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The perils of using encryption without authentication or integrity protection are well known in the cryptographic research community. Yet its exactly the mandatory support for unauthenticated encryption that forms the basis of a serious security flaw in an IPsec implementation we recently discovered. In response, the UK's equivalent to CERT, the National Infrastructure Coordination Centre published a vulnerability advisory about the flaw. Vendors also issued updated recommendations to customers, and we saw a flurry of discussion on Slash-dot and the sci.crypt newsgroup. In the aftermath, we asked ourselves, how did this happen?