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.