On Mon, Jul 15, 2019 at 06:05:37PM -0400, Bruce Momjian wrote:
>On Mon, Jul 15, 2019 at 10:44:34PM +0200, Tomas Vondra wrote:
>> On Mon, Jul 15, 2019 at 03:55:38PM -0400, Bruce Momjian wrote:
>> > The crazy seems more sane now --- "encrypt the page with CRC contents as
>> > zero" (which we probably already do to compute the CRC), then compute
>> > the CRC, and modify the page CRC.
>> >
>>
>> Huh? So you want to
>>
>> 1) set CRC to 0
>> 2) encrypt the page
>> 3) compute CRC
>> 4) set CRC to value computed in (3)
>> 5) encrypt the page again
>>
>> That seems pretty awful from performance POV, and it does not really
>> solve much as we'd still need to decrypt the page while verifying the
>> checksums (because the CRC is in the page header, which is encrypted).
>
>No, I was thinking we would overwrite whatever the encrypted output was
>in the spot that has the CRC with the computed CRC. Yeah, sounds even
>crazier now that I said it --- never mind.
>
Uh, how could that possibly work? Symmetric ciphers are "diffusing" the
bits within the block, i.e. replacing 16 bits in a 128-bit ciphertext
block will affect the whole plaintext block, not just the matching 16
bits of plaintext.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
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