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I often see the suggestion that storing the length of the file too helps secure against hash collisions because it adds the additional requirement that both fil

• hvidgaard 10y ago
  It's not a myth. It is obvious that you can find collisions with this constraint. When you make a collision attack such as this, you start with 2 inputs and have a target size, i.e. with a specific size of the padded data, then you simply iterate over paddings (of the same size), and when you find a collision, it will be of the correct size. But it does add an extra constraint, and it prevent attacks that rely on random length filler data. This is not much of an improvement, but an improvement non the less.
  • AgentME 10y ago
    I can agree it strictly limits the type of attacks that can be done, but it doesn't appear to be any practical increase in security. It's like arguing for adding another step in the manufacturing process of bulletproof armor to add a few extra atoms to the armor's thickness. Sure it theoretically adds some protection against some 0.000...1% of bullets that would have just barely had enough force to get through, but it wouldn't have made a difference to any single previously observed successful attack of a similar nature. The fact that someone would argue that this increase in defense is good enough to obviate the need of switching to a different type of stronger defense seems to imply a misunderstanding of hashing.