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differential oracle

A testing method that checks your code by comparing its output, byte for byte, against a trusted reference implementation.

Instead of hand-writing what every correct answer should be, a differential oracle feeds the same inputs to both your implementation and a known-good one (like OpenSSL) and flags any difference. If they ever disagree, at least one is wrong. It is an especially powerful way to test cryptographic code, where subtle bugs are easy to miss.