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I know from the bottom of my heart how bad the idea of custom-crypto is. I absolutely concur with the recommendation that it should only be done by professionals who specialize in implementing crypto safely.

However, exactly because it's a Bad Idea ™ it's really hard to find good information on this topic. Say one of the aforementioned professionals is looking at a crypto implementation and they want to judge how safe it is. Some things that come to mind:

  • Constant-time implementation (to protect against timing/side-channel attacks)
  • Randomness
    • Source of randomness (true vs. pseudo RNGs)
    • Bits of entropy
  • Zeroing intermediate buffers
  • Appropriate test coverage with test vectors (e.g., NIST, Wycheproof)

What are other areas like this that need to be probed? Is there any documentation/checklist/book that can shed further light on the topic -- something like 1 but more comprehensive?

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    $\begingroup$ It depends on the type of cryptography what kind of attacks can be mounted. Without it all cryptographic attacks ever would fit the bill, because a non-cryptographer (and, quite often, even cryptographers) are bound to repeat past mistakes. $\endgroup$ – Maarten Bodewes Mar 12 at 23:08
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    $\begingroup$ Another one that comes to mind: who wrote it? While not a determining factor, if the software is written by someone known to write good cryptographic code, that's certainly a plus. If it's someone new (like myself), it's reason to be more cautious. $\endgroup$ – Ruben De Smet Mar 13 at 8:07
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One widely accepted standard for information protection is the ISO Common Criteria framework. This set of slides gives some sense of what a Common Criteria evaluation will ask in its analysis.

The US FIPS-140-3 standard also carries a lot of weight, it specifies certain NIST special publications that give criteria beyond ISO.

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