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phantomcraft avatar Crypto

Does random IVs in CTR block cipher mode combined with XOR allow to encrypt number of blocks up to the same numeric count of IV?

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phantomcraft

I read about CTR block cipher mode in Wikipedia: "If the IV/nonce is random, then they can be combined with the counter using any invertible operation (concatenation, addition, or XOR) to produce the actual unique counter block for encryption."

Let's suppose I want to make a enciphering scheme with CTR and AES in which I XOR the counter value with the full IV.

Does it have advantage over combining a Nonce+Counter?

If the block size is 128-bits, does XORing a 128-bit counter onto a random IV allow to encrypt up to 2^128 blocks?

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ctr
kelalaka avatar
in flag
kelalaka
The small problem of x-or is the additional x-or operation per block. Once nonce||counter is set, there is no operation other than the increment. Though the counter in the normal mode limits the number of blocks to encrypt under the same (key,nonce) this is most necessary since AES is a PRP that has a long message distinguished from PRF. Xoring on the other hand has no limit is set explicitly due to the boundaries. In short, there is not much difference.
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