Don't leak how PKCS#7 encryptedKey decryption failed - #15369
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pkcs7_decrypt_der and its PEM and S/MIME variants reported invalid RSA PKCS#1 v1.5 padding, a recovered key of the wrong length, and a wrong key as three distinct errors, and the middle one disclosed the exact recovered length. That is a Bleichenbacher oracle for any caller that decrypts untrusted messages. The wrong-length case also returned before the AES-CBC pass, so the same distinction was observable by timing, proportional to a content size the attacker chooses. Substitute a random key of the expected length on failure, per RFC 3218, and continue down an identical path so that every outcome looks alike. EnvelopedData still authenticates nothing, so revealing whether decryption succeeded remains a CBC padding oracle regardless. That is a property of PKCS#7 rather than of this implementation, and is now documented instead. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Only a ValueError from unwrapping the key is attacker controlled and so deliberately silenced. A private key implemented outside of this library can fail in other ways, and those still reach the caller -- but nothing exercised that arm, leaving it uncovered. Add a DummyRSAPrivateKey double whose decrypt() raises a TypeError, and tests for each of its methods. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
reaperhulk
reviewed
Jul 31, 2026
| .encrypt(serialization.Encoding.DER, []) | ||
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| with pytest.raises(TypeError, match="the smartcard is unplugged"): |
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Let's use a message that makes it clear this is a test rather than a plausible error message.
reaperhulk
approved these changes
Jul 31, 2026
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pkcs7_decrypt_der and its PEM and S/MIME variants reported invalid RSA PKCS#1 v1.5 padding, a recovered key of the wrong length, and a wrong key as three distinct errors, and the middle one disclosed the exact recovered length. That is a Bleichenbacher oracle for any caller that decrypts untrusted messages. The wrong-length case also returned before the AES-CBC pass, so the same distinction was observable by timing, proportional to a content size the attacker chooses.
Substitute a random key of the expected length on failure, per RFC 3218, and continue down an identical path so that every outcome looks alike.
EnvelopedData still authenticates nothing, so revealing whether decryption succeeded remains a CBC padding oracle regardless. That is a property of PKCS#7 rather than of this implementation, and is now documented instead.