo's encryptedKey` in several distinguishable ways, one of which disclosed the exact length recovered from the RSA operation. The same distinction was also observable by timing. An application that decrypts attacker-supplied `EnvelopedData` and reflects the outcome gives the attacker a `Bleichenbacher` oracle against the content-encryption key. Decryption ran as RSA PKCS#1 v1.5 decrypt of `encryptedKey`, build an AES cipher from the result, then AES-CBC decrypt and PKCS#7 unpad. Invalid RSA padding, a valid padding with a bad key length, a correct length with a wrong key, and the real key each failed or succeeded differently. Case 1 is reachable only where the linked library lacks implicit rejection: OpenSSL 3.0 and 3.1, LibreSSL, and `BoringSSL`. Exploitation requires a service that auto-decrypts untrusted `EnvelopedData` matching the victim certificate and answers adaptively at high volume, such as an S/MIME gateway or mail filter. This issue is fixed in 50.0.0.