In Firefox 4, there was a need to implement the HKDF crypto algorithm in a variety of code bases: Java, Javascript, C using OpenSSL, C using NSS. One team coded three implementations. Later, another team coded an implementation. But, that last implementation didn&#x27;t interop with the first three. Of course, the two teams disagreed about which was wrong, each believing that they were right and the other team was wrong.<p>Surely if three different implementations in three different languages agree, then the problem is with the fourth, disagreeing implementation! However, the second team annoyingly pointed out that they had automated tests that covered the official test vectors. Well, maybe the tests were coded wrongly too! Well, could the first team implement the tests that include the official test vectors? What a waste of time! There are automated tests that check that the first three implementations all calculate the same results for many random inputs!<p>It turns out that the very first implementation, in Javascript, had a bug, and that bug was transliterated into two other implementations. But since the second team hated reading Javascript code, and because that team consisted 100% of annoying (but good-looking) pedants, they implemented the algorithm by reading the spec. instead of by copying the first implementation, and so they got it right.<p>Incidentally, the bug was that the key and value in an HMAC calculation were swapped. HKDF uses HMAC in a counterintuitive way, where the key isn&#x27;t (necessarily) a secret, but the value is.