On Sat, Jul 20, 2019 at 03:39:25PM -0400, Sehrope Sarkuni wrote:
> How about storing the CRC of the encrypted pages? It would not leak
> any additional information and serves the same purpose as a
> non-encrypted one, namely I/O corruption detection. I took a look at
> pg_checksum and besides checking for empty pages, the checksum
> validation path does not interpret any other fields to calculate the
> checksum. I think even the offline checksum enabling path looks like
> it may work out of the box. Checksums of encrypted data are not a
> replacement for a MAC and this would allow that validation to run
> without any knowledge of keys.
>
> Related, I think CTR mode should be considered for pages. It has
> performance advantages at 8K data sizes, but even better, allows for
> arbitrary bytes of the cipher text to be replaced. For example, after
> encrypting a block you can replace the two checksum bytes with the CRC
> of the cipher text v.s. CBC mode where that would cause corruption to
> cascade forward. Same could be used for leaving things like
> pd_pagesize_version in plaintext at its current offset. For anything
> deemed non-sensitive, having it readable without having to decrypt the
> page is useful.
Yes, I did cover that here:
https://www.postgresql.org/message-id/20190716002519.yyvgl7qi4ewl6pc2@momjian.us
Yes, it would only work if the checksum was the last part of the page,
or if we used CTR mode, where changing the source bits doesn't affect
the later bits. I am thinking crazy here, I know, but it seemed worth
mentioning in case someone liked it.
https://www.postgresql.org/message-id/20190715194239.iqq5jdj54ru32kmt@momjian.us
If we want to go crazy, we could encrypt, assume zeros for the CRC,
compute the MAC and put it in the place of the CRC is, but then tools
that read CRC would see that as an error, so we don't want to go there.
Yes, crazy.
I know this thread is long so you might have missed it.
I do think CTR mode is the way to go for the heap/index pages and the
WAL, and will reply to another email on that topic now.
> It does not have to be full bytes either. CTR mode operates as a
> stream of bits so its possible to replace nibbles or even individual
> bits. It can be something as small one bit for an "is_encrypted" flag
> or a handful of bits used to infer a derived key. For example, with
> 2-bits you could have 00 represent unencrypted, 01/10 represent
> old/new key, and 11 be future use. Something like that could
> facilitate online key rotation.
Yes, if we do all-cluster encryption, we can just consult pg_control,
but if we do per-table/index, that might be needed. There is another
email suggesting symlink file to a key file could indicate an encrypted
table/index.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
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