Encrypting with CBC then XORing repeatedly a chunk of random data smaller than the plaintext: Is there a gain in security?

Question

If I encrypt a 1MiB file with AES-CBC (or any other cipher) and XOR a 128KiB of (truly) unpredictable random data repeating until the end of file, will I have a security of 1048576-bits (128KiB*8)?

This question is a little weird, but I would like to know if this scheme has a security flaw (maybe known-plaintext attacks).

score 2 · Accepted Answer · answered Nov 28 '22 at 00:21

This will be only as secure as AES-CBC, as the repeating XOR you're describing is massively vulnerable to a myriad of attacks, including known-plaintext attacks. Also remember that you could "cancel out" the 128 KiB by XORing two blocks together, since $P_1 \oplus K \oplus P_2 \oplus K = P_1 \oplus P_2$.

Don't try to chase large key sizes. 256 bits of key material is more than enough.

Encrypting with CBC then XORing repeatedly a chunk of random data smaller than the plaintext: Is there a gain in security?

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