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Forward secure sealingForward secure sealingPosted Aug 25, 2012 20:22 UTC (Sat) by ikm (subscriber, #493)In reply to: Forward secure sealing by nybble41 Parent article: Forward secure sealing
> S[m] = HASH(K[n] + M[m])
May I suggest S[m] = HMAC(K[n], M[m])?
Also, calculating K[n] from V is O(n). If we use the systemd default of 15 minutes per key, we would have 35,040 iterations per year, which doesn't seem bad. If we, however, decide to narrow it down to 10 seconds, as the article suggested we could, we would get a much worse looking number of 3,153,600 iterations per year, which might get a little expensive, especially if the verification is done on an Android device. Other than that, the scheme you've proposed seems fit and may even be the actual scheme systemd uses.
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