Hi, On 2021-05-25 17:29:03 -0400, Bruce Momjian wrote: > So, let me ask --- I thought CTR basically took an encrypted stream of > bits and XOR'ed them with the data. If that is true, then why are > changing hint bits a problem? We already can see some of the bit stream > by knowing some bytes of the page.
A *single* reuse of the nonce in CTR reveals nearly all of the plaintext. As you say, the data is XORed with the key stream. Reusing the nonce means that you reuse the key stream. Which in turn allows you to do: (data ^ stream) ^ (data' ^ stream) which can be simplified to (data ^ data') thereby leaking all of data except the difference between data and data'. That's why it's so crucial to ensure that stream *always* differs between two rounds of encrypting "related" data. We can't just "hope" that data doesn't change and use CTR. Greetings, Andres Freund
