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What is it about a smaller block size that makes it less secure? Wikipedia mentions that it's recommended not to encrypt files larger than 4GB because of the bl
by doomrobo 11y ago
What is it about a smaller block size that makes it less secure? Wikipedia mentions that it's recommended not to encrypt files larger than 4GB because of the block size but it never specifies why.
- tptacek 11y agoTwo that jump immediately to mind: * Constructions like CTR need to divide the block, in CTR's case to hold both a counter and a nonce. The convention is to use half the block for the nonce and half for the counter, which, in an 8-byte block, means you're working with a 32 bit nonce (much too small) and a 32 bit counter. We actually broke an embedded system that, because of protocol headroom issues, used small counters for CTR mode. * (Related) That issue more or less rules out all the modern AEAD modes, all of which are specified on 16+-byte blocks, for that reason. * The smaller block size makes malleability attacks easier. For instance, when you're bitflipping CBC, the edits scramble a block and alter the next. With an 8 byte block, you have much more control over what gets scrambled and what gets edited.