On 11/9/2015 11:15 AM, Dimitry Sibiryakov wrote: > 09.11.2015 16:49, Jim Starkey wrote: >> For CBC mode, the initialization vector is XORed into the first block of >> plaintext. Without this (or something like it), the first 16 bytes of >> every page would have the same encryption, allowing a mapping from >> cryptotext to presumed plaintext, possibly leaking useful information. > XORing of two known values (or doing whatever similar) won't add more > problem for > attacker because the result also will be a known plaintext. To make plaintext > unknown, > initialization vector also must be unknown. > But all this doesn't matter as AES is considered to be practically > invulnerable to > known-plaintext attacks. > It matters because if every page is encrypted with the same key and initial state, information can be learned by building a table of first blocks. If two pages have the same encryption, then an attacker knows that those pages have common prologs. This isn't a known plaintext attack, but an analysis of cryptotext. It doesn't do anything towards breaking the key, only to extract "leaked" information.
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