3 ms·
That's not entirely true for counter mode. For random noise, if you see 2^(n/2) n-bit blocks then there's a ~50% probability of two of those blocks being ident
by caf 13y ago
That's not entirely true for counter mode. For random noise, if you see 2^(n/2) n-bit blocks then there's a ~50% probability of two of those blocks being identical, but if you have that many n-bit blocks from an n-bit block cipher in counter mode the probability of two identical blocks is zero.
This doesn't apply to stream ciphers.
- simias 13y agoInteresting but I don't understand why the probability of two identical blocks in this situation would be 0 for CTR mode. As far as I know theoretically there could be a collision and the cipher could output the same value for two different counter values, unless the block cipher guarantees that such a collision can never occur? I was not aware of such a guarantee.
- caf 13y agoA block cipher cannot produce the same output block for two distinct input blocks because it is reversible.