HKDF in cryptography before 1.5.2 returns an empty byte-string if used with a length less than algorithm.digest_size.
www.openwall.com/lists/oss-security/2016/11/09/2
www.securityfocus.com/bid/94216
www.ubuntu.com/usn/USN-3138-1
cryptography.io/en/latest/changelog
github.com/advisories/GHSA-q3cj-2r34-2cwc
github.com/pyca/cryptography/commit/b924696b2e8731f39696584d12cceeb3aeb2d874
github.com/pyca/cryptography/issues/3211
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