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Apr
27
comment CBC key lifetime, or, “how big is too big?”
Thanks CodeInChaos and cipher! That makes for a good way to frame the problem of collisions. At around 2^64 blocks, it crosses the 50% probability threshold. That point makes for seemingly high risk and very big messages though. With AES’s 128 bit block size, messages can be up to 64GB before breaking out of the 2^32 block length. This method works well highlighting the risk of unexpected disclosure by collisions in messages that are large (by today’s standards). The probability of birthday collision blocks is still less than one in a million for messages of several hundred petabytes.