Zeroise private random data on the stack before returning

[torben-hansen]

CryptoAlg-1524

Description of changes:

Zeroise private random data immediately after use. This is just good secret value hygiene.

Most is stack-allocated. So, good chances it would be overwritten fairly quickly anyway. Looked in both /crypto and /ssl, grepping my way through.

Testing:

CI

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[andrewhop]

Did you consider using sizeof in the calls to OPENSSL_cleanse to ensure all the data is cleansed as expected? If a future change made one of these arrays smaller it would trigger an address sanitizer error as cleanse tries to cleanse past the end, but if the array gets bigger the cleanse would only cleanse the first part.

Also this change introduces an inconsistencies: some places use a new constant (ED25519_SEED_LEN) while others continue to use magic numbers.

[torben-hansen]

Did you consider using sizeof in the calls to OPENSSL_cleanse to ensure all the data is cleansed as expected?

There is an equal issue here if the array is made a pointer that is dynamically allocated. Then sizeof would be bad. So, I think just sticking to current implementation. Let me know if you have other concerns.