12 Jan 2007 ncm   » (Master)

I will not be buying an Apple iPhone. No one in my extended family will be buying an Apple iPhone. I am, however, carefully watching lkcl's progress in putting Free Software on slightly less proprietary gadgets.

It turns out (as I had expected) the idea of combining encipherment and secure hashing in one operation is familiar in the crypto industry. However, the approaches I see published look painfully crude. EAX and CCM, in particular, are no faster than enciphering normally and then hashing with SHA, so what's the point? Kerberos 4 used "propagating CBC" which has the weakness that if you swap two (at-the-time 8-byte) ciphertext blocks, the plaintext is garbled but the checksum is good. That should be fixable with (more-or-less-) simple whitening. E.g, here's 16 bytes of endian-agnostic whitening:


    #define MASK64 0x00ff00ff00ff00ffULL
    #define WASH64 0x005a005a005a005aULL
    sum_lo = ((((sum_lo &  MASK64) +  WASH64) 
                & MASK64)
            | (((sum_lo & ~MASK64) + (WASH64<<8))
                & ~MASK64));
    sum_hi = ((((sum_hi &  MASK64) +  WASH64)
                & MASK64)
            | (((sum_hi & ~MASK64) + (WASH64<<8))
                & ~MASK64));
I kind of prefer the CFB w/checksum approach:

  C(i) = sum(i-1) XOR P(i)
  sum(i) = W(sum(i-1)) XOR AES(C(i))
  sum(0) = AES(IV)
where W is whitening as above.

apenwarr: Automatic variable-type (and argument-type) deduction from initialization value is (1) well-known from Haskell and ML, (2) one of the important new features of C++09, and (3) coming this year or early next to a GNU G++ near you. C# could have had the feature any time, but MS didn't think of adding it until they saw C++ would have it. Of course it's particularly useful only when you have real templates.

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