Re: Pitfalls in CFB mode?

Kristian Gjøsteen wrote:
Couldn't you just shorten the shift register by an appropriate amount?

Sure, but you don't dare shorten it too much. If you shorten it so that
only n bits of ciphertext enter the shift register (and the remaining
128-n bits are constant), then thanks to the birthday paradox, the
resulting scheme will only be secure for encrypting up to around 2^{n/2}
blocks of plaintext. The first time there is any repeat in those n-bit
values, the subsequent block will be encrypted with the equivalent of
the two-time pad, which isn't good. For instance, if you use AES but
with the shift register shortened to n=64 bits, then you'll only be
secure for encrypting up to a few billion blocks of data.

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