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https://ja.hideki.hclippr.com/2015/09/01/line%e3%81%aee2ee%e5%ae%9f%e8%a3%85%e3%80%8cletter-sealing%e3%80%8d%e5%88%9d%e8%a6%8b/
LINEのE2EE実装「Letter Sealing」初見