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Known security limitations ========================== Secure memory wiping -------------------- `Memory wiping`_ is used to protect secret data or key material from attackers with access to deallocated memory. This is a defense-in-depth measure against vulnerabilities that leak application memory. Many ``cryptography`` APIs which accept ``bytes`` also accept types which implement the buffer interface. Thus, users wishing to do so can pass ``memoryview`` or another mutable type to ``cryptography`` APIs, and overwrite the contents once the data is no longer needed. However, ``cryptography`` does not clear memory by default, as there is no way to clear immutable structures such as ``bytes``. As a result, ``cryptography``, like almost all software in Python is potentially vulnerable to this attack. The `CERT secure coding guidelines`_ assesses this issue as "Severity: medium, Likelihood: unlikely, Remediation Cost: expensive to repair" and we do not consider this a high risk for most users. RSA PKCS1 v1.5 constant time decryption --------------------------------------- RSA decryption has several different modes, one of which is PKCS1 v1.5. When used in **online contexts**, a secure protocol implementation requires that peers not be able to tell whether RSA PKCS1 v1.5 decryption failed or succeeded, even by timing variability. ``cryptography`` does not provide an API that makes this possible, due to the fact that RSA decryption raises an exception on failure, which takes a different amount of time than returning a value in the success case. Fixing this would require a new API in ``cryptography``, but OpenSSL does not expose an API for straightforwardly implementing this while reusing its own constant-time logic. See `issue 6167`_ for more information. For this reason we recommend not implementing online protocols that use RSA PKCS1 v1.5 decryption with ``cryptography`` -- independent of this limitation, such protocols generally have poor security properties due to their lack of forward security. .. _`Memory wiping`: https://devblogs.microsoft.com/oldnewthing/?p=4223 .. _`CERT secure coding guidelines`: https://wiki.sei.cmu.edu/confluence/display/c/MEM03-C.+Clear+sensitive+information+stored+in+reusable+resources .. _`issue 6167`: https://github.com/pyca/cryptography/issues/6167#issuecomment-1276151799
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